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Nobody knows exactly when life began. The oldest confirmed fossils, of single-celled microorganisms, are 3.5 billion years old. Life may have begun a bit earlier than that, but probably not while huge rocks were still raining down on Earth. Life may have begun in warm alkaline vents on the seabed, or in open water, or on land. We don't know, and we don't know what the first organisms were like.

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Soon after animals evolved, evolution went through two major growth spurts. In the Cambrian Explosion, it seems almost every group of modern animals appeared within tens of millions of years. This apparent 'explosion' may be partly down to better fossilisation, as many animals now had hard shells. Then 489 million years ago, each animal group expanded in the Great Ordovician Biodiversification Event.

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Boom, you're extinct. 65 million years ago, a huge chunk of rock from outer space smashed into what is now Mexico. The explosion was devastating, but the longer-term effects were worse. Dust was thrown into the upper atmosphere and blocked out sunlight, and in the ensuing cold and darkness Earth suffered its fifth and last mass extinction. The dinosaurs were the most famous casualties, but pterosaurs and giant marine reptiles were also wiped out.

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Smithsonian National Museum of Natural History

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Learn what fossil evidence reveals about the origins of the first life on Earth, from bacteria to animals, including the phyla we know today.

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This story appears in the March 2018 issue ofNational Geographic magazine.

On the southeast coast of Newfoundland, near North America's farthest eastward reach, lies a promontory of rocky cliffs called Mistaken Point. The place got its name from the shipwrecks it helped cause in foggy weather, when sea captains mistook it for somewhere else. Today it represents something quite different: a set of extraordinary clues, recently reinterpreted, to one of the deepest and most puzzling mysteries of life on Earth. After burbling along for more than three billion years as tiny, mostly single-celled things, why did life suddenly erupt into a profusion of complex creatures-multicellular, big, and astonishing? Although these new life-forms spread worldwide, beginning at least 570 million years ago, the earliest evidence of them has been found in one place: Mistaken Point. Paleontologists have been going there for decades. But what the experts think they see now, in small nuances with large implications, is radical and new.

On a cool autumn day I made the journey to Mistaken Point myself, driving south from St. John's, Newfoundland's capital, in a rented Jeep, along a black ribbon of highway through forests of spruce and fir. With me were Marc Laflamme of the University of Toronto Mississauga and his longtime colleague Simon Darroch, an Englishman based at Vanderbilt University in Nashville.

We reached Mistaken Point beneath a blue sky and a blazing sun-rare weather, Laflamme told me, but the strong angled light, especially in late afternoon, helped highlight the subtle fossils we had come to see.

At the Mistaken Point Ecological Reserve, established by the provincial government to protect the fossil beds, we took a gravel road to a broken sea bank and climbed down. Laflamme pointed to a single slab of smooth, purplish gray rock, tilted at about 30 degrees. An image in the stone, like an intricate shadow, suggested the skeleton of a snake, a repeating pattern of ribs and spine, about three feet long. But there was no skeleton here, indeed no bone at all-only the imprint of a soft-bodied creature, dead and buried on the sea bottom a very, very long time ago. It didn't swim; it didn't crawl. It couldn't have lived like any organism alive today. It belonged to a more obscure period, inhabited by cryptic, otherworldly creatures that most people don't realize ever existed. "This is the first time that life got big," Laflamme told me as we knelt on the rock.

The mystery of these life-forms, known as Ediacarans (Ee-dee-AK-arans), begins in the remote Flinders Ranges of South Australia, where a young geologist named Reginald Sprigg, on an assignment to reassess the derelict Ediacara Mines in 1946, noticed some peculiar impressions in exposed sandstone beds. They seemed to him "suggestive of jellyfish." They weren't jellyfish. There were other shapes too, some of them bearing no clear resemblance to any known creature, living or extinct. One figure looked like a fingerprint pressed into the sand.

Sprigg didn't realize (nor did those who had found similar figures in stone before, unsure what to make of them) that the fossils were about 550 million years old-dating to at least 10 million years before a better known evolutionary drama, the famous Cambrian explosion. Scientists until then had believed that the Cambrian explosion was the point when life on Earth opened out, kaboom, like a starburst of wondrous beasts-elaborate and sizable beings (we call them animals), many of whose descendants are still around. Sprigg's discovery proved important as a first signal that the period now called Ediacaran, not the Cambrian just following it, was where the saga of bigness and complexity began.

Then in 1967 a graduate student named S. B. Misra noticed a fossil-rich slab of mudstone at Newfoundland's Mistaken Point. Some of its ancient forms seemed to match the "jellyfish" things from South Australia, others looked like fronds, but several resembled nothing known to science. Other beds nearby, sitting one upon another like layers of Precambrian cake, also proved to contain abundant and various fossils, preserved together as whole communities. Many were still covered with thin crusts of fallen volcanic ash, like icing between each layer of cake. The ash, with its traces of radioactive uranium and the lead into which that decays, allowed for precise radiometric dating of the beds. The Mistaken Point fossils, going back 570 million years, are the earliest evidence on Earth of large, biologically complex beings.

There are now more than 50 different Ediacaran forms known, from nearly 40 localities, on every continent except Antarctica. So what was it, after billions of years of only microbes populating the globe, that allowed the Ediacarans to get big and cover the Earth? And what does their bigness suggest about their internal anatomies, their means of feeding, their ways of living?

Before Ediacaran forms flourished on the planet, evolution worked on a mostly microscopic scale, kept in check by a shortage of oxygen, the element that fuels animal metabolism. Thanks to marine bacteria that generated oxygen as a product of photosynthesis, levels of the gas rose about two billion years ago but stayed relatively low for another billion years. Then, between 717 million and 635 million years ago, a series of glaciations took place, so widespread and severe that they may have frozen over the entire planet, a situation some scientists call a "snowball Earth." During that time oxygen levels bumped up again, for reasons that are still poorly understood.

The great freeze ended as volcanic eruptions spewed carbon dioxide into the atmosphere, creating an early greenhouse effect that warmed the planet and thawed the oceans. Another brief glaciation around 580 million years ago, known as the Gaskiers, may not have been global, but it put Newfoundland, among other places, in a deep freeze. These changes all preceded the earliest appearance of Ediacarans in the fossil record. Were they the causes of what happened next? Did the end of the glaciers, an increase in available oxygen, and the evolution of more complex cells allow the Ediacarans to blossom, like the first crocuses of springtime? Maybe.

Equally enigmatic is their relationship to life today. One eminent German paleontologist, Adolf Seilacher, assigned them to a kingdom all their own, distinct from the animal kingdom, because of what he called their "unique, quilted type of biological construction," so different from most multicellular animals. The "quilted" effect seemed to offer structural stability that might have compensated for the absence of a skeleton. Maybe the quilting, and the frondy shapes, also helped maximize surface area, so they could better absorb nutrients through their skin.

Nutrition would have been problematic for the Ediacarans because, so far as fossil evidence reveals, almost none of them had a mouth. They had no gut, no anus. No head, no eyes, no tail. In some cases there was a sort of anchoring knob or disk at one end, now known as a holdfast, which gripped the sea bottom and allowed the frond to waft upward in the water. Many sea-bottom areas at that time were coated with thick microbial mats, which helped stabilize the sediments like a layer of crusty soil. But the frond wasn't a plant-photosynthesis couldn't have nourished it-because many Ediacarans lived in the depths, thousands of feet underwater, where light didn't penetrate.

If they couldn't eat and they couldn't photosynthesize, how did they nourish themselves? One form, a sluglike thing called Kimberella, may have scratched up and swallowed (this one did have a mouth, major advantage!) sustenance from the microbial mats beneath it. But the leading hypothesis for most Ediacarans is osmotrophy, a fancy word for a very basic process: the uptake of dissolved nutrients by osmosis, or absorption through their outer membrane. It was good enough, maybe, in a simpler world at a simpler time, but it would have been meager sustenance. Some scientists have focused on another fascinating aspect of many Ediacarans: their finer architecture. At a glance they look quilted, but close inspection reveals that their structure is fractal. That is, similar patterns repeat themselves at progressively smaller scales. A big frond was composed of smaller fronds, and those smaller fronds composed of still smaller fronds, all similar except for size. The basic shape echoes itself at three or four scales. Possibly that fractal structuring helps explain how they were able to grow large. It provided some rigidity, it maximized surface area, and perhaps it reflected a genetic shortcut. A simple formula in the genome might have specified: Build a small frondy unit, then repeat that operation over and over, adding one upon another, to make me big.

This sort of fractal structure showed in the snakelike creature Marc Laflamme and I saw in the purplish gray rock at Mistaken Point. It shows too in a number of other Ediacarans, collectively called rangeomorphs, named for a Namibian exemplar of the form, known as Rangea. During our day on the Newfoundland rocks, Laflamme steered my eyes onto many more rangeomorphs, inconspicuous from 10 feet away but spooky when viewed closely. Here was Beothukis mistakensis, a paddle-shaped frond, named for its locale of discovery. Over there was Fractofusus, a spindle-shaped form, tapered at both ends. It lived flat on the sea bottom. When death came to a community of Ediacarans, as when a blizzard of volcanic ash settled through the seawater to smother them or an avalanche of sediment came off a steep slope to bury them, the vertical frondy things sometimes got smashed over (as the fossil evidence shows), but the Fractofusus spindles seem to have died gently where they lay.

Although these rangeomorphs dominated the deep-sea ecosystem at Mistaken Point for millions of years and flourished elsewhere in somewhat shallower water, they all disappeared, leaving no known descendants. By the start of the Cambrian period 541 million years ago, or soon after, they had almost entirely vanished from the fossil record as we know it. That's why some scientists have suggested that the Ediacarans represent "failed experiments" in the early evolution of multicellular life.

Why did the Ediacarans suddenly disappear? Was the extinction absolute, or were there descendants in different forms? And if the end wasn't so abrupt and complete, what finished the Ediacarans as Ediacarans, dying off species by species in obscurity?

Laflamme's colleague Simon Darroch has offered one possible answer. On the afternoon of our visit to Mistaken Point, Darroch reached into his day pack and produced a surprise: small pieces of flat brown stone from the late Ediacaran beds he studies in Namibia. He had brought them from his lab at Vanderbilt to show me some trace fossils. A trace fossil, as distinct from a body fossil, records traces of animal activity-moving, chewing, defecating-as preserved in rock. It's a record of behavior, not of bodily shape. Any such traces are notable in the Ediacaran period, because most Ediacarans couldn't do those things: move, chew, or defecate.

"This is a very static, sessile ecosystem," Darroch said, referring to a famously rich early Ediacaran fossil bed on which we stood.

The later Ediacaran, as revealed in Namibian rocks, was much different. One big difference, he said, was that "for the first time we have complex burrowing." Experts disagree about just when the intricate patterns of burrowing creatures first appeared, but by any judgment those traces signaled a big change from the Ediacaran to the Cambrian. Wormy creatures had long been wriggling along on the sea bottom; now they were tunneling down into it as well. Darroch showed me a little slab marked with dotted-line traces. "They're on the surface, and they disappear, then they come to the surface again." That was evidence of an organism with complicated musculature, allowing it to move about in three dimensions. If it moved that way, it had a front and a rear end. On its front end, probably a mouth. In the mouth, maybe teeth. These were extraordinary new tools and capacities at the time. The worms crawled in, the worms crawled out, disrupting the microbial mats, possibly munching directly on Ediacarans. In a recent paper, Darroch and his co-authors (led by James Schiffbauer, and including Laflamme) have called this early Cambrian time the "Wormworld." It was no place for Ediacarans.

Worminess wasn't the only factor that brought oblivion to the Ediacarans and triggered the Cambrian explosion-there also were changes in ocean chemistry that allowed animals to acquire hard parts (calcium-rich skeletons, teeth, and shells), a generalized increase in modes of mobility (not just burrowing), and the rise of predatory habits, among other things. But the worminess of that transitional time, in the late Ediacaran period, may have played a crucial role. A few weeks after our Mistaken Point outing, I talked with James Gehling, a leading Ediacaran researcher. Go up to the Flinders Ranges in South Australia, near the Ediacara Hills, he told me by phone from his office in Adelaide, and look at the first formation of Cambrian sedimentary layers. "It's just Swiss cheese." Burrowed all through by wormy creatures that had churned the sand and "recycled" the soft-bodied Ediacarans. "That's where the Cambrian begins," Gehling said. "The advent of the musculature to burrow."

Guy Narbonne, at Queen's University in Ontario, largely agrees with the importance of burrowing. But together with his graduate student Calla Carbone, he has taken Wormworld a step further. Based on careful analysis of trace fossils from the late Ediacaran and the early Cambrian, Narbonne and Carbone noticed a significant difference in how those wormy creatures turned. By the early Cambrian, burrowing animals became more systematic in their searches for food, as well as more muscled. They ranged more efficiently, tracking the resources better and crossing their own tracks less. "It reflects the evolution of braininess," Narbonne told me. "Our interpretation," he added, "is that the Cambrian explosion is when behavior became coded on the genome." They titled that paper, "When Life Got Smart."

Most experts would agree that smartness, even on a level expressed by a primitive worm, wasn't a wrench in the Ediacaran tool kit. Those creatures' genomes may have been coded for fractal repetition-at least in the rangeomorphs, where it yielded a simple sort of complexity-but not for responsiveness to circumstances, or efficiency. Still, it's a mistaken point to dismiss the Ediacarans as doomed. People made that error with the dodo, when they branded it an emblem of ill-fated stupidity. But the real dodo, Raphus cucullatus, a large, flightless, fruit-eating bird endemic to the island of Mauritius, had thrived in its peaceable home for many thousands of years-until Homo sapiens and other predators arrived. Likewise the Ediacarans, with their own new threats. You can call them "failed experiments" in evolution if you want, but they succeeded and flourished, within their preferred but challenging environments, for more than 30 million years. We humans should be so steadfast and lucky.

Contributing writer David Quammen's next book, The Tangled Tree, will be published by Simon & Schuster in September. Photographer David Liittschwager has been illuminating the elegance and beauty of the natural world forNational Geographic since 2005.

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First There Were Microbes. Then Life on Earth Got Big.

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This story appears in the March 2018 issue ofNational Geographic magazine. On the southeast coast of Newfoundland, near North America's farthest eastward reach, lies a promontory of rocky cliffs called Mistaken Point. The place got its name from the shipwrecks it helped cause in foggy weather, when sea captains mistook it for somewhere else.

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For most of the nearly 4 billion years that life has existed on Earth, evolution produced little beyond bacteria, plankton, and multi-celled algae. But beginning about 600 million years ago in the Precambrian, the fossil record speaks of more rapid change. First, there was the rise and fall of mysterious creatures of the Ediacaran fauna, named for the fossil site in Australia where they were first discovered. Some of these animals may have belonged to groups that survive today, but others don't seem at all related to animals we know.

Then, between about 570 and 530 million years ago, another burst of diversification occurred, with the eventual appearance of the lineages of almost all animals living today. This stunning and unique evolutionary flowering is termed the "Cambrian explosion," taking the name of the geological age in whose early part it occurred. But it was not as rapid as an explosion: the changes seems to have happened in a range of about 30 million years, and some stages took 5 to 10 million years.

It's important to remember that what we call "the fossil record" is only the available fossil record. In order to be available to us, the remains of ancient plants and animals have to be preserved first, and this means that they need to have fossilizable parts and to be buried in an environment that will not destroy them.

It has long been suspected that the sparseness of the pre-Cambrian fossil record reflects these two problems. First, organisms may not have sequestered and secreted much in the way of fossilizable hard parts; and second, the environments in which they lived may have characteristically dissolved those hard parts after death and recycled them. An exception was the mysterious "small shelly fauna" -- minute shelled animals that are hard to categorize -- that left abundant fossils in the early Cambrian. Recently, minute fossil embryos dating to 570 million years ago have also been discovered. Even organisms that hadn't evolved hard parts, and thus didn't leave fossils of their bodies, left fossils of the trails they made as they moved through the Precambrian mud. Life was flourishing long before the Cambrian "explosion".

The best record of the Cambrian diversification is the Burgess Shale in British Columbia. Laid down in the middle-Cambrian, when the "explosion" had already been underway for several million years, this formation contains the first appearance in the fossil record of brachiopods, with clamlike shells, as well as trilobites, mollusks, echinoderms, and many odd animals that probably belong to extinct lineages. They include Opabinia, with five eyes and a nose like a fire hose, and Wiwaxia, an armored slug with two rows of upright scales.

The question of how so many immense changes occurred in such a short time is one that stirs scientists. Why did many fundamentally different body plans evolve so early and in such profusion? Some point to the increase in oxygen that began around 700 million years ago, providing fuel for movement and the evolution of more complex body structures. Others propose that an extinction of life just before the Cambrian opened up ecological roles, or "adaptive space," that the new forms exploited. External, ecological factors like these were undoubtedly important in creating the opportunity for the Cambrian explosion to occur.

Internal, genetic factors were also crucial. Recent research suggests that the period prior to the Cambrian explosion saw the gradual evolution of a "genetic tool kit" of genes that govern developmental processes. Once assembled, this genetic tool kit enabled an unprecedented period of evolutionary experimentation -- and competition. Many forms seen in the fossil record of the Cambrian disappeared without trace. Once the body plans that proved most successful came to dominate the biosphere, evolution never had such a free hand again, and evolutionary change was limited to relatively minor tinkering with the body plans that already existed.

Interpretations of this critical period are subject of lively debate among scientists like Stephen Jay Gould of Harvard University and Simon Conway Morris of Cambridge University. Gould emphasizes the role of chance. He argues that if one could "rerun the tape" of that evolutionary event, a completely different path might have developed and would likely not have included a humanlike creature. Morris, on the other hand, contends that the environment of our planet would have created selection pressures that would likely have produced similar forms of life to those around us -- including humans.

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For most of the nearly 4 billion years that life has existed on Earth, evolution produced little beyond bacteria, plankton, and multi-celled algae. But beginning about 600 million years ago in the Precambrian, the fossil record speaks of more rapid change.

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Online exhibits : Geologic time scale : Paleozoic Era
The Cambrian Period

The Cambrian Period marks an important point in the history of life on Earth; it is the time when most of the major groups of animals first appear in the fossil record. This event is sometimes called the "Cambrian Explosion," because of the relatively short time over which this diversity of forms appears. It was once thought that Cambrian rocks contained the first and oldest fossil animals, but these are now found in the earlier Ediacaran (Vendian) strata.

Life

Almost every metazoan phylum with hard parts, and many that lack hard parts, made its first appearance in the Cambrian. The only modern phylum with an adequate fossil record to appear after the Cambrian was the phylum Bryozoa, which is not known before the early Ordovician. A few mineralized animal fossils, including sponge spicules and probable worm tubes, are known from the Ediacaran Period immediately preceding the Cambrian. Some of the odd fossils of the biota from the Ediacaran may also have been animals representative of living phyla, although this remains a somewhat controversial topic. However, the Cambrian was nonetheless a time of great evolutionary innovation, with many major groups of organisms appearing within a span of only forty million years. Trace fossils made by animals also show increased diversity in Cambrian rocks, showing that the animals of the Cambrian were developing new ecological niches and strategies — such as active hunting, burrowing deeply into sediment, and making complex branching burrows. Finally, the Cambrian saw the appearance and/or diversification of mineralized algae of various types, such as the coralline red algae and the dasyclad green algae.

This does not mean that life in the Cambrian seas would have been perfectly familiar to a modern-day SCUBA diver! Although almost all of the living marine phyla were present, most were represented by classes that have since gone extinct or faded in importance. For example, the Brachiopoda was present, but greatest diversity was shown by inarticulate brachiopods (like the one pictured below, left). The articulate brachiopods, which would dominate the marine environment in the later Paleozoic, were still relatively rare and not especially diverse. Cambrian echinoderms were predominantly unfamiliar and strange-looking types such as early edrioasteroids, eocrinoids, and helicoplacoids. The more familiar starfish, brittle stars, and sea urchins had not yet evolved, and there is some controversy over whether crinoids (sea lilies) were present or not. Even if present, crinoids were rare in the Cambrian, although they became numerous and diverse through the later Paleozoic. And while jawless vertebrates were present in the Cambrian, it was not until the Ordovician that armored fish became common enough to leave a rich fossil record.

Other dominant Cambrian invertebrates with hard parts were trilobites (like the one pictured above), archaeocyathids (relatives of sponges that were restricted to the Lower Cambrian), and problematic conical fossils known as hyolithids (like the one pictured above, right). Many Early Cambrian invertebrates are known only from "small shelly fossils" — tiny plates, scales, spines, tubes, and so on. Many of these were probably pieces of the skeletons of larger animals.

A few localities around the world that preserve soft-bodied fossils of the Cambrian show that the "Cambrian radiation" generated many unusual forms not easily comparable with anything today. The best-known of these sites is the legendary Burgess Shale (middle Cambrian) in the British Columbian Rocky Mountains. Sites in Utah, southern China, Siberia, and north Greenland are also noted for their unusually good preservation of non-mineralized fossils from the Cambrian. One of these "weird wonders", first documented from the Burgess Shale, is Wiwaxia, depicted at lower left. Wiwaxia was an inch-long, creeping, scaly and spiny bottom dweller that may have been a relative of the molluscs, the annelids, or possibly an extinct animal group that combined features of both phyla.

Stratigraphy

A lot can happen in 40 million years, the approximate length of the Cambrian Period. Animals showed dramatic diversification during this period of Earth's history. This has been called the "Cambrian Explosion". When the fossil record is scrutinized closely, it turns out that the fastest growth in the number of major new animal groups took place during the as-yet-unnamed second and third stages (generally known as the Tommotian and Atdabanian stages) of the early Cambrian, a period of about 13 million years. In that time, the first undoubted fossil annelids, arthropods, brachiopods, echinoderms, molluscs, onychophorans, poriferans, and priapulids show up in rocks all over the world.

Stratigraphic boundaries are generally determined by the occurences of fossils. For instance, the trace fossil Treptichnus pedum marks the base of the Cambrian. This boundary is an unusual case, since stratigraphic boundaries are normally defined by the presence or absence of groups of fossils, called assemblages. In fact, much paleontological work is concerned with questions surrounding when and where stratigraphic boundaries should be defined. At first glance, this may not seem like important work, but consider this: if you wanted to know about the evolution of life on Earth, you would need a fairly accurate timeline. Questions such as: "how long did something stay the same?" or,"how fast did it change?" can only be assessed in the context of time.

Tectonics and paleoclimate

The Cambrian follows the Ediacaran Period, during which time the continents had been joined in a single supercontinent called Rodinia (from the Russian word for "homeland", rodina). As the Cambrian began, Rodinia began to fragment into smaller continents, which did not always correspond to the ones we see today. The reconstruction below shows the rifting of Rodinia during the second stage (Tommotian) of the Cambrian . Green represents land above water at this time, red indicates mountains, light blue indicates shallow seas of the continental shelves, and dark blue denotes the deep ocean basins. (For clarity, the outlines of present-day continents have been superimposed on the map.)

World climates were mild; there was no glaciation. Landmasses were scattered as a result of the fragmentation of the supercontinent Rodinia that had existed in the late Proterozoic. Most of North America lay in warm southern tropical and temperate latitudes, which supported the growth of extensive shallow-water archaeocyathid reefs all through the early Cambrian. Siberia, which also supported abundant reefs, was a separate continent due east of North America. Baltica — what is now Scandinavia, eastern Europe, and European Russia — lay to the south. Most of the rest of the continents were joined together in the supercontinent Gondwana, depicted on the right side of the map; South America, Africa, Antarctica, India, and Australia are all visible. What is now China and east Asia was fragmented at the time, with the fragments visible north and west of Australia. Western Europe was also in pieces, with most of them lying northwest of what is now the north African coastline. The present-day southeastern United States are visible wedged between South America and Africa; they did not become part of North America for another 300 million years. Tectonism affected regions of Gondwana, primarily in what are now Australia, Antarctica, and Argentina. The continental plate movement and collisions during this period generated pressure and heat, resulting in the folding, faulting, and crumpling of rock and the formation of large mountain ranges.

The Cambrian world was bracketed between two ice ages, one during the late Proterozoic and the other during the Ordovician. During these ice ages, the decrease in global temperature led to mass extinctions. Cooler conditions eliminated many warm water species, and glaciation lowered global sea level. However, during the Cambrian there was no significant ice formation. None of the continents were located at the poles so land temperatures remained mild. In fact, global climate was probably warmer and more uniform than it is today. With the retreat of Proterozoic ice, the sea level rose significantly. Lowland areas such as Baltica were flooded and much of the world was covered by epeiric seas. This event opened up new habitats where marine invertebrates, such as trilobites, radiated and flourished.

Plants had not yet evolved, and the terrestrial world was devoid of vegetation and inhospitable to life as we know it. Photosynthesis and primary production were the monopoly of bacteria and algal protists that populated the world's shallow seas.

Also during the Cambrian, the oceans became oxygenated. Although there was plentiful atmospheric oxygen by the beginning of the period, it wasn't until the Cambrian that there was a sufficient reduction in the number of oxygen-depleting bacteria to permit higher oxygen levels in the waters. This dissolved oxygen may have triggered the "Cambrian Explosion" — when most of the major groups of animals, especially those with hard shells, first appeared in the fossil record.

Localities
Resources

Find out more about the Cambrian paleontology and geology of North America at the Paleontology Portal.

See the Wikipedia page on the Cambrian.

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Online exhibits : Geologic time scale : Paleozoic Era The Cambrian Period The Cambrian Period marks an important point in the history of life on Earth; it is the time when most of the major groups of animals first appear in the fossil record.

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Trilobite, any member of a group of extinct fossil arthropods easily recognized by their distinctive three-lobed, three-segmented form. Trilobites, exclusively marine animals, first appeared at the beginning of the Cambrian Period, about 542 million years ago, when they dominated the seas. Although they became less abundant in succeeding geologic periods, a few forms persisted into the Permian Period, which ended about 251 million years ago.

trilobite Photograph by Sandy Grimm. Houston Museum of Natural Science, HMNS 1284

Because trilobites appear fully developed in the Cambrian Period, it appears likely that the ancestral trilobites originated during the Ediacaran Period (630 million to 542 million years ago) of Precambrian times. An organism that may be ancestral to the trilobites, as well as to other arthropods, may be represented by Spriggina, which is known from Precambrian shallow-water marine deposits in Australia. Trilobites are frequently used for stratigraphic correlations.

Trilobites had three body lobes, two of which lay on each side of a longitudinal axial lobe. The trilobite body was segmented and divided into three regions from head to tail: the cephalon, or head region, separated from the thorax, which was followed in turn by the pygidium, or tail region. Trilobites, like other arthropods, had an external skeleton, called exoskeleton, composed of chitinous material. For the animal to grow, the exoskeleton had to be shed, and shed trilobite exoskeletons, or portions of them, are fossils that are relatively common.

trilobite fossil age fotostock/SuperStock

Each trilobite body segment bore a pair of jointed appendages. The forwardmost appendages were modified into sense and feeding organs. Most trilobites had a pair of compound eyes; some of them, however, were eyeless.

trilobite Photograph by Sandy Grimm. Houston Museum of Natural Science, HMNS 1313

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Some trilobites were active predators, whereas others were scavengers, and still others probably ate plankton. Some trilobites grew to large size; Paradoxides harlani, which has been found near Boston in rocks of the Middle Cambrian Epoch (521 million to 501 million years ago), grew to be more than 45 cm (18 inches) in length and may have weighed as much as 4.5 kg (10 pounds). Others were small.

Leslie Jackman/Natural History Photographic Agency

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Trilobite, any member of a group of extinct fossil arthropods easily recognized by their distinctive three-lobed, three-segmented form. Trilobites, exclusively marine animals, first appeared at the beginning of the Cambrian Period, about 542 million years ago, when they dominated the seas. Although

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Dinosaurs are one of the most successful groups of animals to have roamed the planet.

For over 170 million years they dominated the land, from small creatures just a few feet long to some of the largest animals ever to have walked Earth.

But despite their long evolutionary history, the origin of dinosaurs remains shrouded in mystery. When did dinosaurs evolve, and what did the first ones look like?

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Dinosaurs are one of the most successful groups of animals to have roamed the planet. For over 170 million years they dominated the land, from small creatures just a few feet long to some of the largest animals ever to have walked Earth. But despite their long evolutionary history, the origin of dinosaurs remains shrouded in mystery.

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(Image credit: Davide Bonadonna)

Dinosaurs were a successful group of animals that emerged between 240 million and 230 million years ago and came to rule the world until about 66 million years ago, when a giant asteroid slammed into . During that time, dinosaurs evolved from a group of mostly - and -size creatures into the most enormous beasts that ever existed on land.

Some meat-eating dinosaurs shrank over time and evolved into birds. So, in that sense, only the non-avian dinosaurs went extinct. (For the purposes of this article, "dinosaurs" will refer to non-avian dinosaurs, unless otherwise stated.)

During the roughly 174 million years that dinosaurs existed, the world changed greatly. When dinosaurs first appeared in the Triassic period (251.9 million to 201.3 million years ago), they roamed the supercontinent of . But by the time the asteroid hit at the end of the Cretaceous period (145 million to 66 million years ago), the continents were in approximately the same place they are today.

What are dinosaurs?

The oldest unequivocal dinosaur fossils, dating to about 231 million years ago, are from Ischigualasto Provincial Park in northwestern Argentina, and include the genuses Herrerasaurus, Eoraptor and Eodromaeus. Scientists are still debating whether , a genus found in Tanzania that dates to about 240 million years ago, is also an early dinosaur or a dinosauromorph, a group that includes dinosaurs and their close relatives, said Steve Brusatte, a paleontologist at the University of Edinburgh in Scotland.
Related: Photos: Unearthing dinosauromorphs, the ancestors of dinosaurs
Whenever they first appeared, the dinosaurs' unique anatomy set them apart from other animal groups. Dinosaurs are archosaurs, a clade (different groups of animals that share a common ancestor) that includes crocodilians, pterosaurs, dinosaurs and birds. The archosaurs emerged after the end-Permian extinction about 252 million years ago. Over time, some archosaurs, including dinosauromorphs, adapted an upright posture, meaning they had legs under their bodies, rather than out to their sides.

"Sprawling is all well and good for cold-blooded critters that don't need to move very fast. Tucking your limbs under your body, however, opens up a new world of possibilities," Brusatte wrote in "The Rise and Fall of the Dinosaurs: A New History of a Lost World" (William Morrow, 2018). As archosaur progressed, dinosauromorphs gained long tails, big leg muscles and extra hip bones that enabled them to move quickly and efficiently, Brusatte wrote.

Some dinosauromorphs evolved into dinosaurs. The differences between the two are small, but dinosaurs' anatomy offered increased benefits, including arms that could move in and out, neck vertebrae that could support stronger muscles than before, and a joint where the thigh bone meets the pelvis, Brusatte wrote.

This unique anatomy helped dinosaurs become successful. Having an upright posture also freed the hands, allowing dinosaurs such as iguanodonts to grasp branches and carnivorous dinosaurs to claw and kill prey, noted Gregory Erickson, a paleobiologist at Florida State University. Ultimately, having free arms "allowed gliding then flight in birds," he said.

Moreover, dinosaurs were likely warm blooded, according to research on their growth rates. "When you become a warm blooded animal, you can operate 24/7," Erickson told Live Science. "You're not at the whims of the environment in terms of being active."

Initially, dinosaurs were not as diverse as the crocodile-like archosaurs they were living alongside, Brusatte noted. In fact, dinosaurs "didn't become too successful right away; the crocs ruled the Triassic, then the end-Triassic extinction hit and the dinosaurs survived and took over."

The clade Dinosauria (which means "terrible lizard" in Greek) was coined in 1842 by the English paleontologist Richard Owen, who included the meat-eating theropod Megalosaurus, the long-necked sauropodomorph Cetiosaurus and the ornithiscian Iguanodon as the first known species in the clade, according to the book " Dinosaurs Rediscovered" (Thames & Hudson, 2019).

Each of these dinosaurs, it turns out, represents one of the three major dinosaur groups.

Types of dinosaurs

As of 2021, there were 1,545 scientifically described dinosaur species, according to the Paleobiology Database. About 50 previously unknown species are described each year, meaning there's roughly one newfound species described each week, Brusatte said.

All of these dinosaurs fit into one of three groups: Ornithischia, Sauropodomorpha and Theropoda.

Ornithischia dinosaurs include beaked plant-eaters, such as , duck-billed dinosaurs (also called hadrosaurs), as well as horned dinosaurs like and armored dinosaurs like . Some ornithischians walked on four legs, while others walked on two.

Sauropodomorpha dinosaurs were long-necked, pot-bellied dinosaurs that had tiny heads and column-like limbs. This group includes sauropods (such as Diplodocus), their smaller antecedents (including Chromogisaurus) and extra-large sauropods known as titanosaurs (such as and Argentinosaurus), which are among the largest land animals that have ever existed.

Theropoda is a group of meat-eating dinosaurs, although some (such as Chilesaurus diegosuarezi) changed their diet to be herbivorous or omnivorous. Theropods include Tyrannosaurus rex and Velociraptor, as well as birds, which evolved from small theropods.

So, how are these groups related? It's up for debate. Ornithischian dinosaurs have a backward-pointing pubis bone in the hip, earning them the name bird-hipped dinosaurs. (However, they are not the ancestors of birds; theropods are.) Meanwhile, theropods and sauropodomorphs have saurischian or "reptile hips," which are also seen in modern crocodiles and lizards, according to the book "Dinosaurs Rediscovered."

Historically, it was thought that the reptile-hipped theropods and sauropodomorphs were more closely related to each other than to ornithischians. However, a 2017 study in the journal Nature uprooted the dinosaur family tree by suggesting that ornithischians and theropods were more closely related, based on analyses of 74 dinosaur species, Live Science previously reported. Shortly after, another 2017 study in the journal Nature found that neither family tree, as well as a third that is rarely considered, is statistically significant from the other, meaning all the suggested family trees are equally plausible until more evidence comes forth.

When did dinosaurs live?

Dinosaurs lived during most of the Mesozoic era, a geological age that lasted from 252 million to 66 million years ago. The Mesozoic era includes the Triassic, and Cretaceous periods.

Dinosaurs arose from small dinosauromorph ancestors in the Triassic period, when the climate was harsh and dry. They faced "competition from the croc-line archosaurs for tens of millions of years, [but] finally prevailed when Pangaea began to split," Brusatte told Live Science. At this time, volcanoes erupted along the cracks of the supercontinent, causing global warming and mass extinction, he said.

During the Jurassic period (201.3 million to 145 million years ago), dinosaurs rose to dominance and some grew to huge sizes. For example, Vouivria damparisensis, the earliest titanosaur, dates to 160 million years ago. It weighed about 33,000 lbs. (15,000 kilograms) and measured more than 50 feet (15 meters) long. Iconic dinosaurs from this period include Brontosaurus, Brachiosaurus, Diplodocus and Stegosaurus. During the Jurassic, flowering plants and birds, including , first appeared. There was "a small extinction at the end of the Jurassic that we still know little about," Brusatte said.

In the Cretaceous period, dinosaur dominance continued as the continents moved farther apart. Famous dinosaurs from this period include T. rex, Triceratops, Spinosaurus and Velociraptor. The largest dinosaurs on record, including Argentinosaurus, date to the Cretaceous. The Cretaceous period ended with the Cretaceous-Tertiary (K-Pg) extinction event, when a 6-mile-wide (10 kilometers) asteroid collided with Earth, leaving an impact crater more than 110 miles (180 km) in diameter in the Yucatan Peninsula of what is now Mexico.

The impact area, known as the Chicxulub (CHEEK-sheh-loob) crater, has evidence of "shocked quartz" and small glass-like spheres known as tektites, which form when rock is rapidly vaporized and cooled - geologic clues that a space rock struck there with incredible force, Betsy Kruk, an associate paleontologist with Paleo Solutions, a paleontological consulting company based in California, previously told Live Science. Chemical analyses from the sedimentary rock at Chicxulub melted and mixed together at temperatures on par with an asteroid strike about 66 million years ago, she added.

What is the largest dinosaur? The smallest dinosaur?

Some dinosaurs were enormous, but others were pipsqueaks. The smallest dinosaur on record is an avian dinosaur that's alive today: the bee hummingbird ( Mellisuga helenae) from Cuba, which measures just over 2 inches (5 centimeters) long and weighs less than 0.07 ounce (2 grams). As for extinct, non-avian dinosaurs, there are a few contenders for smallest beast, including a bat-like dinosaur from China named Ambopteryx longibrachium that measured 13 inches (32 cm) long and weighed about 11 oz (306 g), according to a 2019 study in the journal .

Titanosaurs were the largest dinosaurs. However, because paleontologists rarely find an entire skeleton, and because soft tissues, such as organs and muscles, rarely fossilize, it's challenging to determine dinosaur mass. However, contenders for the title of world's largest dinosaur include Argentinosaurus, which weighed up to 110 tons (100 metric tons), an unnamed 98 million-year-old titanosaur from Argentina that weighed upward of 69 tons (63 metric tons), and , which also weighed in at 69 tons.

The longest dinosaur is likely Diplodocus or Mamenchisaurus - long and slender sauropod dinosaurs that were about 115 feet (35 m) long. The tallest dinosaur is likely Giraffatitan, a 40-foot-tall (12 m) sauropod dinosaur from the late Jurassic, about 150 million years ago, which lived in what is now Tanzania.

Pterosaurs are not dinosaurs

Many amazing animals lived during the dinosaur age, and some are confused with dinosaurs. The most common misconception is calling pterosaurs dinosaurs: They are not. Pterosaurs are winged reptiles and archosaurs, meaning they are relatives of dinosaurs, but they are not dinosaurs.

The order Crocodilia includes extinct and living crocodiles and their close relatives. Crocodilians are archosaurs, but they are not dinosaurs. Living crocodilians and birds (which are dinosaurs) are the only surviving members of the Archosauria clade.

The Mesozoic oceans teemed with sea life, including predatory reptiles known as mosasaurs (such as ), plesiosaurs and ichthyosaurs. However, none of these reptiles are dinosaurs.

Did dinosaurs have feathers?

Yes, some dinosaurs flaunted feathers, as do their bird descendants. Feathers don't fossilize well, but some remarkable fossils, especially those from Liaoning province in China that were buried in the aftermath of a volcanic eruption, have preserved feathers. Here are a few examples: , and .

It's unclear why dinosaurs first evolved feathers, but they could have been used for the following: as insulation to keep dinosaurs and their incubated eggs warm; for display to use for communication between dinosaurs, such as courtship displays; and for gliding or powered flight, Michael Habib, a research associate at the Dinosaur Institute at the Natural History Museum of Los Angeles County, previously told Live Science.

Initially, it was thought that only theropods and their descendants sported feathers, but researchers have also found downy feathers on the plant-eating ornithischian dinosaur Kulindadromeus zabaikalicus, suggesting that feathers were more widespread than previously thought, a 2014 study in the journal found.

It's possible that pterosaurs had feathers, according to a 2018 study in the journal Nature Ecology & Evolution, but more feathered specimens need to be found and analyzed to say so for sure.

Notably, even T. rex had feathers. However, depictions of dinosaurs rarely have feathers in popular culture, including the "Jurassic Park" movies. Paleontologist Jack Horner, who served as a scientific adviser on some of the "Jurassic Park" movies, remembers telling director Steven Spielberg that the dinosaurs should have feathers.

"Even when 'Jurassic Park' came out [in 1993], we knew that Velociraptors should have feathers, but at that time, it would have been technically difficult to do it, just from a CG [computer-generated] point of view. And Steven wasn't really too excited about it, anyway. When I told him they should be colorful and they should be feathered, and he said, 'Feathered Technicolor dinosaurs aren't scary enough,'" Horner previously told Live Science.

Could dinosaurs fly?

Some dinosaurs could fly, including the earliest known bird - Archaeopteryx - discovered in Germany and dating to about 150 million years ago, during the late Jurassic.

However, unlike most birds today, extinct dinosaurs likely just flew short distances. Research shows that powerful leg muscles, big wings and a relatively small body size were needed for takeoff and flight in ancient birds and bird-like dinosaurs, Habib previously told Live Science. His research suggests that the bird-like dinosaurs Microraptor, Rahonavis, and five avian genuses - Archaeopteryx, Sapeornis, Jeholornis, Eoconfuciusornis and Confuciusornis - would have been able to launch (without running) from the ground to initiate flight.

The bat-like dinosaur Yi qi, dating to China's Jurassic period, could likely glide, according to a 2015 study in the journal .

Why did dinosaurs go extinct?

It's up for debate how well the dinosaurs were doing before the asteroid crashed into Earth. A handful of suggest that in the late Cretaceous, dinosaur extinctions were rising and diversity was declining, especially among herbivorous dinosaurs. But these studies rely on incomplete fossil data and models that may not tell the whole story, Live Science previously reported.

Even if dinosaur diversity was dropping, it's possible they could have bounced back had the asteroid not hit, Brusatte told Live Science. Dinosaurs lived on every continent, including Antarctica, and they filled different rungs in various ecosystems, from plant-eater to apex carnivore. "Dinosaurs had experienced many rises and falls in diversity over their 150-plus million year evolutionary history," he said. If the mass extinction hadn't happened, it's possible "They would still be thriving today as more than birds."

In the aftermath of the asteroid collision, long-term pain followed chaos. The collision caused massive destruction, including a shockwave, heat pulse, wildfires, tsunamis (including an immediate mile-high tsunami), volcanic eruptions, lethal acid rain and earthquakes. Dust and grime that the asteroid kicked up hovered in the air. "This rain of hot dust raised global temperatures for hours after the impact, and cooked alive animals that were too large to seek shelter," according to Kruk. "Small animals that could shelter underground, underwater, or perhaps in caves or large tree trunks, may have been able to survive this initial heat blast."

The dust and particles remained in the air, blocking the sun for several years afterward and causing a nuclear winter that cooled the planet and led to the deaths of countless plants and animals, Brusatte and Kruk said.

"Smaller, omnivorous terrestrial animals, like mammals, lizards, turtles, or birds, may have been able to survive as scavengers feeding on the carcasses of dead dinosaurs, fungi, roots and decaying plant matter, while smaller animals with lower metabolisms were best able to wait the disaster out," Kruk previously said. Moreover, the asteroid also pulverized carbon-rich rocks, which released carbon into the atmosphere and led to "global warming for a few thousand years," after the nuclear winter ended, Brusatte said.

Scientists used to wonder if the Deccan Traps volcanic eruptions in what is now India played a role in the mass extinction. But "show that the Deccan probably had very little impact," Brusatte said. It was "most likely an innocent bystander" - the asteroid is what caused the extinction.

Can dinosaurs be brought back?

In the popular movie franchise "Jurassic Park," scientists find dinosaur preserved in an ancient caught in amber, and then fill in the DNA gaps with DNA. It's an entertaining plot, but the science is far from sound. For instance, amber does not preserve DNA well, and frogs are not at all closely related to dinosaurs; they're not archosaurs, and a 2017 study in the journal Proceedings of the National Academy of Sciences even found that frog evolution took off after the asteroid impact.

For myriad reasons, it's currently impossible to bring extinct dinosaurs back. While dinosaur proteins and blood vessels have been found, scientists have yet to rigorously identify DNA from an extinct dinosaur. DNA begins decaying the moment an organism dies, but parts of it can be preserved in the right circumstances. That said, the oldest sequenced DNA on record belongs to a roughly 1 million-year-old mammoth, and dinosaurs went extinct about 66 million years ago.

Some scientists are studying how to reverse-engineer birds into dinosaurs, including the so-called "dino-chicken," which would have a lengthened tail, teeth, arms and fingers. One group even gave chicken embryos dinosaur-like snouts. However, the "chickenosaurus" wouldn't be a replica of an ancient dinosaur, but rather a dinosaur-like bird, the researchers told Live Science.

Additional resources

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(Image credit: Davide Bonadonna) Dinosaurs were a successful group of animals that emerged between 240 million and 230 million years ago and came to rule the world until about 66 million years ago, when a giant asteroid slammed into .

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Dinosaurs - Extinction, Timeline & Definition

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The prehistoric reptiles known as dinosaurs arose during the Middle to Late Triassic Period of the Mesozoic Era, some 230 million years ago. They were members of a subclass of reptiles called the archosaurs (“ruling reptiles”), a group that also includes birds and crocodiles.

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The fossil record shows that for the first 175 million years of their existence, dinosaurs took on a huge variety of forms as the environment changed and new species evolved that were suited to these new conditions. Dinosaurs that failed to adapt went extinct.

But then 66 million years ago, over a relatively short time, dinosaurs disappeared completely ( except for birds). Many other animals also died out, including pterosaurs, large marine reptiles, and ammonites.

Although the number of dinosaur species was already declining, this suggests a sudden catastrophic event sealed their fate - something that caused unfavourable changes to the environment to occur more quickly than dinosaurs and other creatures could adapt.

What caused the Cretaceous extinction?

The exact nature of this catastrophic event is still open to scientific debate. Evidence suggests an asteroid impact was the main culprit. Volcanic eruptions that caused large-scale climate change may also have been involved, together with more gradual changes to Earth's climate that happened over millions of years.

Whatever the causes, the huge extinction that ended the age of the dinosaur left gaps in ecosystems around the world. These were subsequently filled by the only dinosaurs to survive - birds - and mammals, both of which went on to evolve rapidly.

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The fossil record shows that for the first 175 million years of their existence, dinosaurs took on a huge variety of forms as the environment changed and new species evolved that were suited to these new conditions. Dinosaurs that failed to adapt went extinct.

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The Cretaceous-Tertiary extinction event, or the K-T event, is the name given to the die-off of the dinosaurs and other species that took place some 65.5 million years ago. For many years, paleontologists believed this event was caused by climate and geological changes that interrupted the dinosaurs' food supply. However, in the 1980s, father-and-son scientists Luis (1911-88) and Walter Alvarez (1940-) discovered in the geological record a distinct layer of iridium-an element found in abundance only in space-that corresponds to the precise time the dinosaurs died. This suggests that a comet, asteroid or meteor impact event may have caused the extinction of the dinosaurs. In the 1990s, scientists located the massive Chicxulub Crater at the tip of Mexico's Yucatán Peninsula, which dates to the period in question.

Many Theories, No Proof

Dinosaurs roamed the earth for 160 million years until their sudden demise some 65.5 million years ago, in an event now known as the Cretaceous-Tertiary, or K-T, extinction event. ("K" is the abbreviation for Cretaceous, which is associated with the German word "Kreidezeit.") Besides dinosaurs, many other species of mammals, amphibians and plants died out at the same time. Over the years, paleontologists have proposed several theories for this extensive die-off. One early theory was that small mammals ate dinosaur eggs, thereby reducing the dinosaur population until it became unsustainable. Another theory was that dinosaurs' bodies became too big to be operated by their small brains. Some scientists believed a great plague decimated the dinosaur population and then spread to the animals that feasted on their carcasses. Starvation was another possibility: Large dinosaurs required vast amounts of food and could have stripped bare all the vegetation in their habitat. But many of these theories are easily dismissed. If dinosaurs' brains were too small to be adaptive, they would not have flourished for 160 million years. Also, plants do not have brains nor do they suffer from the same diseases as animals, so their simultaneous extinction makes these theories less plausible.

Did you know? The K-T extinction was not the first such massive die-off in history, nor was it the largest. The Permian-Triassic extinction event, known as the Great Dying, occurred 251.4 million years ago and eradicated 96 percent of all marine species and 70 percent of all terrestrial vertebrates species on earth.

For many years, climate change was the most credible explanation for the dinosaurs' demise. Dinosaurs thrived in the planet's consistently humid, tropical climate. But in the late Mesozoic Era that corresponds with the extinction of the dinosaurs, evidence shows that the planet slowly became cooler. Lower temperatures caused ice to form over the North and South poles and the oceans to become colder. Because the dinosaurs were cold-blooded-meaning they obtained body heat from the sun and the air-they would not have been able to survive in significantly colder climates. Yet some species of cold-blooded animals, such as crocodiles, did manage to survive. Also, climate change would have taken tens of thousands of years, giving the dinosaurs sufficient time to adapt.

It Came From Outer Space

In 1956, Russian astronomer Joseph Shklovsky (1916-85) became the first scientist to consider the extinction was due to a single catastrophic event when he theorized that a supernova (the explosion of a dying star) showered the earth in radiation that could have killed the dinosaurs. Once again, the problem with the theory was explaining why dinosaurs died out and other species did not. Also, scientists said that such an event would have left evidence on the surface of the earth-trace amounts of radiation dating back to the Cretaceous Period. None was found.

Enter Luis Alvarez, a Nobel Prize-winning physicist, inventor and pioneer in the field of radiation and nuclear research. He and his son, noted geologist Walter Alvarez, were conducting research in Italy when they discovered a centimeter-thick layer of iridium-enriched clay at the K-T boundary. Iridium is rare on earth, but more common in space. The Alvarezes published their findings in 1981, postulating that the thin layer of iridium was deposited following the impact of a large meteor, comet or asteroid with the earth. Furthermore, this bolide impact (the meteor, comet or asteroid colliding with the earth's surface) could have caused the extinction of the dinosaurs. At the time, the Alvarez theory was so far removed from prevailing hypotheses that it was ridiculed. Slowly, though, other scientists began finding iridium evidence at various places around the globe that corroborated the Alvarez theory. There was, however, no smoking gun in the form of an impact site.

Then in 1991, a massive meteor crater 110 miles in diameter was discovered on the edge of the Yucatán Peninsula, extending into the Gulf of Mexico. The Chicxulub Crater, as it was dubbed, was named for a nearby village. Scientists believe the bolide that formed it was roughly 6 miles in diameter, struck the earth at 40,000 miles per hour and released 2 million times more energy than the most powerful nuclear bomb ever detonated. The heat would have broiled the earth's surface, ignited wildfires worldwide and plunged the planet into darkness as debris clouded the atmosphere. Miles-high tsunamis would have washed over the continents, drowning many forms of life. Shock waves would have triggered earthquakes and volcanic eruptions.

The resulting darkness could have lasted for months, possibly years. It would have plunged the earth's temperatures into the freezing zone, killing plants and leaving herbivores with nothing to eat. Many dinosaurs would have died within weeks. The carnivores who feasted on the herbivores would have died a month or two later. Overall, the loss of biodiversity would have been tremendous. Only small scavenging mammals that could burrow into the ground and eat whatever remained would have survived. The iridium layer plus the Chicxulub Crater were evidence enough to convince many scientists that the bolide impact theory was credible. It explained much of what previous theories could not.

Still a Theory

Paleontology remains a competitive discipline even though its central mystery appears to have been solved. Agreement over dinosaur extinction is far from unanimous, and fossils continue to be found that add to the body of knowledge about how the dinosaurs lived and died. Only recently have birds been identified as descendants of the dinosaurs, and theories regarding dinosaur intelligence and behavior continue to change. Even long-established truths such as dinosaurs' cold-bloodedness are open for debate. The climate change theory still holds sway over some scientists, who refute that the Chicxulub impact was the sole cause of the extinction. Evidence from the 65-million-year-old lava flows in India hint that a giant, gaseous volcanic plume might have initiated global climate change that threatened the dinosaurs. Scientists' continued research will help paint a more detailed picture of the ever-changing, ever-evolving planet.

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The Cretaceous-Tertiary extinction event, or the K-T event, is the name given to the die-off of the dinosaurs that took place some 65.5 million years ago. For many years, paleontologists believed this event was caused by climate change that interrupted the dinosaurs' food supply, but later, scientists discovered iridium, suggesting a comet, asteroid or meteor impact event may have caused the mass extinction.

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New evidence is emerging that a devastating combo of events — an asteroid impact and supervolcanoes — may be behind the dinosaurs’ demise.

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At the same time that the dinosaurs were spreading and diversifying, the first mammals evolved. Their ancestors were reptiles called cynodonts, whose faces looked a little like those of dogs and may have had fur or whiskers. Early mammals such as Morganucodon were small and shrew-like, and probably only active at night. This may have spurred them to evolve warm-bloodedness: the ability to keep their body temperature constant.

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From the top of Shifting Sands dune in the Serengeti Plain of Africa a million mammals are in motion. Wildebeests. Zebras. Gazelles. The plain is black with them. It is wildebeest calving season, and many of those giant bearded antelope have newborns trailing them. Others walk with the distended bellies of imminent birth. From a distance the movement seems a serene and constant march toward the southeast, where recent rains have made pastures greener. But a closer look reveals details of high drama.

A young Grant's gazelle suddenly dashes between the clusters of wildebeests, followed closely by its mother. A hyena races in pursuit. The mother slows and moves evasively to distract the hungry predator. But the inexperienced fawn makes a panicky turn. Within moments it falls victim to the jaws of the hyena. A few yards away, ears twitching, the mother stands helpless. Then, as if in frustration, she charges two jackals on the sidelines of the kill.

"She must be feeling emotion, but there's no way to prove it," says Patricia Moehlman, the wildlife biologist who has brought me to Shifting Sands, a 12-foot-high (3.5-meter-high) dune that is itself slowly migrating across the plain. Continues Moehlman, "She's a mother. Her brain may not work like ours, but I think there's pain. I think there's fear. And certainly stress. We feel connected to her because she's a fellow mammal."

Local Masai women regard the dune as a sacred fertility site. Moehlman calls it "a place of pilgrimage." Indeed, no place on Earth offers a more spectacular abundance of our fur-bearing, breast-feeding brethren, especially when the wildebeests are on the march. But the wildebeests are only part of the scene. Myriad mammal species graze, gallop, prowl, and wallow in this part of Africa.

In the nearby Ngorongoro Crater a mother hippopotamus nuzzles her pink newborn in a muddy pond, while a pair of lions leisurely copulate along the roadside. In a grove of acacia trees a group of giraffes, members of a family of mammals that until 20 million years ago were small forest dwellers, nibble at the top branches. A few miles away elephants-which scientists are just now realizing may come from one of the oldest of the modern mammalian lineages-lumber toward a midday bath in a rain-swollen stream. Quick-witted vervet monkeys dash down from the trees to steal food through the open door of a tourist van. Meanwhile, one of the few surviving black rhinoceroses in the area wanders stealthily through a stand of high grasses.

So many mammals-and such varied shapes and behaviors-throng this land that it's hard to believe any two could have descended from the same ancestor. Nonetheless, the amphibious hippo, with its lawnmower-like diet of up to a hundred pounds (45 kilograms) of grass a night, shares a common lineage with the three-inch-long (7.5-centimeter-long) naked mole rat-a subterranean, tuber-chomping hot dog with teeth, which lives like a termite in large colonies dominated by a queen.

Deep in their bones, all mammals are related. The earliest known mammals were the morganucodontids, tiny shrew-size creatures that lived in the shadows of the dinosaurs 210 million years ago. They were one of several different mammal lineages that emerged around that time. All living mammals today, including us, descend from the one line that survived. During the next 145 million years of evolution, the dominance of dinosaurs ensured that our distant mammalian ancestors remained no larger than a cat. But when a catastrophic asteroid or comet-maybe a few comets, as some scientists are now arguing-finished off the dinosaurs 65 million years ago, mammals got the most important evolutionary opportunity they would ever have. With dinosaurs gone, mammals could exploit the planet's resources themselves. Within a few million years of the impact the fossil record shows an explosion in mammalian diversity.

How did those little creatures transform into not only the hippo and the mole rat but also today's vast panorama of mammals with fur, hooves, and fangs, as well as others that swim hairless through deep oceans-or ride, like me, in a Land Rover across this grassland?

Only humans can ask that question, or hope to answer it. We are, in a sense, the ultimate mammals. To be sure, we share defining traits with the first mammals-traits that were evolving even as the morganucodontids scrambled for food among the dinosaurs: We are warm-blooded. We have specialized jaws, whose hinges came together early in our evolution to create the ear bones that let us hear better than other animals. We have complex teeth that let us grind and chew our food so that we get more nutrition out of it. We have hair. We are superb mothers whom evolution has supplied with physical adaptations-such as breasts and placental birth-that give mammalian young an important head start. We humans are among the most recent to evolve, and we use our big mammalian brains to reason and solve problems and struggle for goals beyond our basic needs. We ask about our past and wonder what it might tell us about the future.

From scratching around in the dirt to deciphering DNA-how did we get from there to here?

That question has never had an easy answer, but today new fossil discoveries and important new tools are illuminating our distant past more clearly than ever before. Less than half a century ago, trying to understand mammalian evolution was like exploring the universe with a primitive telescope. But now high-speed analysis of genetic evidence, painstaking reconstruction of past climates and continental movements, and dogged work with often minuscule bones are creating insights that are challenging some cherished assumptions.

In the late 1960s, evidence emerged that the world's landmasses were once assembled into one great continent, called Pangaea. Around 225 million years ago Pangaea began to split into a northern continent, called Laurasia, and its southern counterpart, Gondwana. Each continent carried its own cargo of animals. Based on the known fossil record, scientists believed that the ancestors of mammals alive today emerged in the north, and then migrated south, all the way to Antarctica and Australia, as land bridges episodically developed between the continents.

André Wyss, a paleontologist at the University of California, Santa Barbara, says that's known as the "Sherwin-Williams model of evolution," a reference to the paint company logo that shows paint dripping over a globe from north to south.

Recently paleontologists have dug deeper into the fossil record of southern continents. They are finding evidence of advanced mammals far older than any known in the north, perhaps turning the Sherwin-Williams world upside down.

On another front, geneticists comparing the genes of living mammals have found that certain groups thought to be very distant cousins-hippos and whales, say-are in fact next of kin. They have also found evidence that mammals began to diversify into today's 18 living orders much earlier than the fossil record shows. Fossils suggest that most modern groups appeared around 60 million years ago, after the dinosaurs were gone. Molecular data suggest they actually began diversifying about 100 million years ago.

"It's been a complete upheaval, says Mark Springer, an evolutionary geneticist at the University of California, Riverside. "We've come up with a very different family tree for mammals."

Many paleontologists angrily reject the DNA findings, arguing there must be something wrong with the molecular clocks the geneticists use to date their findings. The geneticists counter that paleontologists just haven't found the right fossils yet.

Scientists who trust the fossils and those who look to the genes agree on at least one thing: Mammals were starting to come into their own around the time of the morganucodontids. Their tiny jawbones-about an inch (2.5 centimeters) long-show just how different the mammalian form was from the giant reptile world.

Their jawbones were beginning to fuse into one piece. "This is very different from reptile jaws, which are made up of several bones," says paleontologist Rich Cifelli of the Sam Noble Oklahoma Museum of Natural History. "Modern mammals' bones migrated backward to become the small bones of the middle ear. That's why mammals hear so much better than reptiles."

The separation of the jaw and the ear bones allowed the skulls of later mammals to expand sideways and backward-enabling mammals to develop bigger brains. The teeth of the morganucodontids were another important innovation that later mammals would improve upon. The upper and lower molars of morganucodontid jawbones interlocked, letting them slice their food into pieces. That released more calories and nutrients.

"Reptiles don't cut up their food," says Cifelli. "They grab and gulp. But these little guys were so active they had to get every calorie they could out of what they ate. The more they could process their food in their mouths, the more energy it gave them."

Scientists believe mammary glands began as sweat glands at the bases of hairs. Both sweat glands and mammary glands produce water, salts, and proteins, all of which a newborn needs to survive.

The duck-billed platypus of Australia gives us a glimpse of how those primitive mammary glands worked. The platypus and the spiny anteater are the only surviving examples of a mammalian subgroup called monotremes.

"The platypus female doesn't have nipples," explains Peter Temple-Smith, a platypus specialist at the Melbourne Zoo. "Rather, there is a region where milk ducts come together and secrete milk onto hairs. The young then lick or suck the milk off the hairs."

Nipples, which concentrate milk ducts, probably emerged with the branch of mammals we know as marsupials-a group that includes kangaroos, koalas, and opossums. "The advantage of nipples is that they give the young something to hold on to," says Temple-Smith. "The marsupial mother can therefore continue to roam about and feed freely, carrying her baby wherever she goes in her pouch."

Back on the Serengeti, we see again how mammals emphasize maternal care. A newborn wildebeest stands between the legs of its mother, its skin still wet from birth. Suddenly the air fills with the cries of vultures. They descend and with their fierce beaks begin to tear into the placenta lying a few yards away. The mother wildebeest jerks her head. Their arrival has announced to every scavenger for miles around that there is fresh young meat here, and she urges her calf into as fast a gallop as its wobbly legs can manage.

"She's being a good mother," says Moehlman, the wildlife biologist. "If you aren't a good mom, your lineage dies out. That's what being a mammal is all about."

The bloody wildebeest placenta, which the scavenging birds fight over so aggressively, illustrates the physical investment that advanced mammalian mothers make in their young. Metabolically speaking, the placenta is very expensive for the mother to maintain. Yet it is invaluable. It not only nourishes the fetus in the womb; it also isolates the developing fetus from the mother's immune system. Otherwise, her immune cells would attack the fetus as a foreign object-after all, half its genes come from the father.

Reptiles and birds avoid immune system attack by surrounding the fetus in an eggshell and moving it out of the body. Monotremes such as the platypus still lay eggs. And marsupials solve the immune problem by delivering their embryos early.

Recent DNA studies suggest placental mammals began to diverge from marsupials as early as 175 million years ago. Thus far, the fossil record has not shown this, perhaps because paleontologists trying to date the split have only little teeth and jawbones to work with. The major differences between placentals and marsupials lie in the reproductive tract-which doesn't leave much fossil evidence. But the spectacularly complete new fossil of a protoplacental species found in China's Liaoning Province has given a concrete example that strengthens DNA researchers' claims that placentals began evolving much earlier than previously thought.

"This is the mother of all placental mammals," says Zhe-Xi Luo, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh, proudly presenting a fossil of what resembles a pressed mouse with a long snout. It is so well preserved that some of its fur remains visible. "We call it Eomaia, which means 'dawn mother' in Greek.'

Luo and his colleagues estimate the fossil's age at 125 million years and have found anatomic markers that suggest that Eomaia, while not fully placental, was well on its way to becoming so. That placental development was so far along 125 million years ago makes it easier for paleontologists to accept the genetic evidence that says the first protoplacentals began to evolve 50 million years earlier.

Eomaia's mousy appearance makes it a pretty modest prize by today's mammal standards, but the little creature was the leading edge of a wave of mammalian evolution that had begun with the morganucodontids. Eomaia's placental progeny represented a huge leap, opening up evolutionary options that marsupials' pouch approach constrained. For instance, marsupials develop their forelimbs early in order to climb into the pouch. But placentals' extra time in the womb lets specializations such as the bat's wing and the seal's flipper evolve. The placenta also transports nutrients much more efficiently than milk ducts do. As a result, placental babies grow faster in utero and are more mature when they leave the womb.

For those reasons, most scientists regard the pouch strategy as archaic, and perhaps inferior, pointing out that placental mammals have dominated most of the world for the past 65 million years.

However, there are some dissenters. Marilyn Renfree, a marsupial specialist at the University of Melbourne, says that biologically speaking, "marsupials are every bit as good as other mammals-and in some ways superior." Marsupials have lower metabolic rates and can therefore survive in a broader range of conditions.

Mike Archer, director of the Australian Museum, also believes that the pouch has its advantages. "For marsupials there is such a thing as being a little bit pregnant," he says. After having two eggs fertilized, a kangaroo mother may have only one egg fully develop. Should food or water become scarce and the firstborn infant die, the embryo-in-reserve can implant itself after conditions improve. In an arid land such as Australia, these conditional pregnancies can be the best strategy.

But marsupials remain much less common than other mammals. Opossums and other marsupials exist in North and South America, but Australia is the only continent where marsupials-and monotremes-still rule.

Kangaroos, koalas, platypuses, and wombats: Why does Australia retain these supposedly antiquated mammals? According to the Sherwin-Williams model, marsupials, advanced mammals themselves 100 million years ago, migrated into Gondwana ahead of placentals.

They simply got on board the Antarctic-Australian landmass before it broke away from the rest of Gondwana. Placentals arrived too late-the Australian ship had already sailed.

That theory made a lot of sense until the late 1990s, when some revealing fossils began turning up in various parts of the old Gondwana-Patagonia, Madagascar, and Australia.

The new evidence, once again, came in the form of jawbones and teeth-a particular type known as tribosphenic molars. Such teeth work like a mortar and pestle, a further improvement on the slicing teeth of earlier mammals.

The ancestor of marsupials and placentals had tribosphenic teeth. Thus the discovery in the Southern Hemisphere of tribosphenic teeth as old as 167 million years, or 25 million years older than any found in the north, complicates the north-south model. Some explain the presence of these southern tribosphenic teeth by saying they must have developed independently in both hemispheres. Others say the innovation was too intricate to have evolved twice and that mammals must have evolved in the south, with subsequent generations moving north.

"It's good to remember that the evidence is still slim," says Oklahoma's Rich Cifelli. "I like to say that anyone who really stands up strongly for either theory is either nuts or thinks too highly of himself."

The tribosphenic controversy gets even deeper in Australia, where the husband-and-wife team of Tom Rich of the Museum of Victoria and Pat Vickers-Rich of Monash University have turned up three different mammals with tribosphenic teeth dating back 110 million years. The Riches say that these mammals weren't simply on the way to becoming placental, they were placental-something like hedgehogs, in fact.

Opponents of the Riches' theory argue that placentals-and certainly not the relatively advanced hedgehogs-were not supposed to be anywhere near Australia so long ago. Eomaia, that early forerunner of placentals, lived in Asia. If the Riches are right, we have to rethink how placentals traveled from Asia to the Southern Hemisphere. Rather than traveling down the Americas, Eomaia may have found an island-hopping shortcut to Australia. Or perhaps placentals were widespread much earlier than we think now, and there's just no record of them. They could even have originated in Gondwana and spread out from there. Placentals, suggest the Riches, might even have become extinct with the dinosaurs in Australia, making room for the marsupials to move in later.

Rich himself concedes, "Most radical ideas are wrong. It's wise to be wary of them-especially when they are your own."

Even more radical to many paleontologists has been the marriage of plate tectonics evidence and the placental family tree proposed by evolutionary geneticist Mark Springer and his colleagues. Springer is part of a new generation of researchers who examine the strands of an animal's DNA rather than scraping dirt from fossils at a dig. These molecular biologists read the sequences of genes in a living animal's DNA like an evolutionary history book. The scientists can then determine how closely these animals are related genetically and how long ago their ancestors diverged.

Troubling as it is to many paleontologists, Springer's reading of mammals' genetic history fits remarkably well with what geologists now know about the breaking up and subsequent motion of ancient continents. The oldest group of living placental mammals, according to Springer and his colleagues, arose in Africa just before the continent finished breaking away from the rest of Gondwana around 110 million years ago. Springer calls these animals afrotheres. They include elephants, aardvarks, manatees, and hyraxes. When Africa floated off, it carried these animals away to evolve on their own for tens of millions of years.

The fossil record for Africa from this period is almost blank. Nevertheless, Emmanuel Gheerbrant, a researcher for the National Center for Scientific Research in France, speculates that Africa "must have been a laboratory for some very peculiar animals."

One species Gheerbrant has discovered from this period in Africa is the oldest and most primitive known member of the elephant group, the proboscideans. The 55-million-year-old fossil of Phosphatherium escuilliei was discovered in Morocco. It was the size of a fox, and although it lacked a trunk, it had many dental and cranial features strikingly similar to modern elephants. Paleontologists had long thought elephants were one of the younger modern groups, evolving from ungulates that originated in Asia. But Gheerbrant's fossil, like the genetic evidence, suggests that proboscideans are in fact one of the oldest of the modern ungulate mammals.

One of the few fossil-rich regions in Africa-the Faiyûm Depression of Egypt-has not only these early elephants but also a strange assortment of hyraxes. Today hyraxes resemble guinea pigs. But 35 million years ago hyraxes took many forms. Some were the size of rhinoceroses; others had long legs like gazelles.

Most mammals on the African ark began to disappear around 20 million years ago, after Africa came into contact with the rest of the world again. But Africa wasn't the only ark. An ancient seaway split South America from Eurasia and North America for millions of years, and South America became home to what geneticist Springer calls xenarthrans, another group of placental mammals. South America's fossil record during its isolation is far better than Africa's, and includes such xenarthrans as sloths, armadillos, and anteaters.

Springer's data, in other words, indicate that the most recent common ancestor of placental mammals is Gondwanan. Contrary to more than a century of northern chauvinism, the northern continents have the youngest placental mammals. One group, the laurasiatheres, includes seals, cows, horses, whales, and hedgehogs. The other group, euarchontoglires, includes rodents, tree shrews, monkeys, and humans.

These genetic findings reveal more than simply which came first. They also redefine relationships among placental mammals. For one, anatomists have always assumed that bats were in the same superorder as tree shrews, flying lemurs, and primates. But genetic data place bats with pigs, cows, cats, horses, and whales.

The data further show that these superorders of living mammals started to diversify much earlier than the fossil record suggests. What gets fossilized is a record of an animal's shape. But geneticists contend that genes in an organism's mitochondria, the parts of a cell that are used to trace and date lineages, can be evolving rapidly without changing what would be left behind in the fossil record.

"An animal's shape may be heavily affected by its environment," says Úlfur Érnason, a geneticist at Sweden's University of Lund. "Crocodiles haven't changed much physically in 250 million years, yet they have a high rate of change in their mitochondrial DNA. Birds have a slow rate, yet they can evolve physically very rapidly."

However surprising the claims of geneticists seem at first, paleontologists and DNA researchers are finding that their theories can be complementary. Some stunning new fossils have confirmed a previously controversial DNA finding about whales. Most paleontologists long believed that whales and dolphins-or cetaceans-descended from an extinct line of carnivorous mammals that for unknown reasons became aquatic between 50 and 45 million years ago.

At the time of these fossils' discovery, molecular biologists were maintaining that new DNA work indicated the cetaceans were actually aligned closely with artiodactyls, an order that includes even-toed ungulates such as pigs, camels, deer, and hippopotamuses.

Paleontologists first dismissed this unlikely connection because nothing in the fossil record supported it. Then in September 2001 two teams of fossil hunters published finds that backed up the claims of the biologists. A group led by Hans Thewissen of Northeastern Ohio Universities College of Medicine found two species of the earliest known whales in 50-million-year-old deposits in Pakistan. Both had ear bones unique to whales, but the legs and anklebones of artiodactyls. "The first whales, it turns out, were fully terrestrial and good runners," Thewissen says.

Almost simultaneously, a group from the University of Michigan led by Philip Gingerich announced similar fossils from Pakistan that had the same dual traits. The evolutionary transition among major groups of mammals is rarely illustrated so clearly. And no other discoveries have linked fossils to DNA findings with such precision.

Until 65 million years ago dinosaurs dominated the land. The oceans swarmed with huge sharks and voracious marine reptiles. The dinosaurs and other large predators occupied the richest and most obvious evolutionary niches, keeping mammals at the margins.

Then an event occurred whose scale is still hard to comprehend. An object six miles (9.5 kilometers) across crashed near the present-day Yucatán Peninsula, punching out a crater 110 miles (177 kilometers) across. That impact may have been one of many over the next several hundred thousand years, each adding to the destruction. But the damage done by the Yucatán impact alone is impressive: Tsunamis 500 feet (150 meters) high battered North America. The temperature reached 500 degrees in parts of the world.

"Everything big bit it," says Kirk Johnson of the Denver Museum of Nature & Science. "The key to survival was to be small." Mammals fit that profile. They suddenly found themselves in a world without large carnivores. Restraints were off. Within 270,000 years they were diversifying and growing bigger.

Still, the majority of mammals didn't get much larger than a pig until the Eocene epoch, which began about 55 million years ago. Then a rapid increase in global temperature encouraged the spread of forests around the world-even near both Poles. This abundance of rich vegetation opened yet more ecological niches for mammals to exploit. Mammal diversity soared. One of the newcomers in the fossil record was our own order, the primates. The earliest primates belonged to the lemur branch. Today lemurs are confined to the island of Madagascar, where one species made it from Africa perhaps 50 million years ago, probably on rafts of storm-tossed debris.

A few million years later, more advanced primates appear in the fossil record of eastern Asia. These higher primates are anthropoids-monkeys, apes, and humans. Chris Beard, a specialist in primate origins at the Carnegie Museum of Natural History, has unearthed in China what may be the earliest known example, called Eosimias. These creatures evolved in the mid-Eocene as the world was cooling and concentrated in the midlatitudes where forests remained lush.

Beard says they "must have been frenetic little animals. Kind of caffeinated. They probably ate all the time. When you are that small, you have to. They probably lived in troops and maybe never left the tree they were born in." Despite its primitive anatomy, Eosimias had already adopted the monkeylike habit of walking along the tops of branches rather than leaping from tree to tree like earlier primates.

About 34 million years ago smarter, bigger, and more aggressive monkeys evolved. Fossils from the Faiyûm Depression, where Elwyn Simons of Duke University has led a dig since 1961, reveal how anthropoids were changing. Catopithecus, one of many anthropoids his team has uncovered, has a skull the size of a small monkey's, a relatively flat face, and a bony enclosure at the rear of its eye sockets. It is the first anthropoid to show the same arrangement of teeth humans have-two incisors, one canine, two premolars, and three molars-leading Simons to argue, "This is the first chapter of human history."

At the start of the long Miocene epoch-23.5 million to 5.3 million years ago-yet another major climate change occurred. The world was warming again and more seasonal climate patterns may have emerged. At higher latitudes, forests gradually gave way in many places to grassland meadows and savannas. Because grass is abrasive, some mammals developed new dentition. Horses, for instance, emerged as little leaf-eaters in the forests but later developed molars that are much better adapted to eating grass. Horses' crowns extend into the jawbones. As the crown gets ground down, new crown will emerge from the jaw to replace it.

Early in the Miocene, Africa's long isolation ended when it and Arabia came back into contact with Eurasia. That's when the ancestors of many mammals we think of as native to Africa arrived there. First came the ancestors of antelope, cats, giraffes, and rhinos. Later, around ten million years ago, North American mammals-camels, horses, and dogs-began to arrive. Almost every animal that roams the Serengeti today is a relative newcomer to the continent.

Africa gave back as well. Apes moved into Eurasia and flourished. Elephants and their relatives spread across the globe, reaching as far as the tip of Patagonia.

But geology and climate changed the world once again for mammals as the Miocene drew to a close. The Earth grew colder and drier still. Ice caps formed in the Arctic. The Sahara began taking over North Africa, and savannas spread across much of the continent.

The changing climate restricted the range of the primates to the equatorial zone. The surviving apes became larger and more specialized. Then, around seven million years ago, at least one offshoot of the African apes began walking on two legs.

As that bipedal ape evolved into what would become us, other mammals came and went. Most had to adapt to yet another global climate change about 2.5 million years ago, triggered in part by the formation of the Isthmus of Panama. Its formation blocked east-west ocean circulation and encouraged the Gulf Stream to grow stronger. As the Gulf Stream pumped more warm water closer to the North Pole, precipitation increased. Heavy snows became glaciers two miles (three kilometers) thick, which advanced and retreated in a series of more than 20 ice ages. Because big bodies retain heat better, many mammals, such as the woolly mammoth, grew larger. Even in the temperate zones of Australia, animals became immense. Australia was soon home to big meat-eating kangaroos, wombat-like creatures the size of trucks, and a marsupial lion twice as big as a leopard.

"It was a kick-ass big predator," says paleontologist Steve Wroe of the University of Sydney, as he admires a foot-long (0.3-meter-long) fossil skull of a marsupial lion from 40,000 years ago. "These big shearing teeth make it more highly adapted as a carnivore than any other known mammal." The teeth, explains Wroe, are "for butchery only. The animal would starve to death in a fruit and veggie shop."

Those big mammals, such as the marsupial lion and the killer kangaroo, disappeared between 100,000 and 20,000 years ago. Few controversies rage more fiercely in paleontology than why the megafauna vanished-not just in Australia but also in North America, where mammoths, horses, camels, and dozens of other large Ice Age mammals all vanished by about 11,000 years ago. Many scientists cite climate change. Others say it was humans, arguing that newly arrived Homo sapiens killed off the giants with their spears.

We humans may or may not have killed off the giant mammals of the Ice Age. But we are unquestionably threatening innumerable species today, as we expand relentlessly into ever more of their habitats. Signs of this encroachment appear all around the world. Manatees in Florida chopped up by boat propellers. Rhinos in the Ngorongoro Crater poached. Vast rain forests in Southeast Asia obliterated. All this done by the most intelligent of mammals. Evolution has given us this gift of intelligence, but are we too smart for our own good? If somehow we could rewind time to the dawn of anthropoids, what different path could we have gone down?

One answer lies some 5,000 miles (8,000 kilometers) from the Serengeti's vibrant mammalian spectacle, in the rain forests of Indonesia, Borneo, and the Philippines. There lives the tarsier, which the Carnegie Museum's Beard cites as an example of the primate road not taken. "Tarsiers are pretty weird," says Beard, "They can turn their heads 270 degrees. They're the primates' version of an owl. But they're the nearest living relatives of higher primates."

Tarsiers share a common ancestor with all anthropoids. We know this because like all higher primates, tarsiers lack a tapetum lucidum-the reflective layer in the eyes of nocturnal animals. The tapetum lucidum is critical to vision in low light levels and is what makes the eyes of night creatures glow when a flashlight shines on them.

Unlike most of their anthropoid relatives, tarsiers went back to a nocturnal lifestyle at some point and had to compensate by evolving enormous, spooky eyes. Shine a flashlight in a lemur's eyes at night, and they'll glow back at you. A tarsier's won't.

For humans, tarsiers represent what might have been. But because of us, today they're hard to find. Hunting, development, and the destruction of the rain forests have constricted the tarsiers' habitat.

In Borneo, where tarsiers are considered bad luck, few villagers worry about that. "If I see a tarsier, I go home," says a villager in Kampung Duras in Sarawak. Another local, Lemon Ales, agrees.

"They frighten people because of their big eyes. Also, they may leap on you and bite."

Other villagers regard the tarsier as totems, because the small agile creatures sometimes are seen in rice paddies holding on to the rice stems, as if guarding them.

Lemon and I head into the forest at twilight. The world looked like this in the Eocene, when primates were evolving. The forest is full of the same kinds of fruit-bearing trees that helped primates thrive in the vast forests then emerging around the world. The heavy air seems to press moisture into my skin, and my pores fight back with gushes of sweat. With flashlights we stumble on for several hours in the dark. But the tarsiers never show up. Or maybe they do: The locals warn that if the creatures don't move, you won't see them.

The Singapore zoo has tried to make sure its patrons won't be similarly disappointed. Its tarsiers are behind a glass wall in a simulated forest. "Only six zoos in the world have tarsiers," says C. S. Menon, an animal management officer. "Usually they stress out and die in captivity."

Eager, often pushy, visitors chatter in languages ranging from Dutch to Hindi to Japanese while waiting to board zoo trams that will take them to see what few can now see in the wild at night.

Many get off the tram, pause for a moment at the tarsier exhibit, and move on. That's because even here, under the best nighttime viewing conditions-the simulated light of a full moon-tarsiers are difficult to spot. I try to be patient.

Finally, like lightning, one flashes out of nowhere to grasp a cricket with both hands and land gracefully on a slender branch. It sits upright, munching and rotating its little head in an improbably wide arc. You can't look into the big eyes of our distant cousin without feeling awed by the distance we have traveled away from each other. Our intelligence may pose grave dangers to the world's wildlife and to ourselves, but it also lets us feel wonder. And concern. The jury's still out on where that will take us.

Then, faster than a moth's flutter, the tarsier is gone.

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Rise of Mammals Article, Mammal Evolution Information, Facts -- National Geographic

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From the top of Shifting Sands dune in the Serengeti Plain of Africa a million mammals are in motion. Wildebeests. Zebras. Gazelles. The plain is black with them. It is wildebeest calving season, and many of those giant bearded antelope have newborns trailing them. Others walk with the distended bellies of imminent birth.

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Night is falling in the early Jurassic 185 million years ago, and the Kayentatherium is tending to her newly hatched brood. Heavy rains pummel the bank above her den as she looks over her dozens of tiny young. She is about the size of a large cat and could easily pass for a mammal, but her large jawbone, characteristic teeth and lack of external ears give her away: she is a cynodont, a member of the group from which mammals evolved. At some point without warning, the sodden bank collapses, entombing the hatchlings and their mother in mud.

There they remained until the summer of 2000, when a fossil-hunting crew led by Timothy Rowe at the University of Texas at Austin chanced upon their scattered bones among rocks of the Kayenta Formation in northern Arizona.

That initial encounter with the fossils did little to impress the palaeontologists. They dug up the block and shipped it back to the laboratory for safekeeping. It wasn't until nine years later that a specialist preparing the fossil for study noticed something startling: embedded in the block were tiny teeth, and jawbones just 1 centimetre in length. "Immediately they stopped the preparation and thought about ways of non-destructively examining the babies," says Eva Hoffman, at Texas with Rowe at the time and now a palaeontologist at the American Museum of Natural History in New York City. Instead of breaking into the rock, Hoffman and Rowe digitally extracted the bones with a microcomputed tomography (microCT) scanner, which uses X-rays to create fine-grained 3D images.

What they found inside the rock were the first known babies of mammals or their relatives from the Jurassic - and not just one, but 38 of them, placing this among the most significant discoveries related to mammal origins made in the past decade. Kayentatherium is at the cusp of mammalhood - and researchers say that it provides crucial insights into which traits define mammals and which were present in their earlier relatives.

Kayentatherium 's skeleton is mammal-like in many ways, but the fossil suggested that it still reproduced very much like a reptile, giving birth to large litters of small-brained offspring. By contrast, "mammal moms invest a lot in a smaller number of babies, each of which has a better chance of surviving", says Hoffman. Mammal babies spend longer under their parents' care, developing relatively large brains, whereas these fossil hatchlings had well-developed bones and teeth, hinting that they could fend for themselves and were not nourished by milk, as all mammals are today.

The find is among a mass of discoveries in the past 10-20 years that are illuminating milestones in mammalian evolution. Although major finds are emerging all over the world, the largest number are coming out of China; together, they have overturned the now dated belief that dinosaur-era mammals were small, unremarkable insectivores, eking out a life in the shadows of the giant reptiles.

The fossils have revealed that early mammals were ecologically diverse and experimenting in gliding, swimming, burrowing and climbing. The discoveries are also starting to reveal the evolutionary origins of many of the key traits of mammals - such as lactation, large brains and superbly keen senses.

"The explosion of early-mammal discoveries, particularly from China, over the last two decades has been eye-opening, mind-numbing and absolutely dazzling," says David Krause, a vertebrate palaeontologist at the Denver Museum of Nature and Science in Colorado.

This avalanche of discovery is also stirring up debate: some researchers disagree over which fossil groups are true mammals and the shape of the mammal family tree. "We want to understand our early history in the language of evolutionary biology, and that's what fires me up," says Zhe-Xi Luo, a palaeontologist at the University of Chicago in Illinois. "That's why this entire field is so interesting, because the fossil record is getting better and better, and we are starting to really tackle some of these questions."

Out of the shadows

In 1824, at the Geological Society of London, naturalist William Buckland presented bones from one of the first known dinosaurs, Megalosaurus. At the same talk, he revealed tiny mammalian jaws that had been found in the same fossil deposit. Their presence suggested that mammals had a very deep history, but as would happen repeatedly, the dinosaur discoveries completely overshadowed the mammal ones.

The slow trickle of mammal finds from around the world continued for 150 years. Then in 1997, researchers described the first ancient mammal from the fossil-rich rocks of Liaoning in northeastern China, and the floodgates opened. Since then, 50 or more near-complete and "beautiful specimens" have been found there, according to Jin Meng, a palaeontologist at the American Museum of Natural History. Like the dinosaur fossils, they are dug up by local farmers and sold on to museums.

But the dinosaurs continued to get the vast majority of the attention, says palaeontologist Steve Brusatte at the University of Edinburgh, UK. "It's only that very recently, through the work of Luo, Meng and others, that the mammals are getting their due."

Most of China's mammal fossils were formed when volcanoes buried the animals in ash - and they are exquisitely detailed. Typical mammal fossils from the Mesozoic era (252 million to 66 million years ago) are little more than teeth and jaw fragments, but Chinese specimens often have entire skeletons, with fur, skin and internal organs. "We have a lot of detail to answer scientific questions," says Meng, He is interested in understanding the evolution of the mammalian ear, for instance.

The finds overturned previous dogma. "We used to say that during the time of dinosaurs, mammals were totally unspectacular. That they were just these little mousey things scampering around in the shadows," says Brusatte. But these animals "were undergoing their own evolutionary explosion", he says.

Mammals first appeared at least 178 million years ago, and scampered amid the dinosaurs until the majority of those beasts, with the exception of the birds, were wiped out 66 million years ago. But mammals didn't have to wait for that extinction to diversify into many forms and species. "These new discoveries document a huge, hitherto-undreamed-of ecological diversity," says Richard Cifelli, a palaeontologist at the University of Oklahoma in Norman.

Among the first innovations that researchers began to find in fossil form were those to do with locomotion. In 2006, Meng's team reported the first gliding mammal, 164-million-year-old Volaticotherium, which had wing membranes made of furry skin, like today's flying squirrels. In 2017, Luo's team added Vilevolodon and Maiopatagaium,, which lived at around the same time and belonged to a group called the haramiyids. These animals swooped between the trees alongside some of the first flying dinosaurs, taking advantage of previously unexploitable food resources.

Researchers found other specializations that they assumed had evolved only later: Agilodocodon could climb trees and gnawed into bark to feast on sap; the platypus-sized river-dweller Castorocauda had webbed feet and a beaver-like tail for swimming; and Docofossor had paws and claws for digging, and looked like a modern mole.

These mammals had also adapted to a multitude of diets, much more diverse than previously assumed. In 2014, Krause described the groundhog-like Vintana from Madagascar, a herbivore that perhaps fed on roots and seeds. And the wolverine-sized carnivore Repenomamus, which Meng's team reported in 2005, had baby dinosaur bones in its stomach. Many of these new-found fossil mammals belong to long-extinct subgroups, says Meng. In contrast to the panoply that existed in the Mesozoic, mammals today come in just three varieties: placentals, which make up the majority of species and include humans; marsupials, such as kangaroos and koalas, in which gestation in the womb is brief and development continues in a pouch; and the egg-laying monotremes, represented only by the platypus and several echidnas. "But in geological history, there were many other groups such as multituberculates, triconodonts and haramiyids," says Meng. "Mammals were actually very diverse in the Jurassic."

Some, such as the shrew-like Juramaia - described by Luo's team in 2011 and dated to 160 million years ago - are among the earliest placental mammals and therefore have the potential to be our ancestors.

And a few dinosaur-era mammals were much bigger than suspected, too. Repenomamus was 12-14 kilograms, and the racoon-sized Vintana weighed in at 9 kg. "It's exciting that we kind of busted the old myths that early mammals came from a very humble generalized ancestor," says Luo.

The finds are not solely from China. Important fossils are also coming from the United States, Spain, Brazil, Argentina, Madagascar and Mongolia. Some of the most intriguing and oldest fossils - as well as the biggest gaps in our knowledge - relate to the southern continents, where only five genera of Mesozoic mammals and their relatives are known, compared with more than 70 genera from northern latitudes. In the past two decades, Brazil has yielded several Triassic fossils that are more than 200 million years old. Guillermo Rougier, a palaeontologist at the University of Louisville in Kentucky, describes them as "incredible discoveries" that are right on the cusp between mammals and their cynodont ancestors. "These forms really show a very transitional progression from things that are typically non-mammalian, to things that pretty much have all the features of early mammals."

Mammal must-haves

The latest finds are also offering clues to the evolution of key mammal features. For instance, the keen hearing of mammals is partly down to tiny bones in the middle ear - the malleus, incus and ectotympanic. But in the reptilian ancestors of mammals, these bones were part of the jaw, and were used for chewing instead of hearing. Mammal forerunners, such as shrew-like Morganucodon from 205 million years ago, sported a prototype of the mammal arrangement that allowed for both functions.

In 2011, Meng reported an intermediary: a 120-million-year-old specimen from China belonging to a group of mammals called eutriconodonts and named Liaoconodon hui (see 'Mammal hallmarks'). The rat-sized fossil revealed three middle-ear bones, but they were still attached to the jaw by cartilage. "The hearing function and the chewing function were still not completely separated," he explains. This was hard evidence of the evolutionary transition from jaw to ear.

Another unique trait of mammals is the sophisticated way they chew and ingest food in small parcels, rather than swallowing things whole as snakes and alligators do. To make that possible, mammals evolved a wide variety of complex teeth for biting and grinding food.

But as babies, mammals are nourished another way - by suckling from their mother's mammary glands. "Our whole group is named after this incredible biological innovation," says Luo. Drinking milk is made possible by the ability to suck and swallow, aided by the hyoid bones in the throat and muscles that support them. This apparatus also forms the voice box.

In July, Luo published a paper revealing a 165-million-year-old vole-sized docodont - a close relative of true mammals - that had the hyoid bones of its throat preserved. Microdocodon gracilis is the earliest animal known to have been able to suckle like a modern mammal.

This level of detail is rare, and - similar to the study of the Kayentatherium hatchlings - the work on both the ear and throat bones has been made possible only through advances in microCT scanning techniques, says Krause. The technique has also revealed details about the olfactory abilities and brains of early mammals. These revelations are "breathing life into these early mammals in ways that were previously impossible and almost inconceivable", he says.

Much of the constellation of features we think of as defining mammals - complex teeth, excellent senses, lactation, small litter size - might actually have evolved before true mammals, and quite quickly. "More and more it looks like it all came out in a very short burst of evolutionary experimentation," Luo says. By the time mammal-like creatures were roaming around in the Mesozoic, he says, "the lineage has already acquired its modern look and modern biological adaptations".

Family drama

Although the experts concur on many points, there is still much debate about how early mammal groups are related, and which groups are true mammals. That leads to uncertainty about how key traits evolved, says Hoffman.

One sticking point between Meng and Luo, who have each developed their own evolutionary trees, is the haramiyids. Meng thinks this early group belongs with true mammals, whereas Luo is convinced it's a side branch. The oldest known haramiyids are from 208 million years ago in the Triassic. If they are true mammals, then mammal origins date back at least that far - if not, then the oldest known mammal is 178 million years old, well into the Jurassic.

More fossils will help to resolve such questions, and bring more surprises. Krause and Meng say they are both studying exciting fossils, but are yet to publish their findings on them, and tens of unstudied specimens lie piled in the offices of their Chinese colleagues.

Palaeontologists have many items on their wish lists. One characteristic that Luo wants to understand is growth rates. Reptiles grow slowly throughout their lives, whereas mammals grow in bursts in youth and then plateau. He'd love to find a series of fossils from babies to adults to watch this happening. "That is one of the most critical features in mammals that help to define our biology," he says.

Both Hoffman and Meng agree that embryos and more babies would be significant finds - and, like the Kayentatherium discovery with its dozens of hatchlings, they would help us to pinpoint the date that mammal-style small litter sizes appeared. Meng's dream is to find a pregnant mammal. "This is always in my mind that I will find a mammal that inside the skeleton you can see some very delicate skeleton, which is either an egg that hasn't hatched, or it's a more interesting fetus," he says.

If the flurry of discoveries has taught researchers anything, it's that every fossil find has the potential to add a chapter to evolutionary history or even flip the prevailing narrative on its head. "We're really in this exciting, almost manic phase of lots of new evidence coming in, and it's going to take time to synthesize," says Brusatte.

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How the earliest mammals thrived alongside dinosaurs

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An explosion of fossil finds reveals that ancient mammals evolved a wide variety of adaptations allowing them to exploit the skies, rivers and underground lairs. An explosion of fossil finds reveals that ancient mammals evolved a wide variety of adaptations allowing them to exploit the skies, rivers and underground lairs.

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Ask the average person (or high-schooler) on the street, and he or she will guess that the first mammals didn't appear on the scene until after the dinosaurs went extinct 65 million years ago--and, moreover, that the last dinosaurs evolved into the first mammals. The truth, though, is very different: in fact, the first mammals evolved from a population of vertebrates called therapsids ("mammal-like reptiles") at the end of the Triassic period and coexisted with dinosaurs throughout the Mesozoic Era.

But part of this folk tale has a grain of truth: it was only after the dinosaurs went kaput that mammals were able to evolve beyond their tiny, quivering, mouselike forms into the widely specialized species that populate the world today.

These popular misconceptions about the mammals of the Mesozoic Era are easy to explain: scientifically speaking, dinosaurs tended to be very, very big and early mammals tended to be very, very small. With a couple of exceptions, the first mammals were tiny, inoffensive creatures, rarely more than a few inches long and a few ounces in weight, about on a par with modern shrews. Thanks to their low profiles, these hard-to-see critters could feed on insects and small reptiles (which bigger ​ raptors and tyrannosaurs tended to ignore), and they could also scurry up trees or dig into burrows to avoid getting stomped on by larger ornithopods and sauropods.

The Evolution of the First Mammals

Before discussing how the first mammals evolved, it's helpful to define what distinguishes mammals from other animals, especially reptiles.

Female mammals possess milk-producing mammary glands with which they suckle their young; all mammals have hair or fur during at least some stage of their life cycles; and all are endowed with warm-blooded (endothermic) metabolisms. In terms of the fossil record, paleontologists can distinguish ancestral mammals from ancestral reptiles by the shape of their skull and neck bones, as well as the presence, in mammals, of two small bones in the inner ear (in reptiles, these bones constitute part of the jaw).

As mentioned above, the first mammals evolved toward the end of the Triassic period from a population of therapsids, the "mammal-like reptiles" that arose in the early Permian period and produced such uncannily mammal-like beasts as Thrinaxodon and Cynognathus. By the time they went extinct in the mid- Jurassic period, some therapsids had evolved proto-mammalian traits (fur, cold noses, warm-blooded metabolisms, and possibly even live birth) that were further elaborated upon by their descendants of the later Mesozoic Era.

As you can imagine, paleontologists have a hard time distinguishing between the last, highly evolved therapsids and the first, newly evolved mammals. Late Triassic vertebrates like Eozostrodon, Megazostrodon and Sinoconodon appear to have been intermediate "missing links" between therapsids and mammals, and even in the early Jurassic period, Oligokyphus possessed reptilian ear and jaw bones at the same time as it showed every other sign (rat-like teeth, the habit of suckling its young) of being a mammal. (If this seems confusing, bear in mind that the modern-day platypus is classified as a mammal, even though it lays reptilian, soft-shelled eggs rather than giving birth to live young!)

The Lifestyles of the First Mammals

The most distinctive thing about the mammals of the Mesozoic Era is how small they were. Although some of their therapsid ancestors attained respectable sizes (for example, the late Permian Biarmosuchus was about the size of a large dog), very few early mammals were larger than mice, for a simple reason: dinosaurs had already become the dominant terrestrial animals on earth. The only ecological niches open to the first mammals entailed a) feeding on plants, insects and small lizards, b) hunting at night (when predatory dinosaurs were less active), and c) living high up in trees or underground, in burrows. Eomaia, from the early Cretaceous period, and Cimolestes, from the late Cretaceous period, were fairly typical in this regard.

This isn't to say that all early mammals pursued identical lifestyles.

For example, the North American Fruitafossor possessed a pointed snout and mole-like claws, which it clearly used to dig for insects (and probably to hide deep underground when predators were afoot), and the late Jurassic Castorocauda was built for a semi-marine lifestyle, with its long, beaverlike tail and hydrodynamic arms and legs. Perhaps the most spectacular deviation from the basic Mesozoic mammalian body plan was Repenomamus, a three-foot-long, 25-pound carnivore that is the only mammal known to have fed on dinosaurs (a fossilized specimen of Repenomamus has been found with the remains of a Psittacosaurus in its stomach).

Recently, paleontologists discovered conclusive fossil evidence for the first important split in the mammal family tree, the one between placental and marsupial mammals. (Technically, the first, marsupial-like mammals of the late Triassic period are known as metatherians; from these evolved the eutherians, which later branched off into placental mammals.) The type specimen of Juramaia, the "Jurassic mother," dates to about 160 million years ago, and demonstrates that the metatherian/eutherian split occurred at least 35 million years before scientists had previously estimated.

The Age of Giant Mammals

Ironically, the same characteristics that helped mammals maintain a low profile during the Mesozoic Era also allowed them to survive the K/T Extinction Event that doomed the dinosaurs. As we now know, that giant meteor impact 65 million years ago produced a kind of "nuclear winter," destroying most of the vegetation that sustained the herbivorous dinosaurs, which themselves sustained the carnivorous dinosaurs that preyed on them.

Because of their tiny size, early mammals could survive on much less food, and their fur coats (and warm-blooded metabolisms) helped keep them warm in an age of plunging global temperatures.

With the dinosaurs out of the way, the Cenozoic Era was an object lesson in convergent evolution: mammals were free to radiate into open ecological niches, in many cases taking on the general "shape" of their dinosaur predecessors (giraffes, as you may have noticed, are eerily similar in body plan to ancient sauropods like Brachiosaurus, and other mammalian megafauna pursued similar evolutionary paths). Most important, from our perspective, early primates like Purgatorius were free to multiply, populating the branch of the evolutionary tree that led eventually to modern humans.

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Yes, a Tiny, Tree-dwelling Triassic Shrew Is Your Great, Great Grandma

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Ask the average person (or high-schooler) on the street, and he or she will guess that the first mammals didn't appear on the scene until after the dinosaurs went extinct 65 million years ago--and, moreover, that the last dinosaurs evolved into the first mammals.

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An ice age is a period of colder global temperatures and recurring glacial expansion capable of lasting hundreds of millions of years. Thanks to the efforts of geologist Louis Agassiz and mathematician Milutin Milankovitch, scientists have determined that variations in the Earth's orbit and shifting plate tectonics spur the waxing and waning of these periods. There have been at least five significant ice ages in Earth's history, with approximately a dozen epochs of glacial expansion occurring in the past 1 million years. Humans developed significantly during the most recent glaciation period, emerging as the dominant land animal afterward as megafauna such as the wooly mammoth went extinct.

An ice age is a period of colder global temperatures that features recurring glacial expansion across the Earth's surface. Capable of lasting hundreds of millions of years, these periods are interspersed with regular warmer interglacial intervals in which at least one major ice sheet is present. Earth is currently in the midst of an ice age, as the Antarctic and Greenland ice sheets remain intact despite moderate temperatures.

These global cooling periods begin when a drop in temperature prevents snow from fully melting in some areas. The bottom layer turns to ice, which becomes a glacier as the weight of accumulated snow causes it to slowly move forward. A cyclical pattern emerges in which the snow and ice traps the Earth's moisture, fueling the growth of these ice sheets as the sea levels simultaneously drop.

An ice age causes enormous changes to the Earth's surface. Glaciers reshape the landscape by picking up rocks and soil and eroding hills during their unstoppable push, their sheer weight depressing the Earth's crust. As temperatures drop in areas adjacent to these ice cliffs, cold-weather plant life is driven to southern latitudes. Meanwhile, the dramatic drop in sea levels enables rivers to carve out deeper valleys and produce enormous inland lakes, with previously submerged land bridges appearing between continents. Upon retreating during warmer periods, the glaciers leave behind scattered ridges of sediment and fill basins with melted water to create new lakes.

Scientists have recorded five significant ice ages throughout the Earth's history: the Huronian (2.4-2.1 billion years ago), Cryogenian (850-635 million years ago), Andean-Saharan (460-430 mya), Karoo (360-260 mya) and Quaternary (2.6 mya-present). Approximately a dozen major glaciations have occurred over the past 1 million years, the largest of which peaked 650,000 years ago and lasted for 50,000 years. The most recent glaciation period, often known simply as the "Ice Age," reached peak conditions some 18,000 years ago before giving way to the interglacial Holocene epoch 11,700 years ago.

At the height of the recent glaciation, the ice grew to more than 12,000 feet thick as sheets spread across Canada, Scandinavia, Russia and South America. Corresponding sea levels plunged more than 400 feet, while global temperatures dipped around 10 degrees Fahrenheit on average and up to 40 degrees in some areas. In North America, the region of the Gulf Coast states was dotted with the pine forests and prairie grasses that are today associated with the northern states and Canada.

The origins of ice age theory began hundreds of years ago, when Europeans noted that glaciers in the Alps had shrunk, but its popularization is credited to 19th century Swiss geologist Louis Agassiz. Contradicting the belief that a wide-ranging flood killed off such megafauna as the wooly mammoth, Agassiz pointed to rock striations and sediment piles as evidence of glacier activity from a destructive global winter. Geologists soon found evidence of plant life between glacial sediment, and by the close of the century the theory of multiple global winters had been established.

A second important figure in the development of these studies was Serbian mathematician Milutin Milankovitch. Seeking to chart the Earth's temperature from the past 600,000 years, Milankovitch carefully calculated how orbital variations such as eccentricity, precession and axial tilt affected solar radiation levels, publishing his work in the 1941 book Canon of Insolation and the Ice Age Problem. Milankovitch's findings were corroborated when technological improvements in the 1960s allowed for the analyzation of deep sea ice cores and plankton shells, which helped pinpoint periods of glaciation.

Along with solar radiation levels, it is believed that global warming and cooling is connected to plate tectonic activity. The shifting of the Earth's plates creates large-scale changes to continental masses, which impacts ocean and atmospheric currents, and triggers volcanic activity that releases carbon dioxide into the air.

One significant outcome of the recent ice age was the development of Homo sapiens. Humans adapted to the harsh climate by developing such tools as the bone needle to sew warm clothing, and used the land bridges to spread to new regions. By the start of the warmer Holocene epoch, humans were in position to take advantage of the favorable conditions by developing agricultural and domestication techniques. Meanwhile, the mastodons, saber-toothed cats, giant ground sloths and other megafauna that reigned during the glacial period went extinct by its end.

The reasons for the disappearance of these giants, from human hunting to disease, are among the ice age mysteries that have yet to be fully explained. Scientists continue to study the evidence of these important periods, both to gain more insight into the Earth's history and to help determine future climatic events.

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An ice age is a period of colder global temperatures and recurring glacial expansion capable of lasting hundreds of millions of years. Thanks to the efforts of geologist Louis Agassiz and mathematician Milutin Milankovitch, scientists have determined that variations in the Earth's orbit and shifting plate tectonics spur the waxing and waning of these periods.

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Kids learn about the ice ages and Earth sciences including glacial and interglacial periods, major ice ages, interesting facts, and their causes.

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Scientists are not sure what caused the ice ages. Changes in Earth's orbit around the sun may be one cause. These changes would have caused parts of Earth to receive less heat from the sun at certain times. Another possible cause is changing amounts of dust and gases in the air. More dust and fewer gases could have caused Earth's surface to cool.

Whatever its cause, the latest ice age left lasting marks on the land. The creeping ice carried gravel, sand, soil, clay, and even boulders. When the ice melted, these materials were left behind in ridges, piles, and other formations. Many lakes formed in places where the ice melted. Melting ice also caused sea levels to rise. As the land near the sea flooded, coastlines moved inland. Finally, the huge weight of the ice sheets caused land to sink in some places. As the glaciers moved back, this land slowly rose back up.

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Scientists are not sure what caused the ice ages. Changes in Earth's orbit around the sun may be one cause. These changes would have caused parts of Earth to receive less heat from the sun at certain times. Another possible cause is changing amounts of dust and gases in the air.

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What is the Ice Age?

Earth has experienced many periods of warming and cooling episodes throughout history. The Ice Age is a period on Earth's geological history that started around 70,000 years ago and is well-documented through animal and plant fossils and artifacts left by humans. The peak of the Ice Age is thought to be around 19,000 years ago, and the Ice Age ended around 10,000 years ago. As the name suggests, this was a time when the climate was much, much colder than it is today. In fact, much of the Earth's surface was covered in sheets of ice.

It is estimated that about 1/3 of the Earth was covered in ice, and some of the ice sheets were up to a mile thick. New York state was covered in a sheet of ice approximately 1,000 feet thick. That's almost as high as New York City's famed Empire State Building. Because much of Earth's water was locked up in ice sheets, scientists estimate that sea level was over 300 feet lower than it is today.

What Caused the Ice Age?

There is geological evidence for many warming and cooling periods throughout Earth's over 4-billion-year history. The Ice Age was not caused by one singular event but many factors contributed to the Earth warming or cooling. These included the Earth's position to the sun, the Earth's tilt, and/or small changes in the Earth's atmosphere or ocean currents. Small changes over a long period of time can create dramatic changes to the climate.

Life in the Ice Age

Imagine living during the Ice Age. Your home would look very different than it does today, and evidence suggests that people lived in caves during the Ice Age. Hunters would set out on expeditions to find food. Instead of modern weapons we think of hunters using today, people of the Ice Age had only stone tools that included knives and spears made out of rocks shaped into sharp points. It was no easy task to kill a giant mammoth, cave bear, or saber tooth cat with a spear!

Megafauna

The Ice Age is known for having been a time when giant animals known as megafauna roamed the Earth. Today's megafauna includes large animals like elephants, hippopotamus, lions, and tigers. Megafauna of the Ice Age were quite spectacular and included armadillos the size of small cars called glyptodons, giant sloths as tall as a house, and giant beavers standing over 6 feet tall.

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ice age lesson for kids: history & facts | study.com

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Ice Age Lesson for Kids: History & Facts | Study.com

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Earth has experienced many periods of warming and cooling episodes throughout history. The Ice Age is a period on Earth's geological history that started around 70,000 years ago and is well-documented through animal and plant fossils and artifacts left by humans.

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The first apes appeared in Africa around 25 million years ago. Then at some point, the group split into the ancestors of modern humans and the ancestors of modern apes. It's hard to say exactly when, but thanks to modern genetics and a host of fossil discoveries, we have a rough idea. The oldest known hominid was Sahelanthropus tchadensis, which lived about 7 million years ago.

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The study of human origins, or beginnings, involves figuring out how and when human beings began to exist. Scientists have many different theories about human origins. But…

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Many modern animals have close relatives - other species that are in their same genus. House cats, for instance, belong to the same genus as the European mountain cat, the jungle cat and more. Dogs are in the same genus as coyotes and jackals. But humans? People are alone. We are the last surviving member of the genus Homo.

We weren't always alone. Our family, the hominids, included other primates that walked the Earth on two legs. Some of them were our ancestors. We know them from the fossils, footprints and tools they left behind.

One famous hominid fossil goes by the name "Lucy." This member of Australopithecus afarensis walked upright 3.2 million years ago in what is now Ethiopia. A closer relative to modern humans, Homo naledi, might have roamed South Africa at the same time as members of our own species. Another famous relative - Homo neanderthalensis, or Neandertals - lived alongside modern humans. Neandertals used medicine and tools just as humans of the time did.

Over time, though, these other species died out. Modern humans spread all over the world, from our first home in Africa to Australia and the Americas. Now, Homo sapiens is all that's left of our family tree.

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'Cousin' Lucy may have fallen from a tree to her death 3.2 million years ago: A contested study suggests that Lucy, a famous fossil ancestor of humans, fell from a tree to her death. (8/30/2016) Readability: 7.4

This hominid may have shared Earth with humans: Newfound fossils in South Africa point to a far more recent age for Homo naledi than had been accepted. If correct, this hominid might have coexisted with humans - even interacted with our species. (5/10/2017) Readability: 7.8

This cave hosted the oldest known human remains in Europe: Bone fragments, tools and other finds in Bulgaria suggest that Homo sapiens moved rapidly into Eurasia as early as 46,000 years ago. (6/12/2020) Readability: 7.2

Who were our ancient human ancestors? Meet the other members of our genus, Homo.

Explore more
Scientists Say: Archaeology Explainer: How a fossil forms Cool Jobs: Drilling into the secrets of teeth H DNA reveals clues to the Siberian ancestors of the first Americans Neandertals: Ancient Stone Age builders had tech skills Ancient footprints surface in Britain Fossils hint ancient humans passed through a green Arabia obbits: Our tiny cousins
Be a detective in an early human crime scene. An interactive from the Smithsonian National Museum of Natural History offers close looks at ancient bones to show how early humans ate - and got eaten.

Power Words
More About Power Words

archaeology: (also archeology) The study of human history and prehistory through the excavation of sites and the analysis of artifacts and other physical remains. Those remains can range from housing materials and cooking vessels to clothing and footprints. People who work in this field are known as archaeologists.

Australopithecus: An extinct genus of hominids that lived in East Africa from about 4 million to 2 million years ago.

coyote: This relatively long-legged member of the dog family ( Canis latrans) is sometimes referred to as the prairie wolf. It is, however, notably smaller and its build more scrawny than a true wolf. Found from Alaska down into Central America, coyotes have lately expanded their range into all 50 U.S. states. Many now hang out in urban areas where they have no predators and can easily dine on rodents and scavenge trashed food.

DNA: (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.

Eurasia: That part of the globe covered by Europe and Asia.

family: A taxonomic group consisting of at least one genus of organisms.

fossil: Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called "body fossils." Things like footprints are called "trace fossils." Even specimens of dinosaur poop are fossils. The process of forming fossils is called fossilization.

genus: (plural: genera) A group of closely related species. For example, the genus Canis - which is Latin for "dog" - includes all domestic breeds of dog and their closest wild relatives, including wolves, coyotes, jackals and dingoes.

hominid: A primate within the animal family that includes humans and their ancient upright-walking relatives. Except for humans, all other hominids are known only from fossils.

Homo: A genus of species that includes modern humans ( Homo sapiens). All had large brains and used tools. This genus is believed to have first evolved in Africa and over time evolved and radiated throughout the rest of the world.

Homo naledi: An extinct human relative of unknown age. The bones of this species were found in a cave system in South Africa, beginning in 2013.

Neandertal: A species ( Homo neanderthalensis) that lived in Europe and parts of Asia from about 200,000 years ago to roughly 28,000 years ago.

primate: The order of mammals that includes humans, apes, monkeys and related animals (such as tarsiers, the Daubentonia and other lemurs).

species: A group of similar organisms capable of producing offspring that can survive and reproduce.

Stone Age: A prehistoric period when weapons and tools were made of stone or of materials such as bone, wood, or horn. This period lasted millions of years and came to an end around 10,000 to 12,000 years ago.

tool: An object that a person or other animal makes or obtains and then uses to carry out some purpose such as reaching food, defending itself or grooming.

Bethany Brookshire was a longtime staff writer at Science News Explores. She has a Ph.D. in physiology and pharmacology and likes to write about neuroscience, biology, climate and more. She thinks Porgs are an invasive species.

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Many modern animals have close relatives - other species that are in their same genus. House cats, for instance, belong to the same genus as the European mountain cat, the jungle cat and more. Dogs are in the same genus as coyotes and jackals. But humans? People are alone.

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Homo sapiens evolved from their early hominid predecessors between 200,000 and 300,000 years ago and developed a capacity for language about 50,000 years ago.

Homo sapiens, the first modern humans, evolved from their early hominid predecessors between 200,000 and 300,000 years ago. They developed a capacity for language about 50,000 years ago.

The first modern humans began moving outside of Africa starting about 70,000-100,000 years ago.

Humans are the only known species to have successfully populated, adapted to, and significantly altered a wide variety of land regions across the world, resulting in profound historical and environmental impacts.

Where do we begin?

Before we tell the stories that make up world history, it is useful to ask: where do we begin? Where did our human stories start?

Homo sapiens is part of a group called hominids, which were the earliest humanlike creatures. Based on archaeological and anthropological evidence, we think that hominids diverged from other primates somewhere between 2.5 and 4 million years ago in eastern and southern Africa. Though there was a degree of diversity among the hominid family, they all shared the trait of bipedalism, or the ability to walk upright on two legs.

Scientists have several theories about why early hominids evolved. One, the aridity hypothesis, suggests that early hominids were more suited to dry climates and evolved as the Africa's dry savannah regions expanded.

Hominids continued to evolve and develop unique characteristics. Their brain capacities increased, and approximately 2.3 million years ago, a hominid known as Homo habilis began to make and use simple tools. By a million years ago, some hominid species, particularly Homo erectus, began to migrate out of Africa and into Eurasia, where they began to make other advances like controlling fire.

Though there were once many kinds of hominids, only one remains: Homo sapiens. Extinction is a normal part of evolution, and scientists continue to theorize why other hominid species didn't survive. We do have some clues as to why some species were less successful at surviving than others, such as an inability to cope with competition for food, changes in climate, and volcanic eruptions.

Migration and the Peopling of the Earth

Between 70,000 and 100,000 years ago, Homo sapiens began migrating from the African continent and populating parts of Europe and Asia. They reached the Australian continent in canoes sometime between 35,000 and 65,000 years ago.

Adaptation and effects on nature

When humans migrated from Africa to colder climates, they made clothing out of animal skins and constructed fires to keep themselves warm; often, they burned fires continuously through the winter. Sophisticated weapons, such as spears and bows and arrows, allowed them to kill large mammals efficiently. Along with changing climates, these hunting methods contributed to the extinction of giant land mammals such as mammoths, giant kangaroos, and mastodons. Fewer giant mammals, in turn, limited hunters' available prey.

What do you think?

Why do you think Homo sapiens is the only surviving human species?

What effects do you think the environment had on human evolution? What effects did human evolution have on the environment?

What skills were needed for human species to populate the Earth?

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First humans: Homo sapiens & early human migration (article) | Khan Academy

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Learn for free about math, art, computer programming, economics, physics, chemistry, biology, medicine, finance, history, and more. Khan Academy is a nonprofit with the mission of providing a free, world-class education for anyone, anywhere.