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The current geological era, that started 66 million years ago with the extinction of non-avian dinosaurs. It is divided into 3 periods (the Paleogene, Neogene, and Quaternary) and seven epochs (the Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, and Holocene)

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The current geological era, that started 66 million years ago with the extinction of non-avian dinosaurs. It is divided into 3 periods (the Paleogene, Neogene, and Quaternary) and seven epochs (the Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, and Holocene)

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"When the solar system settled into its current layout about 4.6 billion years ago, Earth formed when gravity pulled swirling gas and dust in to become the third planet from the Sun."

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"When the solar system settled into its current layout about 4.6 billion years ago, Earth formed when gravity pulled swirling gas and dust in to become the third planet from the Sun."

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Earth - our home planet - is the third planet from the Sun, and the fifth largest planet. It's the only place we know of inhabited by living things.

Some of the features of our planet that make it great for sustaining life are changing due to the ongoing effects of climate change.

Earth is the only planet that has a single moon. Our Moon is the brightest and most familiar object in the night sky. In many ways, the Moon is responsible for making Earth such a great home. It stabilizes our planet's wobble, which has made the climate less variable over thousands of years.

1. Measuring Up - If the Sun were as tall as a typical front door, Earth would be the size of a nickel.

2. We're On It - Earth is a rocky planet with a solid and dynamic surface of mountains, canyons, plains and more. Most of our planet is covered in water.

3. Breathe Easy - Earth's atmosphere is 78 percent nitrogen, 21 percent oxygen and 1 percent other ingredients-the perfect balance to breathe and live.

4. Our Cosmic Companion - Earth has one moon.

5. Ringless - Earth has no rings.

6. Orbital Science - Many orbiting spacecraft study the Earth from above as a whole system-observing the atmosphere, ocean, glaciers, and the solid earth.

7. Home, Sweet Home - Earth is the perfect place for life as we know it.

8. Protective Shield - Our atmosphere protects us from incoming meteoroids, most of which break up in our atmosphere before they can strike the surface.

Discover More Topics From NASA

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Earth is the third planet from the Sun, and the fifth largest planet. It's the only place we know of inhabited by living things.

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At first, the planet was likely completely molten. Over a few hundred million years, the planet began to cool and the oceans initially formed.

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At first, the planet was likely completely molten. Over a few hundred million years, the planet began to cool and the oceans initially formed.

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Our planet began as part of a cloud of dust and gas. It has evolved into our home, which has an abundance of rocky landscapes, an atmosphere that supports life, and oceans filled with mysteries.

Subjects

Chemistry, Earth Science, Astronomy, Geology

Image

Manicouagan Crater

Asteroids were not only important in Earth's early formation, but have continued to shape our planet. A five-kilometer (three-mile) diameter asteroid is theorized to have formed the Manicouagan Crater about 215.5 million years ago.

NASA photo

We live on Earth's hard, rocky surface, breathe the air that surrounds the planet, drink the water that falls from the sky, and eat the food that grows in the soil. But Earth did not always exist within this expansive universe, and it was not always a hospitable haven for life.

Billions of years ago, Earth, along with the rest of our solar system, was entirely unrecognizable, existing only as an enormous cloud of dust and gas. Eventually, a mysterious occurrence-one that even the world's foremost scientists have yet been unable to determine-created a disturbance in that dust cloud, setting forth a string of events that would lead to the formation of life as we know it. One common belief among scientists is that a distant star collapsed, creating a supernova explosion, which disrupted the dust cloud and caused it to pull together. This formed a spinning disc of gas and dust, known as a solar nebula. The faster the cloud spun, the more the dust and gas became concentrated at the center, further fueling the speed of the nebula. Over time, the gravity at the center of the cloud became so intense that hydrogen atoms began to move more rapidly and violently. The hydrogen protons began fusing, forming helium and releasing massive amounts of energy. This led to the formation of the star that is the center point of our solar system-the sun-roughly 4.6 billion years ago.

Planet Formation

The formation of the sun consumed more than 99 percent of the matter in the nebula. The remaining material began to coalesce into various masses. The cloud was still spinning, and clumps of matter continued to collide with others. Eventually, some of those clusters of matter grew large enough to maintain their own gravitational pull, which shaped them into the planets and dwarf planets that make up our solar system today.

Earth is one of the four inner, terrestrial planets in our solar system. Just like the other inner planets-Mercury, Venus, and Mars-it is relatively small and rocky. Early in the history of the solar system, rocky material was the only substance that could exist so close to the Sun and withstand its heat.

In Earth's Beginning

At its beginning, Earth was unrecognizable from its modern form. At first, it was extremely hot, to the point that the planet likely consisted almost entirely of molten magma. Over the course of a few hundred million years, the planet began to cool and oceans of liquid water formed. Heavy elements began sinking past the oceans and magma toward the center of the planet. As this occurred, Earth became differentiated into layers, with the outermost layer being a solid covering of relatively lighter material while the denser, molten material sunk to the center.

Scientists believe that Earth, like the other inner planets, came to its current state in three different stages. The first stage, described above, is known as accretion, or the formation of a planet from the existing particles within the solar system as they collided with each other to form larger and larger bodies. Scientists believe the next stage involved the collision of a protoplanet with a very young planet Earth. This is thought to have occurred more than 4.5 billion years ago and may have resulted in the formation of Earth's moon. The final stage of development saw the bombardment of the planet with asteroids.

Earth's early atmosphere was most likely composed of hydrogen and helium. As the planet changed, and the crust began to form, volcanic eruptions occurred frequently. These volcanoes pumped water vapor, ammonia, and carbon dioxide into the atmosphere around Earth. Slowly, the oceans began to take shape, and eventually, primitive life evolved in those oceans.

Contributions from Asteroids

Other events were occurring on our young planet at this time as well. It is believed that during the early formation of Earth, asteroids were continuously bombarding the planet, and could have been carrying with them an important source of water. Scientists believe the asteroids that slammed into Earth, the moon, and other inner planets contained a significant amount of water in their minerals, needed for the creation of life. It seems the asteroids, when they hit the surface of Earth at a great speed, shattered, leaving behind fragments of rock. Some suggest that nearly 30 percent of the water contained initially in the asteroids would have remained in the fragmented sections of rock on Earth, even after impact.

A few hundred million years after this process-around 2.2 billion to 2.7 billion years ago-photosynthesizing bacteria evolved. They released oxygen into the atmosphere via photosynthesis and, in a few hundred million years, were able to change the composition of the atmosphere into what we have today. Our modern atmosphere is comprised of 78 percent nitrogen and 21 percent oxygen, among other gases, which enables it to support the many lives residing within it.

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October 19, 2023

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Our planet began as part of a cloud of dust and gas. It has evolved into our home, which has an abundance of rocky landscapes, an atmosphere that supports life, and oceans filled with mysteries.

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This artistic visualization shows Earth today (right) vs Earth 4.5 billion years ago (left). (Image credit: Tobias Stierli/NCCR PlanetS) [FROM THE SITE]

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"Collisions between these cosmic bodies [asteriods] likely deposited much of the water on Earth's surface. Our planet lies in what is known as the Goldilocks zone, a region surrounding a star that is close enough for liquid water to exist on a planet's surface, with water neither freezing nor evaporating. Many scientists think that being in this zone, and the presence of liquid water, plays a key role in the existence of life."

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"Collisions between these cosmic bodies [asteriods] likely deposited much of the water on Earth's surface. Our planet lies in what is known as the Goldilocks zone, a region surrounding a star that is close enough for liquid water to exist on a planet's surface, with water neither freezing nor evaporating. Many scientists think that being in this zone, and the presence of liquid water, plays a key role in the existence of life."

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(Image credit: Tobias Stierli/NCCR PlanetS)

Earth's formation remains a strange, scientific mystery.

We live on a planet in a solar system with seven other planets and have discovered thousands of exoplanets to date. But how planets like Earth form still remains a subject of great debate.

Currently, there are two leading theories on planetary formation. Scientists continue to study planets in and out of our solar system in an effort to better understand which of these theories most accurately describes how the solar system and its planets formed.

The first and most widely accepted theory is the core accretion model, which works well to explain the formation of terrestrial planets like Earth but doesn't fully account for giant planets. The second theory, called the disk instability method, may account for the creation of larger planets. These two leading theories are joined by the pebble accretion theory which helps to additionally explain how different objects might form.
Related: Earth's layers: Exploring our planet inside and out
What is the core accretion model?

Approximately 4.6 billion years ago, our solar system was just a cloud of dust and gas known as a solar nebula. Gravity collapsed the material in on itself as it began to spin, condensing the matter and forming the sun in the center of the nebula.

With the sun beginning to form, the remaining material started to clump up. Small particles drew together, bound by the force of gravity, into larger particles. The solar wind, a constant stream of charged particles that emanate from the sun's upper atmosphere, swept away lighter elements, such as hydrogen and helium.

This left behind heavy, rocky materials that formed smaller terrestrial worlds like Earth. And farther away from the sun, the solar wind had less of an impact on lighter elements which allowed these elements to coalesce into gas giants. This process created our solar system's asteroids, comets, planets and moons.

Earth's rocky core formed first, with heavy elements colliding and binding together. Dense material sank to the protoplanet's center while lighter material built up the crust. Earth's magnetic field is thought to have likely formed around this time.

Early in its evolution, Earth suffered an impact by a large body that catapulted pieces of the young planet's mantle into space. Gravity pulled many of these pieces together to form the moon, which took up orbit around its creator.

The flow of the mantle beneath Earth's crust causes plate tectonics, the movement of the large plates of rock on the planet's surface. Collisions and friction gave rise to mountains and volcanoes, which began to spew gases.

When Earth first formed it had barely any atmosphere. Its atmosphere began to form as the planet started to cool and gravity captured gases from Earth's volcanoes.

While the population of comets and asteroids passing through the inner solar system is sparse today, they were more abundant when the planets and sun were young. Collisions between these cosmic bodies likely deposited much of the water on Earth's surface.

Our planet lies in what is known as the Goldilocks zone, a region surrounding a star that is close enough for liquid water to exist on a planet's surface, with water neither freezing nor evaporating. Many scientists think that being in this zone, and the presence of liquid water, plays a key role in the existence of life.

In observing exoplanets, scientists think that this core accretion model fits as the dominant formation process.

Stars with more "metals" - a term astronomers use for all chemical elements heavier than hydrogen and helium - in their cores host more giant planets than their metal-poor cousins. According to NASA (opens in new tab), core accretion suggests that small, rocky worlds should be more common than the more massive gas giants.

One finding that has helped to strengthen core accretion's legitimacy as an explanation for planet formation is the 2005 discovery of a giant planet with a massive core orbiting the sun-like star HD 149026.

"This is a confirmation of the core accretion theory for planet formation and evidence that planets of this kind should exist in abundance," said Greg Henry in a press (opens in new tab) release (opens in new tab). Henry, an astronomer at Tennessee State University, Nashville, detected the dimming of the star.

In 2019, the European Space Agency launched the CHaracterising ExOPlanet Satellite (CHEOPS), which was designed to study exoplanets ranging in sizes from super-Earths to Neptune. With missions like this and others, scientists aim to study distant worlds to grow their understanding of how planets in different solar systems likely formed.

"In the core accretion scenario, the core of a planet must reach a critical mass before it is able to accrete gas in a runaway fashion," said the CHEOPS team (opens in new tab). "This critical mass depends upon many physical variables, among the most important of which is the rate of planetesimals accretion."

Related: How fast is Earth moving?

What is the disk instability model?

While the core accretion model works for terrestrial planets, gas giants would need to evolve rapidly to grab hold of the significant mass of lighter gases they contain. But simulations with that model have not been able to account for this rapid formation. In those simulations, the process takes several million years, which is longer than light gases were available in the early solar system.

But the core accretion model isn't the only explanation for how planets might come to be.

According to a newer theory, disk instability, clumps of dust and gas bind together early in the solar system's existence. Over time, these clumps can slowly compact into a giant planet. These planets can form faster than those that form within the core accretion explanation, sometimes in as little as a thousand years, which allows them to trap the rapidly-vanishing lighter gases. These planets also quickly reach an orbit-stabilizing mass that keeps them from death-marching into the sun.

According to exoplanetary astronomer Paul Wilson (opens in new tab), if disk instability dominates the formation of planets, it should produce a wide number of worlds at large orders. The four giant planets orbiting at significant distances around the star HD 9799 provides observational evidence for disk instability.

Fomalhaut b (opens in new tab), an exoplanet with a 2,000-year orbit around its star, could serve as an example of a world formed through disk instability, though the planet could also have been ejected due to interactions with its neighbors.

What is pebble accretion?

The disk instability model contends with the core accretion model's issue with time; specifically how quickly massive gas giants would have to grab lighter components. But another, recent model known as pebble accretion, also helps to fill in this explanatory gap.

In this model, researchers have shown how smaller, pebble-sized objects could have fused together to build giant planets up to 1000 times faster than in other explanations.

"This is the first model that we know about that you start out with a pretty simple structure for the solar nebula from which planets form, and end up with the giant-planet system that we see," Harold Levison, an astronomer at the Southwest Research Institute (SwRI) in Colorado and lead study author of a paper describing and exploring the model, told Space.com in 2015.

A few years earlier, in 2012, researchers Michiel Lambrechts and Anders Johansen from Lund University in Sweden proposed that tiny pebbles, once written off, held the key to rapidly building giant planets.

"They showed that the leftover pebbles from this formation process, which previously were thought to be unimportant, could actually be a huge solution to the planet-forming problem," Levison said.

Levison and his team built on that research to model more precisely how tiny pebbles could form planets seen in the galaxy today. In previous simulations, both large and medium-sized objects consumed their pebble-sized cousins at a relatively constant rate, but Levison's simulations suggest that the larger objects acted more like bullies, snatching away pebbles from the mid-sized masses to grow at a much faster rate.

"The larger objects now tend to scatter the smaller ones more than the smaller ones scatter them back, so the smaller ones end up getting scattered out of the pebble disk," study co-author Katherine Kretke, also from SwRI, told Space.com. "The bigger guy basically bullies the smaller one so they can eat all the pebbles themselves, and they can continue to grow up to form the cores of the giant planets."

As scientists continue to study planets inside and outside of the solar system, they will better understand how Earth and its siblings formed.
This article was updated on Jan. 5, 2022 by Space.com senior writer Chelsea Gohd
Additional resources

# data[].attributes.attachment_link.description
(Image credit: Tobias Stierli/NCCR PlanetS) Earth's formation remains a strange, scientific mystery. We live on a planet in a solar system with seven other planets and have discovered thousands of exoplanets to date. But how planets like Earth form still remains a subject of great debate.

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The Sun and the planets formed together around 4.6 billion years ago. It formed from a cloud of gas and dust called the solar nebula. A shock wave from a nearby supernova explosion likely started the formation of the solar system.

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The Sun and the planets formed together around 4.6 billion years ago. It formed from a cloud of gas and dust called the solar nebula. A shock wave from a nearby supernova explosion likely started the formation of the solar system.

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https://www.amnh.org/exhibitions/permanent/the-universe/planets/formation-of-our-solar-system

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The Precambrian takes up 7/8ths of the Earth's history and consists of 3 eons (the Hadean, the Archean, and the Proterozoic)

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The Precambrian takes up 7/8ths of the Earth's history and consists of 3 eons (the Hadean, the Archean, and the Proterozoic)

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There is little known about this time, which is why it's grouped into one unit. Some synonyms include the Eozoic, the Cryptozoic, etc.

https://en.wikipedia.org/wiki/Precambrian

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The Paleozoic is the first of three geological eras of the Phanerozoic Eon and began about 538.8 million years ago with the Cambrian Explosion and ends with the Permian Extinction (otherwise known as The Great Dying)

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The Paleozoic is the first of three geological eras of the Phanerozoic Eon and began about 538.8 million years ago with the Cambrian Explosion and ends with the Permian Extinction (otherwise known as The Great Dying)

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https://en.wikipedia.org/wiki/Paleozoic#Permian_Period

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Lasting from 252 to 66 million years ago and is composed of the Triassic, Jurassic, and Cretaceous

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Lasting from 252 to 66 million years ago and is composed of the Triassic, Jurassic, and Cretaceous

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(Also called the Age of Reptiles and famously known for its dinosaurs)

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The Proterozoic era was the foundation of life as we know it, with the formation of cells, plants, and the first animals. 

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The Proterozoic era was the foundation of life as we know it, with the formation of cells, plants, and the first animals. 

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Proterozoic Eon, the younger of the two divisions of Precambrian time, the older being the Archean Eon. The Proterozoic Eon extended from 2.5 billion to 541 million years ago and is often divided into the Paleoproterozoic (2.5 billion to 1.6 billion years ago), the Mesoproterozoic (1.6 billion to 1 billion years ago), and the Neoproterozoic (1 billion to 541 million years ago) eras. Proterozoic rocks have been identified on all the continents and often constitute important sources of metallic ores, notably of iron, gold, copper, uranium, and nickel. During the Proterozoic the atmosphere and oceans changed significantly. Proterozoic rocks contain many definite traces of primitive life-forms-the fossil remains of bacteria and blue-green algae as well as the first oxygen-dependent animals, the Ediacara fauna.

Life and oxygenation of Earth's atmosphere

Oxygen is a by-product of photosynthesis. Free oxygen in the atmosphere increased significantly as a result of biological activity during the Proterozoic. The most important period of change occurred between 2.3 billion and 1.8 billion years ago when free oxygen began to accumulate in the atmosphere. Oxygen levels fluctuated during this time, coinciding with the peak deposition period of banded-iron formations, which removed surplus oxygen from the atmosphere throughout the world. Ferrous iron (Fe ²⁺) in the oceans combined with atmospheric oxygen and, after oxidizing to Fe ₂O ₃, precipitated as the mineral hematite on the ocean floor. Continued biological activity allowed atmospheric oxygen concentrations to increase.

By the time eukaryotes became established in the environment, atmospheric oxygen pressure had risen from low values to about 10 percent of the present atmospheric level (PAL). Megascopic eukaryotes first appeared about 2.3 billion years ago and became widespread by about 1.8 billion years ago. Eukaryotes employed a form of respiration and oxidative metabolism; they had a central nucleus that could split into separate sex cells, and so for the first time a mixed and variable genetic code could be passed to younger generations.

Early organisms on Earth flourished most easily in the shallow water of continental margins. Such stable continental shelf environments, which were rare in the Archean, developed after 2.5 billion years ago, facilitating the growth of photosynthetic organisms and thus oxygen production. Evidence of the rapid rise in the oxygen content includes the first appearance on continental margins of red sandstones. Their colour is caused by the coating of quartz grains with hematite. Other evidence is provided by the occurrence of hematite-rich fossil soil beds that date to about 2.5 billion years ago. The formation of these beds is consistent with a drastic rise in oxygen pressure to 0.1 atmosphere (100 millibars) between 2.2 billion and 2.0 billion years ago.

Evidence of tiny sponge-like fossils in rocks dated to 890 million years ago suggests that the earliest metazoans (animals made up of more than one type of cell) may have emerged a little more than halfway through the Proterozoic; however, the earliest undisputed metazoans appeared much later. By 600 million to 543 million years ago the multicellular Ediacara fauna had appeared. They required oxygen for growth and were the precursors of organisms with skeletons, the appearance of which marked the end of the Proterozoic and the beginning of the Phanerozoic Eon.

Supercontinent assembly and breakup

The history of the Proterozoic Eon is dominated by the formation and breakup of supercontinents. By the time of the Archean-Proterozoic boundary about 2.5 billion years ago, many small cratons (stable interior portions of continents) dominated by island arcs had coalesced into one large landmass, or supercontinent. The breakup of this landmass is indicated by the intrusion of abundant transcontinental swarms of dolerite (a type of fine-grained igneous rock) dikes during the period of 2.4 billion to 2.2 billion years ago. These dikes resulted from the impingement of mantle plumes onto the base of the continental crust. This was the fundamental cause of the breakup of the initial supercontinent. During the period between 2.1 billion and 1.8 billion years ago, these fragments again coalesced, by collision tectonics, into a new supercontinent called Columbia. Modern plate-tectonic processes were in operation by at least 2.1 billion to 2.0 billion years ago, as shown by two of the world's oldest well-preserved ophiolites (fragments of oceanic crust), located in the Purtuniq complex in Labrador and the Jourma complex in Finland. The fragmentation of Columbia gave rise to many smaller continents that eventually assembled into another supercontinent, or a group of several large continental pieces in close proximity to one another, by about 1.0 billion years ago. This assemblage is called Rodinia.

Rodinia was intruded by many basaltic dikes after 1.0 billion years ago. These dikes contributed to the supercontinent's fragmentation and were associated with the formation of the Iapetus Ocean about 600 million years ago. Other indications of plume activity and continental breakup are vast piles of basalts and transcontinental rifts. A key example is the 1.1-billion-year-old Keweenawan Rift in North America that extends from Michigan via Lake Superior to Kansas. This rift, which is 2,000 km (about 1,200 miles) long and 160 km (100 miles) wide, contains a pile of basaltic lavas 25 km (about 16 miles) thick.

Many mountain belts formed during the Proterozoic, in particular during the intervals between 2.1 and 1.8 billion, 1.3 and 1.0 billion, and 800 and 500 million years ago, associated with the breakup of supercontinents and the subsequent collision of their fragments. New ocean basins were created by the rifting apart of the continents and were subsequently destroyed in subduction zones similar to those under modern-day Japan. The closure of these oceans enabled continental blocks to collide, giving rise to major mountain belts such as the Grenville belt in eastern North America. This belt, which is 1.3 to 1.0 billion years old and 4,000 km (about 2,500 miles) long, was very similar in origin to the Himalayan Mountains that formed in recent geological time. Other major Proterozoic mountain belts created by continental collisions include the Wopmay Orogen in northwest Canada (2.1 billion years old), the Trans-Hudson in Canada (1.8 billion years old), the Svecofennian in Finland (1.9 to 1.8 billion years old), the Ketilidian orogen (1.8 billion years old) in southwestern Greenland, and the Braziliano, Namibian, and Mozambique belts, which are all about 900 to 500 million years old. In contrast, mountain belts such as the 2.1-billion-year-old Birimian in West Africa and the 1-billion-year-old to 500-million-year-old belts of the Arabian-Nubian Shield developed by the addition of new material largely derived from Earth's mantle. Thus, they include many island arcs similar to those found in modern-day Japan as well as many ophiolite sequences.

Many Phanerozoic basins contain thick piles of sediments and lie partly to completely on top of Proterozoic mountain belts, obscuring the underlying geological relationships. Some Phanerozoic mountain belts such as the Himalayas contain blocks of Proterozoic rocks many tens of kilometres in size that have been heavily reworked by later tectonic activity.

Brian Frederick Windley The Editors of Encyclopaedia Britannica

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Proterozoic Eon | Oxygen Crisis, Animals, & Facts

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Proterozoic Eon, the younger of the two divisions of Precambrian time, extending from 2.5 billion to 541 million years ago. During the Proterozoic, the atmosphere and oceans changed significantly. Its rocks contain the fossil remains of bacteria and blue-green algae as well as the first oxygen-dependent animals.

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History of Life on Earth (That We Know Of)

Lindsay Nicole

[I can't link the video because it's unavailable on the computer] [Her whole series was my inspiration and main resource for this research]

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The evolution of organs, organ systems, biomineralization (shells and bones), etc. The formation of Arthropods, Mollusks, Echinoderms, and Chordates (That's us!)

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The evolution of organs, organ systems, biomineralization (shells and bones), etc. The formation of Arthropods, Mollusks, Echinoderms, and Chordates (That's us!)

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Cambrian explosion, the unparalleled emergence of organisms between 541 million and approximately 530 million years ago at the beginning of the Cambrian Period. The event was characterized by the appearance of many of the major phyla (between 20 and 35) that make up modern animal life. Many other phyla also evolved during this time, the great majority of which became extinct during the following 50 to 100 million years. Ironically, many of the most successful modern phyla (including the chordates, which encompass all vertebrates) are rare elements in Cambrian assemblages; phyla that include the arthropods and sponges contained the most numerically dominant taxa (taxonomic groups) during the Cambrian, and those were the taxa that became extinct.

The beginning of the Cambrian Period is marked by the evolution of hard body parts such as calcium carbonate shells. These body parts fossilize more easily than soft tissues, and thus the fossil record becomes much more complete after their appearance. Many lineages of animals independently evolved hard parts at about the same time. The reasons for this are still debated, but a leading theory is that the amount of oxygen in the atmosphere had finally reached levels that allowed large, complex animals to exist. Oxygen levels may also have facilitated the metabolic processes that produce collagen, a protein building block that is the basis for hard structures in the body.

Other major changes that occurred in the Early Cambrian (541 to 510 million years ago) include the development of animal species that burrowed into the sediments of the seafloor, rather than lying on top of it, and the evolution of the first carbonate reefs, which were built by spongelike animals called archaeocyathids.

By the Early Cambrian the bulk of the biosphere was confined to the margins of the world's oceans; no life was found on land (except possibly cyanobacteria [formerly known as blue-green algae] in moist sediment), relatively few pelagic species (biota living in the open sea) existed, and no organisms inhabited the ocean depths. Life in the shallow regions of the seafloor, however, was already well diversified. This early aquatic ecosystem included the relatively large carnivore Anomalocaris, the deposit-feeding trilobites (early arthropods) and mollusks, the suspension-feeding sponges, various scavenging arthropods, and possibly even parasites such as the onychophoran Aysheaia. Thus, it seems likely that a well-developed aquatic ecosystem was already in operation in the ocean shallows by this time.

Encyclopædia Britannica, Inc.

Following the Cambrian Period, the biosphere continued to expand relatively rapidly. In the Ordovician Period (485.4 million to 443.4 million years ago), the classic Paleozoic marine faunas-which included bryozoans, brachiopods, corals, nautiloids, and crinoids-developed. Many marine species died off near the end of the Ordovician because of environmental changes. The Silurian Period (443.4 million to 419.2 million years ago) marks a time when a rapid evolution of many suspension-feeders in the oceans occurred. As a result, pelagic predators such as nautiloids became abundant. Gnathostome fishes, the oldest craniates, became common near the end of Silurian times.

Timothy Fridtjof Flannery

Learn More in these related Britannica articles:

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Cambrian explosion, the unparalleled emergence of organisms between 541 million and approximately 530 million years ago at the beginning of the Cambrian Period. The event was characterized by the appearance of many of the major phyla (between 20 and 35) that make up modern animal life. Many other

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The Cambrian explosion - Understanding Evolution

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Around 530 million years ago, a wide variety of animals burst onto the evolutionary scene in an event known as the Cambrian explosion. In perhaps as few as 10 million years, marine animals evolved most of the basic body forms that we observe in modern groups.

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© Daniel Eskridge//Shutterstock.com

One day 66 million years ago, an asteroid the size of a mountain struck near the Yucatán Peninsula with an explosive force equivalent to 100 trillion tons of TNT. In that cataclysmic instant, the 165-million-year reign of the dinosaurs came to an end.

The asteroid theory of the dinosaurs' demise was first suggested in 1980. More than a decade later, the identification of the Chicxulub Crater in the Gulf of Mexico established where and when.

Using an impact calculator developed by geophysicists from Purdue University and Imperial College London, researchers obtained a pretty good idea of what occurred at the moment of impact and immediately after-what is known as the K-Pg (Cretaceous-Paleogene) extinction event. The asteroid struck the Earth at 40,000 miles (64,000 kilometres) per hour, creating a crater more than 115 miles across and instantly vaporizing thousands of cubic miles of rock. Any creature close enough to witness the strike was immediately incinerated, along with all trees and brush. In coastal regions, the impact triggered tsunamis as high at 1,000 feet (305 metres) as well as earthquakes more severe than anything experienced by modern humans.

But the devastation was just beginning. Minutes after the initial impact, red-hot debris began raining down, covering the ground with deadly ash and dirt. Around the impact zone, the ground was likely covered with hundreds of feet of rocky rubble. Less than an hour after impact, a horrific wind raced through the region, knocking down anything still standing.

Then ash, smoke, and debris in the atmosphere spread around the planet, turning daylight into a constant twilight that lasted for months and possibly years. Temperatures dropped, and food became increasingly scarce. Entire ecosystems collapsed. When it was all over, between 75 and 80 percent of life on Earth had perished.

It is assumed by many that the dinosaurs went extinct very quickly following the asteroid strike. But, while a great many animals died at the moment of impact and in the weeks immediately following-particularly near ground zero-global mass extinction took a while, and it affected certain species more dramatically than others. Many of the small mammals that lived among the dinosaurs, for example, were able to survive because they lived in burrows and could eat just about anything. In addition, species that lived in fresh water typically fared better than those that lived on land.

Many researchers now believe that the K-Pg extinction event came at a time when the world was in environmental flux and life was already struggling. Times were tough for the dinosaurs: their world was starting to cool, and they faced significant competition for dwindling food supplies. Ecological diversity shrank as species after species finally succumbed.
[Find out how many Tyrannosaurus rex lived on Earth before the species went extinct.]
Paleontologists admit that they still have many questions about the K-Pg extinction event and its impact on the prehistoric world. They don't know why some species died quickly while others managed to hang on or exactly how the event affected individual ecosystems around the globe-especially those far from the asteroid's impact. Research in regions such as the American West, where exposed rock offers unique evidence of that terrible event, may one day provide the answers.

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One day 66 million years ago, an asteroid the size of a mountain struck near the Yucatán Peninsula with an explosive force equivalent to 100 trillion tons of TNT. In that cataclysmic instant, the 165-million-year reign of the dinosaurs came to an end. The asteroid theory of the dinosaurs' demise was first suggested in 1980.

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Permian Period, in geologic time, the last period of the Paleozoic Era. The Permian Period began 298.9 million years ago and ended 252.2 million years ago, extending from the close of the Carboniferous Period to the outset of the Triassic Period.

At the beginning of the period, glaciation was widespread, and latitudinal climatic belts were strongly developed. Climate warmed throughout the Permian times, and, by the end of the period, hot and dry conditions were so extensive that they caused a crisis in Permian marine and terrestrial life. This dramatic climatic shift may have been partially triggered by the assembly of smaller continents into the supercontinent of Pangea. Most of Earth's land area was incorporated into Pangea, which was surrounded by an immense world ocean called Panthalassa.

Terrestrial plants broadly diversified during the Permian Period, and insects evolved rapidly as they followed the plants into new habitats. In addition, several important reptile lineages first appeared during this period, including those that eventually gave rise to mammals in the Mesozoic Era. The largest mass extinction in the Earth's history occurred during the latter part of the Permian Period. This mass extinction was so severe that only 10 percent or less of the species present during the time of maximum biodiversity in the Permian survived to the end of the period.

Permian rocks are found on all present-day continents; however, some have been displaced considerable distances from their original latitudes of deposition by tectonic transport occurring during the Mesozoic and Cenozoic eras. Some beds dated from the latest Permian ages are renowned for their fossils; strata (rock layers) in the Russian Platform contain a remarkable vertebrate faunal assemblage as well as fossil insects and plants.

The Permian Period derives its name from the Russian region of Perm, where rocks deposited during this time are particularly well developed.

The Permian environment

The Permian Period constitutes an important crossroads both in the history of the Earth's continents and in the evolution of life. The principal geographic features of the Permian world were a supercontinent, Pangea, and a huge ocean basin, Panthalassa, with its branch, the Tethys Sea (a large indentation in the tropical eastern side of Pangea).

Distribution of land

During the Early Permian (Cisuralian) Epoch, northwestern Gondwana collided with and joined southern Laurussia (a craton also known as Euramerica), resulting in the Alleghenian orogeny, occurring in the region that would become North America, and the continuance of the Hercynian orogeny, its northwestern European counterpart. The assembly of Pangea was complete by the middle of the Early Permian Epoch following its fusion to Angara (part of the Siberian craton) during the Uralian orogeny.

On the periphery of Pangea was Cathaysia, a region extending beyond the eastern edge of Angara and comprising the landmasses of both North and South China. Cathaysia lay within the western Panthalassic Ocean and at the eastern end of Tethys (sometimes called Paleo-Tethys) Sea. The Panthalassa and Tethys also contained scattered fragments of continental crust (microcontinents), basaltic volcanic island arcs, oceanic plateaus, and trenches. The island arcs featured extensive fringing limestone reefs and platforms that were subsequently displaced by seafloor spreading. These isolated landmasses were later welded onto the margins of Pangea, forming accreted terranes.

Sea level

Evidence of sea-level rise and fall is well displayed in Permian strata. Fluctuations in sea level are often associated with changes in climate. Some fluctuations with large magnitudes and short durations, such as near the base of the Permian Period, are likely the result of glaciation. For others, the possibility that Milankovitch cycles (adjustments in Earth's axis and the long-term orbital patterns of Earth about the Sun) directly affect sea level is still being investigated, though their periodic occurrence has been linked to episodes of glaciation. Global sea-level events are marked by four long lowstands (times when sea level falls below the level of the continental shelf) within the Early Permian Epoch, a major lowstand near the base of the Middle Permian (Guadalupian) Epoch, and four long lowstands within and at the top of the Middle Permian Epoch. Lowstands are also recorded at various times within the Late Permian (Lopingian) Epoch and at the terminus of the Permian Period. Extended global withdrawal of seas from continental shelves and platforms led to significant unconformities (gaps in the geologic record) and to extensive evolutionary turnovers (events of species diversification and extinction) in shallow marine faunas at the family and superfamily levels.

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Permian Period | Plants, Animals, Extinction, & Facts

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Permian Period, in geologic time, the last period of the Paleozoic Era, lasting from 298.9 million to 252.2 million years ago. The climate was warming throughout Permian times, and, by the end of the period, hot and dry conditions were so extensive that they caused a crisis in Permian marine and terrestrial life.