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(S#) = Segment Number
(E#) = Event Number
Segment = 200 MYA after the last segment
Yellow = Time Period/Segment
Red = Era
Purple = Eon
Green = Events
White = Resources

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A picture of a solar nebula that may have formed our solar system

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The Beginning of the Solar System (~4.8 (started forming) - ~4.5(finished forming) BYA) (E#1)

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The Beginning of the Solar System (~4.8 (started forming) - ~4.5(finished forming) BYA) (E#1)

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The solar system formed about 4.5 billion years ago from a dense cloud of gas and dust. The cloud collapsed, due to the shock wave of a nearby supernova. When this dust cloud collapsed, it formed a solar nebula a spinning disk of material. In the center, gravity pulled lots of material in. Eventually, the pressure in the core was so big that hydrogen atoms began to combine and form helium, releasing a huge amount of energy. The sun was formed because of this.

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The Earth formed over 4.6 billion years ago out of a mixture of dust and gas around the sun. It grew larger due to countless collisions between dust particles, asteroids, and other growing planets, including one giant impact that threw enough rock, gas, and dust into space to form the moon. During this period of bombardment, the pounding generated enough heat to vaporize all the available water and prevent seas from forming. Most geologists now agree that this bombardment phase ended about 3.9 BYA.

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The Hadean  is a geological eon of Earth's history preceding the Archean. On Earth, the Hadean began with the planets formation about 4.6 billion years ago. Hadean rocks are very rare, largely consisting of zircons from one locality in Western Australia. Hadean geophysical models remain controversial among geologists: it appears that plate tectonics and the growth of continents may have started in the Hadean.

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The Precambrian is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the Phanerozoic Eon, which is named after Cambria, the Latinised name for Wales, in which rocks from this age were first studied. The Precambrian accounts for 88% of the Earth's geologic time.

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The Earth during the Archean was mostly a water world: there was continental crust, but much of it was under an ocean deeper than today's ocean. Except for some trace minerals, today's oldest continental crust dates back to the Archean. Much of the geological detail of the Archean has been destroyed by subsequent activity. The earliest known life started in the Archean. Life was simple throughout the Archean, mostly represented by shallow-water microbial mats called stromatolites, and the atmosphere lacked free oxygen.

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This fossilized prokaryote, about 3.5 billion years old, is an example of ancient bacteria collected in western Australia.

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The oldest fossils of organisms that have been found so far are inside rocks from western Australia that are 3.5 BYO (billion years old). These microfossils resemble certain prokaryotic bacteria that still exist today. For these prokaryotic bacteria, so complex to have evolved 3.5 billion years ago, it is a reasonable hypothesis that life originated much earlier, maybe as early as 3.9 BYA.

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Cyanobacteria found in the pacific ocean

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The only photosynthetic prokaryotes that generate O2 are called cyanobacteria. They most likely evolved over 2.7 years ago. They lived in all types of water and are unicellular.

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Oxygen Appears in Earths Atmosphere (2.7 BYA) (E#5)

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Oxygen Appears in Earths Atmosphere (2.7 BYA) (E#5)

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At the time, cyanobacteria were the only photosynthetic prokaryotes that produced O2. It used the CO2 in the atmosphere and emitted oxygen as waste into the oceans. Since they lived in the oceans, the oxygen concentration increased in the ocean.

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The well-identified events of this eon were the transition to an oxygenated atmosphere during the Paleoproterozoic; the evolution of eukaryotes; several glaciations, which produced the hypothesized Snowball Earth during the Cryogenian Period in the late Neoproterozoic Era; and the Ediacaran Period (635 to 538.8 MYA) which is characterized by the evolution of abundant soft-bodied multicellular organisms and provides us with the first obvious fossil evidence of life on earth.

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The Huronian glaciation was a period where several ice ages occurred during the deposition of the Huronian Supergroup, rather than a single continuous event as it is commonly misrepresented to be. The deposition of this group extended from 2.4 billion years ago (BYA) to 2.1 BYA. This led to the deposition of several diamictites. Most of the deposits of the Huronian are typical passive margin deposits in a marine setting. The diamictites within the Huronian are on par in thickness with Quaternary analogs. Evidence for the Huronian comes from glacial deposits identified within the stratigraphic record of the Huronian Supergroup. Within it are three distinct formations of diamictite, from the oldest to youngest, the Ramsay, Bruce, and Gowganda Formations. Although there are other glacial deposits recognized throughout the world, the Huronian is restricted to the North American Midwest. Other similar deposits are known from South Africa.

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These are some of the extremely old eukaryotic fossils found.

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Eukaryotic Life Appears (~2.1 BYA) (E#7)

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Eukaryotic Life Appears (~2.1 BYA) (E#7)

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Eukaryotic cells are generally larger and much more complex than prokaryotic cells. There is evidence that eukaryotic cells evolved from a symbiotic community of prokaryotes living within larger prokaryotic cells. The mitochondria of our cells
and those of all other eukaryotes are descendants of some of those "endosymbionts" (symbiotic cells living within larger host cells). And so are the chloroplasts of plants and algae. The oldest putative fossils of eukaryotes are 2.2 billion-year-old corkscrew-shaped organisms that look like relatively simple single-celled algae. The oldest fossils that are large
enough to be eukaryotic to the satisfaction of most researchers are only about 2.1 billion years old. However, some researchers postulate a much earlier eukaryotic origin based on certain chemical traces dating back 2. 7 billion years. This range of vintages places the earliest eukaryotes during a time when the oxygen revolution was changing Earth's environments dramatically. Chloroplasts may be part of the explanation for this temporal correlation.

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Fossilized multicellular algae from 1.2 BYA

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Appearance of Multicellular Organisms (1.2 BYA) (E#8)

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Appearance of Multicellular Organisms (1.2 BYA) (E#8)

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The origin of the more complex cells set the stage for the diversification of eukaryotic life. A great range of unicellular forms evolved, giving rise to a diversity of protists and their single-celled descendants, that continue to flourish today. But multicellular forms also evolved. Their descendant's plants, fungi, and animals. A multicellular organism, such as an animal, generally develops from a single cell. Cell division and cell differentiation help transform the single cell into a multicellular adult with many types of specialized cells. Molecular clocks date the common ancestor of multicellular eukaryotes back to 1.5 BYA. However, the oldest known fossils of multicellular eukaryotes are of relatively small algae that lived about 1.2 BYA. Larger organisms, including animals such as jellyfish and worms, do not appear in the fossil record until several hundred million years later.

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Cryogenian Ice Age (850 - 635 MYA) (E#9)

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Cryogenian Ice Age (850 - 635 MYA) (E#9)

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The Cryogenian is generally considered to be divisible into at least two major worldwide glaciations. The Sturtian glaciation persisted from 720 to 660 million years ago, and the Marinoan glaciation which ended approximately 635 MYA, at the end of the Cryogenian. The deposits of glacial tillite also occur in places that were at low latitudes during the Cryogenian, a phenomenon which led to the hypothesis of deeply frozen planetary oceans called "Snowball Earth".

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Picture of what snowball earth may have looked like from space

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The Snowball Earth hypothesis proposes that, during one or more of Earth's icehouse climates, the planet's surface became entirely or nearly entirely frozen. It is believed that this occurred sometime before 650 M.Y.A. (million years ago) during the Cryogenian ice age. Proponents of the hypothesis argue that it best explains sedimentary deposits that are generally believed to be of glacial origin at tropical paleolatitudes and other enigmatic features in the geological record. Opponents of the hypothesis contest the geological evidence for global glaciation and the geophysical feasibility of an ice - or slush-covered ocean.

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Picture of the fossil of one of the first terrestrial fungus fossils.

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The earliest terrestrial fungus fossils, or at least fungus-like fossils, have been found in South China from around 635 million years ago. The researchers who reported on these fossils suggested that these fungus-like organisms may have played a role in oxygenating Earth's atmosphere in the aftermath of the Cryogenian glaciations.

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The Great Oxygenation Event caused the ozone layer to be formed. It formed because ultraviolet light began to split oxygen molecules. The ozone later helped trap oxygen which allowed organisms to develop and evolve.

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The time span of the Phanerozoic starts with the sudden appearance of fossilized evidence of a number of animal phyla; the evolution of those phyla into diverse forms; the emergence and development of complex plants; the evolution of fish; the emergence of insects and tetrapods; and the development of modern fauna. Plant life on land appeared in the early Phanerozoic eon. During this time span, tectonic forces which move the continents had collected them into a single landmass known as Pangaea (the most recent supercontinent), which then separated into the current continental landmasses.

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The Paleozoic was a time of dramatic geological, climatic, and evolutionary change. The Cambrian witnessed the most rapid and widespread diversification of life in Earth's history, known as the Cambrian explosion, in which most modern phyla first appeared. Arthropods, molluscs, fish, amphibians, reptiles, and synapsids all evolved during the Paleozoic. Life began in the ocean but eventually transitioned onto land, and by the late Paleozoic, great forests of primitive plants covered the continents, many of which formed the coal beds of Europe and eastern North America. Towards the end of the era, large, sophisticated synapsids and diapsids were dominant and the first modern plants (conifers) appeared

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The Cambrian marked a profound change in life on Earth: prior to the Cambrian, the majority of living organisms on the whole were small, unicellular and simple (Ediacaran fauna being notable exceptions). Complex, multicellular organisms gradually became more common in the millions of years immediately preceding the Cambrian, but it was not until this period that mineralized – hence readily fossilized – organisms became common.

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Appearance of Invertebrates (541 - 525 MYA) (E#13)

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Appearance of Invertebrates (541 - 525 MYA) (E#13)

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The first invertebrates developed in the oceans. They were soft-bodied animals with a shell or carapace, such as trilobites. They appeared during the Cambrian Explosion.

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Appearance of Vertebrates (~525 - 500 MYA) (E#14)

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Appearance of Vertebrates (~525 - 500 MYA) (E#14)

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Having a spine is a major feature of vertebrates—back-boned animals. Vertebrates first appear in the fossil record about 500 million years ago. These first vertebrates looked like small fishes but didn't have the elaborate fins of modern fishes. Some of these early vertebrates (or their close relatives) include Pikaia and Haikouichthys. These creatures looked a bit like a mix between a fish and a flatworm, and they lived 525 to 500 million years ago.

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Life continued to flourish during the Ordovician as it did in the earlier Cambrian Period, although the end of the period was marked by the Ordovician–Silurian extinction events. Invertebrates, namely mollusks and arthropods, dominated the oceans, with members of the latter group probably starting their establishment on land during this time, becoming fully established by the Devonians. The first land plants are known from this period. The Great Ordovician Biodiversification Event considerably increased the diversity of life.

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Andean-Saharan Ice Age (460 - 430 MYA) (E#15)

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Andean-Saharan Ice Age (460 - 430 MYA) (E#15)

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The major glaciation during this period, which was formerly thought only to consist of the Hirnantian glaciation itself, but has now been recognized as a longer, more gradual event that began as early as the Darriwilian, is widely considered to be the leading cause of the Ordovician-Silurian extinction event. Evidence of this glaciation can be seen in places such as Arabia, North Africa, South Africa, Brazil, Peru, Bolivia, Chile, Argentina, and Wyoming. More evidence derived from isotopic data is that during the Late Ordovician, tropical ocean temperatures were about 5 °C cooler than present day; this would have been a major factor that aided in the glaciation process.

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First Appearance of Insects (~480 MYA) (E#16)

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First Appearance of Insects (~480 MYA) (E#16)

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An international study has revealed that insects evolved at the same time as the earliest land plants around 480 million years ago. The earliest fossil evidence for insects is dated at around 400 million years old, but the new study uses genetic techniques to corroborate estimates that they evolved much earlier. Insects are the largest and most diverse group of animals on the planet and played a crucial role in shaping Earth’s early ecosystems.

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First appearance of Land Plants (470 MYA) (E#17)

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First appearance of Land Plants (470 MYA) (E#17)

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The evidence of plant evolution changes dramatically in the Ordovician with the first extensive appearance of spores in the fossil record (Cambrian spores have been found, also). The first terrestrial was probably in the form of tiny plants resembling liverworts when, around the Middle Ordovician, evidence for the beginning of the terrestrialization of the land is found in the form of tetrads of spores with resistant polymers in their outer walls. These early plants did not have to conduct tissues, severely limiting their size. With spores, plants on land could have sent out large numbers of spores that could grow into adult plants when sufficient environmental moisture was present.

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One important event in this period was the initial establishment of terrestrial life in what is known as the Silurian-Devonian Terrestrial Revolution: vascular plants emerged from more primitive land plants, dikaryan fungi started expanding and diversifying along with glomeromycotan fungi,[14] and three groups of arthropods (myriapods, arachnids and hexapods) became fully terrestrialized.

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Ordovician-Silurian Mass Extinction (443 MYA) (E#18)

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Ordovician-Silurian Mass Extinction (443 MYA) (E#18)

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The Ordovician-Silurian mass extinction occurred 443 million years ago and wiped out approximately 85% of all species. Scientists think it was caused by temperatures plummeting and huge glaciers forming, which caused sea levels to drop dramatically. This was followed by a period of rapid warming. Many small marine creatures died out.

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The first significant adaptive radiation of life on dry land occurred during the Devonian. Free-sporing vascular plants began to spread across dry land, forming extensive forests which covered the continents. By the middle of the Devonian, several groups of plants had evolved leaves and true roots, and by the end of the period the first seed-bearing plants appeared. The arthropod groups of myriapods, arachnids and hexapods also became well-established early in this period, after starting their expansion to land at least from the Ordovician period.

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Restoration of Eusthenopteron, a fully aquatic lobe-finned fish

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The first major groups of amphibians developed in the Devonian period, around 370 million years ago, from lobe-finned fish which were similar to the modern coelacanth and lungfish. These ancient lobe-finned fish had evolved multi-jointed leg-like fins with digits that enabled them to crawl along the sea bottom. Some fish had developed primitive lungs that help them breathe air when the stagnant pools of the Devonian swamps were low in oxygen. They could also use their strong fins to hoist themselves out of the water and onto dry land if circumstances so required. Eventually, their bony fins would evolve into limbs and they would become the ancestors of all tetrapods, including modern amphibians, reptiles, birds, and mammals. Despite being able to crawl on land, many of these prehistoric tetrapodomorph fish still spent most of their time in the water. They had started to develop lungs, but still breathed predominantly with gills

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A fossil of one of the first tetrapods, Ichthyostega.

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The first animals that walked on land were called tetrapods. It is believed that the first tetrapods walked the parts of our planet where Scotland is situated today. Fossils of four-legged animals were found near Chirnside in Scotland, in a place called Willie’s Hole. These remains were believed to be from 360 million years ago, the time when animals with backbones started making the transition from sea to land.

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It was the second major icehouse period of the Phanerozoic. The tectonic assembly of the continents of Euramerica and Gondwana into Pangaea, in the Hercynian-Alleghany Orogeny, made a major continental land mass within the Antarctic region, and the closure of the Rheic Ocean and Iapetus Ocean saw disruption of warm-water currents in the Panthalassa Ocean and Paleotethys Sea and an increase in carbon sequestration via silicate weathering, which led to progressive cooling of summers, and the snowfields accumulating in winters, which caused mountainous alpine glaciers to grow, and then spread out of highland areas. That made continental glaciers, which spread to cover much of Gondwana.

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Devonian Mass Extinction (374 MYA) (E#22)

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Devonian Mass Extinction (374 MYA) (E#22)

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The Devonian mass extinction event took place 374 million years ago and killed about three-quarters of the world's species, most of which were marine invertebrates that lived at the bottom of the sea. This was a period of many environmental changes, including global warming and cooling, a rise and fall of sea levels and a reduction in oxygen and carbon dioxide in the atmosphere. We don't know exactly what triggered the extinction event.

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Terrestrial animal life was well established by the Carboniferous Period. Tetrapods (four-limbed vertebrates), which had originated from lobe-finned fish during the preceding Devonian, became pentadactylous in and diversified during the Carboniferous, including early amphibian lineages such as temnospondyls, with the first appearance of amniotes, including synapsids (the group to which modern mammals belong) and reptiles during the late Carboniferous. The period is sometimes called the Age of Amphibians, during which amphibians became dominant land vertebrates and diversified into many forms including lizard-like, snake-like, and crocodile-like.

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Reptiles first arose from earlier tetrapods in the swamps of the middle Carboniferous. Increasing evolutionary pressure and the vast untouched niches of the land powered the evolutionary changes in amphibians to gradually become more and more land-based. Environmental selection propelled the development of certain traits, such as a stronger skeletal structure, muscles, and more protective coating (scales) became more favorable; the basic foundation of reptiles were founded. The evolution of lungs and legs are the main transitional steps towards reptiles, but the development of hard-shelled external eggs replacing the amphibious water bound eggs is the defining feature of the class Reptilia and is what allowed these amphibians to fully leave water. Another major difference from amphibians is the increased brain size, more specifically, the enlarged cerebrum and cerebellum.

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The Permian witnessed the diversification of the two groups of amniotes, the synapsids and the sauropsids (reptiles). The world at the time was dominated by the supercontinent Pangaea, which had formed due to the collision of Euramerica and Gondwana during the Carboniferous. Pangaea was surrounded by the superocean Panthalassa. The Carboniferous rainforest collapse left behind vast regions of desert within the continental interior. Amniotes, which could better cope with these drier conditions, rose to dominance in place of their amphibian ancestors.

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The era began in the wake of the Permian–Triassic extinction event, the largest well-documented mass extinction in Earth's history, and ended with the Cretaceous–Paleogene extinction event, another mass extinction whose victims included the non-avian dinosaurs, pterosaurs, mosasaurs, and plesiosaurs. The Mesozoic was a time of significant tectonic, climatic, and evolutionary activity. The era witnessed the gradual rifting of the supercontinent Pangaea into separate landmasses that would move into their current positions during the next era. The climate of the Mesozoic was varied, alternating between warming and cooling periods. Overall, however, the Earth was hotter than it is today.

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The Triassic began in the wake of the Permian–Triassic extinction event, which left the Earth's biosphere impoverished; it was well into the middle of the Triassic before life recovered its former diversity. Three categories of organisms can be distinguished in the Triassic record: survivors from the extinction event, new groups that flourished briefly, and other new groups that went on to dominate the Mesozoic Era. Reptiles, especially archosaurs, were the chief terrestrial vertebrates during this time. A specialized subgroup of archosaurs, called dinosaurs, first appeared in the Late Triassic but did not become dominant until the succeeding Jurassic Period. Archosaurs that became dominant in this period were primarily pseudosuchians, ancestors of modern crocodilians, while some archosaurs specialized in flight, the first time among vertebrates, becoming the pterosaurs.

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Permian Mass Extinction (~250 MYA) (E#24)

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Permian Mass Extinction (~250 MYA) (E#24)

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The Permian mass extinction, which happened 250 million years ago, was the largest and most devastating event of the five. Also known as the Great Dying, it eradicated more than 95% of all species, including most of the vertebrates which had begun to evolve by this time. Some scientists think Earth was hit by a large asteroid which filled the air with dust particles that blocked out the Sun and caused acid rain. Others think there was a large volcanic explosion that increased carbon dioxide and made the oceans toxic.

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Fossil structure of one of the earliest dinosaurs, Eorapter and Herrerasaurus

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When dinosaurs appeared, they were not the dominant terrestrial animals. The terrestrial habitats were occupied by various types of archosauromorphs and therapsids, like cynodonts and rhynchosaurs. Their main competitors were the pseudosuchians, such as aetosaurs, ornithosuchids and rauisuchians, which were more successful than the dinosaurs. Most of these other animals became extinct in the Triassic, in one of two events. First, at about 215 million years ago, a variety of basal archosauromorphs, including the protorosaurs, became extinct. This was followed by the Triassic–Jurassic extinction event (about 201 million years ago), that saw the end of most of the other groups of early archosaurs, like aetosaurs, ornithosuchids, phytosaurs, and rauisuchians. Rhynchosaurs and dicynodonts survived (at least in some areas) at least as late as early –mid Norian and late Norian or earliest Rhaetian stages, respectively, and the exact date of their extinction is uncertain.

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Picture of one of the earliest mammals, Morganucodon.

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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.

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The start of the Jurassic was marked by the major Triassic–Jurassic extinction event, associated with the eruption of the Central Atlantic Magmatic Province. The beginning of the Toarcian Stage started around 183 million years ago and is marked by an extinction event associated with widespread oceanic anoxia, ocean acidification, and elevated temperatures likely caused by the eruption of the Karoo-Ferrar large igneous provinces. The end of the Jurassic, however, has no clear boundary with the Cretaceous and is the only boundary between geological periods to remain formally undefined.

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Triassic Mass Extinction (~200 MYA) (E#28)

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Triassic Mass Extinction (~200 MYA) (E#28)

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The Triassic mass extinction event took place 200 million years ago, eliminating about 80% of Earth's species, including many types of dinosaurs. This was probably caused by colossal geological activity that increased carbon dioxide levels and global temperatures, as well as ocean acidification.

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The pollen bearing organs of the early "flower" Crossotheca

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The main function of a flower is reproduction, which, before the evolution of the flower and angiosperms, was the job of microsporophylls and megasporophylls. A flower can be considered a powerful evolutionary innovation because its presence allowed the plant world to access new means and mechanisms for reproduction. The flowering plants have long been assumed to have evolved from within the gymnosperms; according to the traditional morphological view, they are closely allied to the Gnetales. However, as noted above, recent molecular evidence is at odds with this hypothesis, and further suggests that Gnetales are more closely related to some gymnosperm groups than angiosperms.

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This is the fossil of the dinosaur that birds evolved from and may be a bird itself, Archaeopteryx.

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There is significant evidence that birds emerged within theropod dinosaurs, specifically, that birds are members of Maniraptora, a group of theropods that includes dromaeosaurs and oviraptorids, among others. As more non-avian theropods that are closely related to birds are discovered, the formerly clear distinction between non-birds and birds becomes less so.

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The Cretaceous was a period with a relatively warm climate, resulting in high eustatic sea levels that created numerous shallow inland seas. These oceans and seas were populated with now-extinct marine reptiles, ammonites, and rudists, while dinosaurs continued to dominate on land. The world was ice free, and forests extended to the poles. During this time, new groups of mammals and birds appeared. During the Early Cretaceous, flowering plants appeared and began to rapidly diversify, becoming the dominant group of plants across the Earth by the end of the Cretaceous, coincident with the decline and extinction of previously widespread gymnosperm groups.

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Cretaceous Mass Extinction (66 MYA) (E#30)

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Cretaceous Mass Extinction (66 MYA) (E#30)

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The Cretaceous mass extinction event occurred 66 million years ago, killing 78% of all species, including the remaining non-avian dinosaurs. This was most likely caused by an asteroid hitting the Earth in what is now Mexico, potentially compounded by ongoing flood volcanism in what is now India.

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The Cenozoic is also known as the Age of Mammals because the terrestrial animals that dominated both hemispheres were mammals – the eutherians (placentals) in the northern hemisphere and the metatherians (marsupials, now mainly restricted to Australia) in the southern hemisphere. The extinction of many groups allowed mammals and birds to greatly diversify so that large mammals and birds dominated life on Earth. The continents also moved into their current positions during this era.

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This period consists of the Paleocene, Eocene, and Oligocene epochs. The end of the Paleocene (56 Mya) was marked by the Paleocene–Eocene Thermal Maximum, one of the most significant periods of global change during the Cenozoic, which upset oceanic and atmospheric circulation and led to the extinction of numerous deep-sea benthic foraminifera and on land, a major turnover in mammals. The term 'Paleogene System' is applied to the rocks deposited during the 'Paleogene Period'.

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During this period, mammals and birds continued to evolve into modern forms, while other groups of life remained relatively unchanged. Some continental movements took place, the most significant event being the connection of North and South America at the Isthmus of Panama, late in the Pliocene. This cut off the warm ocean currents from the Pacific to the Atlantic Ocean, leaving only the Gulf Stream to transfer heat to the Arctic Ocean.

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Humans are primates. Physical and genetic similarities show that the modern human species, Homo sapiens, has a very close relationship to another group of primate species, the apes. Humans and the great apes (large apes) of Africa -- chimpanzees (including bonobos, or so-called “pygmy chimpanzees”) and gorillas -- share a common ancestor that lived between 8 and 6 million years ago. Humans first evolved in Africa, and much of human evolution occurred on that continent. The fossils of early humans who lived between 6 and 2 million years ago come entirely from Africa. Most scientists currently recognize some 15 to 20 different species of early humans. Scientists do not all agree, however, about how these species are related or which ones simply died out. Many early human species -- certainly the majority of them – left no living descendants. Scientists also debate over how to identify and classify particular species of early humans, and about what factors influenced the evolution and extinction of each species.

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The major geographical changes during this time period included the emergence of the Strait of Bosphorus and Skagerrak during glacial epochs, which respectively turned the Black Sea and Baltic Sea into fresh water lakes, followed by their flooding (and return to salt water) by rising sea level; the periodic filling of the English Channel, forming a land bridge between Britain and the European mainland; the periodic closing of the Bering Strait, forming the land bridge between Asia and North America; and the periodic flash flooding of Scablands of the American Northwest by glacial water. The current extent of Hudson Bay, the Great Lakes and other major lakes of North America are a consequence of the Canadian Shield's readjustment since the last ice age; different shorelines have existed over the course of Quaternary time

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Quarternary Ice Age (2.6 MYA - Today) (E#32)

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Quarternary Ice Age (2.6 MYA - Today) (E#32)

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Geologists describe this entire period up to the present as an "ice age", in popular culture this term usually refers to the most recent glacial period, or to the Pleistocene epoch in general. Since Earth still has polar ice sheets, geologists consider the Quaternary glaciation to be ongoing, though currently in an interglacial period. During the Quaternary glaciation, ice sheets appeared, expanding during glacial periods and contracting during interglacial periods. Since the end of the last glacial period, only the Antarctic and Greenland ice sheets have survived, with other sheets formed during glacial periods, such as the Laurentide Ice Sheet, completely melted. The major effects of the Quaternary glaciation have been the continental erosion of land and the deposition of material; the modification of river systems; the creation of millions of lakes, including the development of pluvial lakes far from the ice margins; changes in sea level; the isostatic adjustment of the Earth's crust; flooding; and abnormal winds.