History of Earth

The history of the Earth is a summary of the most important events and fundamental stages in the development and evolution that has taken place on the planet Earth from its formation. It covers the leading, most current scientific theories and nearly all branches of natural science have contributed to the understanding of the main events of the Earth's past.

The age of the Earth has been determined to be 4.54 billion years (4,540,000,000 years),[1] corresponding approximately to one third of the age of the universe, and immense geological and biological changes and upheavals have occurred during that time span. The formation of the Earth and the simultaneous formation of the Sun and other bodies of the solar system resulted from the contraction of a solar nebula. The nebula developed into a protoplanetary disk with the Sun forming in its centre and the planets forming by accretion of material in orbit around it. The Earth was formed 10 million years after the beginning of contraction. Heat generated by impacts and contraction meant that it was totally molten, and a differentation took place, forming an inner core of the heaviest elements and a mantle and proto-crust of the lighter elements. Around this time the Moon was formed, possibly by a giant impact between the Earth and a smaller planet. The Earth was steadily cooling and acquired a solid crust that gave rise to the first continents. A major bombardment from icy meteorites and comets supplied the Earth with an enormous amount of water that created the oceans, while volcanic activity and water vapor created an atmosphere, devoid of oxygen. The continents floated on the molten mantle of the planet and through plate tectonics assembled into supercontinents that later broke up again in a process that has been repeated several times during the passing of the billions of years.

Chemical reactions created organic molecules which interacted to create still more elaborate and complicated structures, and finally gave rise to a molecule that was able to create copies of itself. This abilitiy initiated the evolution and led to the creation of life. At first life started in the form of one-celled organisms but later multicellularity developed, and a major evolution was the process of photosynthesis which supplied oxygen to the atmosphere and led to the creation of an ozone layer. The life forms branched into many species and became still more advanced, colonizing the land and gradually filling all the habitats of the Earth. Ice ages, volcanic eruptions, and major impacts of meteorites have caused several mass extinctions of life forms, but the remaining species have developed in new directions and have created a lasting biosphere.

About six million years ago, a split of branches of what was then the evolutionary family tree ultimately led to the modern man. The ability to walk upright, an increase in brain size, and improvement of communication skills were crucial factors. Man learned to control fire, developed agriculture and began systematic husbandry of animals and plants. This improved living conditions and societies and civilizations with religious and cultural characteristics formed. Through progress in science, writing, organization of governments, transportation and communication, man has become the dominant species on Earth and influences the environment and all other life forms. The scope of human activity and an increasing population now require mankind to apply a global perspective on major concerns and problems like protection of the environment, exploitation of natural resources, protection of wildlife, and climate change.
Hadean and Archaean

The first eon in the Earth's history is called the Archaean. It lasted until 2.5 Ga (billion years ago). The oldest rocks found on Earth date to about 4.0 Ga.[2] The timespan between the age of those oldest rocks and the formation of the Earth is sometimes seen as a separate eon, called the Hadean. Because no material from this time is preserved, little is known about Hadean times. The Earth's surface must have been under an intense bombardment of meteorites and volcanism must have been severe due to the large heat flow and geotherm. Sometimes sporadic detrital zircon crystals are found older than 4.0 Ga, and they show evidence of having been in contact with liquid water 4.3 Ga. This is proof that the planet already had oceans or seas at that time. From crater counts on other celestial bodies it is known that the intense meteorite bombardment (Late Heavy Bombardment) came to an end about 3.8 Ga.[3] At the beginning of the Archaean eon, the Earth had cooled considerably. Due to the composition of the atmosphere, life would have been impossible for most present life forms, because of the lack of oxygen and absence of an ozone layer.

Origin of the solar system

The Solar System (including the Earth) formed from a large, rotating cloud of interstellar dust and gas called the solar nebula. It was composed of hydrogen and helium produced in the Big Bang, as well as heavier elements ejected by supernovas.[4] About 4.6 Ga, the solar nebula began to contract, possibly due to the shock wave of a nearby supernova. Such a shock wave would have caused the nebula to gain angular momentum. As the cloud began to accelerate its rotation, gravity and inertia flattened it into a protoplanetary disk oriented perpendicularly to its axis of rotation. Most of the mass concentrated in the middle and began to heat up, but small perturbations due to collisions and the angular momentum of other large debris created the means by which protoplanets up to several kilometres in size began to form.

The infall of material, increase in rotational speed and the crush of gravity created an enormous amount of kinetic heat at the center. Its inability to transfer that energy away through any other process at a rate capable of relieving the build-up resulted in the disk's center heating up. Ultimately, nuclear fusion of hydrogen into helium began, and eventually, after contraction, a T Tauri star ignited to create the Sun. Meanwhile, as gravity caused matter to condense around the previously perturbed objects outside the gravitational grasp of the new sun, dust particles and the rest of the protoplanetary disk began separating into rings. Successively larger fragments collided with one another and became larger objects, ultimately becoming protoplanets.[5] These included one collection approximately 150 million kilometers from the center: Earth. The planet formed about 4.54 billion years ago (within an uncertainty of 1%),[6][7][8][9] and the planet was largely completed within 10–20 million years.[10] The solar wind of the newly formed T Tauri star cleared out most of the material in the disk that had not already condensed into larger bodies.

Computer simulations have shown that planets with distances equal to the terrestrial planets in our solar system can be created from a protoplanetary disk.[11]

[edit]
Origin of the Earth's core and first atmosphere
See also: planetary differentiation

The Proto-Earth grew by accretion, until the inner part of the protoplanet was hot enough to melt the heavy, siderophile metals. Due to their larger densities such (now liquid) metals began to sink to the Earth's center of mass. This so called iron catastrophe resulted in a separation of a primitive mantle and a (metallic) core only 10 million years after the Earth began to form. This produced the layered structure of Earth and also set up the formation of Earth's magnetic field.

During the accretion of material to the protoplanet, a cloud of gaseous silica must have surrounded the Earth, to condense afterwards as solid rocks on the surface. What was left surrounding the planet was an early atmosphere of light (atmophile) elements from the solar nebula, mainly hydrogen and helium, but the solar wind and Earth's heat would have driven off this atmosphere.

This changed when Earth was about 40% its present radius, and gravitational attraction retained an atmosphere which included water.


The giant impact

A rare characteristic of our planet is its large natural satellite, the Moon. During the Apollo program, rocks from the Moon's surface were brought back to Earth. Radiometric dating of these rocks has shown the Moon to be 4527 ± 10 million years old,[13] about 30 to 55 million years younger than other bodies in the solar system.[14] Another special feature is the relatively low density of the Moon, which must mean it does not have a large metallic core, like all other terrestrial bodies in the solar system. In fact, the Moon has a bulk composition closely resembling the Earth's mantle and crust together, without the Earth's core. This has led to the giant impact hypothesis, the idea that the Moon was formed during a giant impact of the proto-Earth with another protoplanet. The Moon formed by accretion of the material blown off the mantles of the proto-Earth and impactor.[15]

The impactor, sometimes named Theia, is thought to have been a little smaller than the current planet Mars. It could have formed by accretion of matter about 150 million kilometres from both the Sun and Earth, at their fourth or fifth Lagrangian point. Its orbit may have been stable at first, but destabilized as Earth's mass increased due to accretion of more and more matter. Theia swung back and forth relative to Earth until it finally collided with Earth an estimated 4.533 Ga.[16]

Models show that when an impactor this size struck the proto-Earth at a low angle, a lot of material from the mantles (and proto-crusts) of the proto-Earth and the impactor was ejected into space, where much of it stayed in orbit around the Earth. This material would eventually form the Moon. However, the metallic cores of the impactor would have sunk through the Earth's mantle to fuse with the Earth's core, depleting the Moon of metallic material.[17] The giant impact hypothesis thus explains the Moon's abnormal composition.[18] The ejecta in orbit around the Earth could have condensed into a single body within a couple of weeks. Under the influence of its own gravity, the ejected material became a more spherical body: the Moon.[19]

The radiometric ages show the Earth existed already for at least 10 million years before the impact, enough time to allow for differentiation of the Earth's primitive mantle and core. Then, when the impact occurred, only material from the mantle was ejected, leaving the Earth's core of heavy siderophile elements untouched.

The impact had some important consequences for the young Earth. It released a gigantic amount of energy, causing both the Earth and Moon to be completely molten. Immediately after the impact, the Earth's mantle was vigorously convecting, the surface was a large magma ocean. Due to the enormous amount of energy released, the planet's first atmosphere must have been completely blown off.[20] The impact is also thought to have changed Earth’s axis to produce the large 23.5° axial tilt that is responsible for Earth’s seasons (a simple, ideal model of the planets’ origins would have axial tilts of 0° with no recognizable seasons). It may also have sped up Earth’s rotation.

[edit]
Origin of the oceans and atmosphere

Because the Earth lacked an atmosphere immediately after the giant impact, cooling must have been fast. Within 150 million years a solid crust with a basaltis composition must have formed. The felsic continental crust of today did not yet exist. Within the Earth, further differentiation could only begin when the mantle had at least partly solidified again. Nevertheless, during the early Archaean (about 3.0 Ga) the mantle was still much hotter than today, probably around 1600°C. This means its fraction that was partially molten was still much larger than today.

Steam escaped from the crust, and more gases were released by volcanoes, completing the second atmosphere. Additional water was imported by bolide collisions, probably from asteroids ejected from the outer asteroid belt under the influence of Jupiter's gravity.

The large amount of water on Earth can never have been produced by volcanism and degassing alone. It is assumed the water was derived from impacting comets that contained ice.[21] Though most comets are today in orbits further away form the Sun than Neptune, computer simulations show they were originally far more common in the inner parts of the solar system. However, most of the water on Earth was probably derived from small impacting protoplanets, objects comparable with today's small icy moons of the outer planets.[22] Impacts of these objects can have enriched the terrestrial planets (Mercury, Venus, the Earth and Mars) with water, carbon dioxide, methane, ammonia, nitrogen and other volatiles. If all water in the Earth's oceans was derived from comets alone, a million impacting comets are required to explain the oceans. Computer simulations show this is not an unreasonable number.

As the planet cooled, clouds formed. Rain gave rise to the oceans. Recent evidence suggests the oceans may have begun forming by 4.2 Ga.[23] At the start of the Archaean eon, the Earth was already covered with oceans. The new atmosphere probably contained ammonia, methane, water vapor, carbon dioxide, and nitrogen, as well as smaller amounts of other gases. Any free oxygen would have been bound by hydrogen or minerals on the surface. Volcanic activity was intense and, without an ozone layer to hinder its entry, ultraviolet radiation flooded the surface.
 
Lithified stromatolites on the shores of Lake Thetis (Western Australia). Stromatolites are formed by colonies of single celled organisms like cyanobacteria or chlorophyta. These colonies of algae entrap sedimentary grains, thus forming the draped sedimentary layers of a stromatolite. Archaean stromatolites are the first direct fossil traces of life on Earth, even though little preserved fossilized cells have been found inside them. The Archaean and Proterozoic oceans could have been full of algal mats like these.

[edit]
The first continents

Mantle convection, the process that drives plate tectonics today, is a result of heat flow from the core to the Earth's surface. It involves the creation of rigid tectonic plates at mid-oceanic ridges. These plates are destroyed by subduction into the mantle at subduction zones. The inner Earth was warmer during the Hadean and Archaean eons, so convection in the mantle must have been faster. When a process similar to present day plate tectonics did occur, this will have gone faster too. Most geologist think that in the Hadean and Archaean subduction zones were more common, and therefore tectonic plates were smaller.

The initial crust that formed when the Earth's surface first solidified totally disappeared from a combination of this fast Hadean plate tectonics and the intense impacts of the Late Heavy Bombardment. It is however assumed that this crust must have been basaltic in composition like today's oceanic crust, because little crustal differentiation had yet taken place. The first larger pieces of continental crust, which is a product of differentiation of lighter elements during partial melting in the lower crust, appeared at the start of the Archaean, about 4.0 Ga. What is left of these first small continents are called cratons. These pieces of Archaean crust form the cores around which today's continents grew.

The oldest rocks on Earth are found in the North American craton of Canada. They are tonalites and about 4.0 Ga. They show traces of metamorphism by high temperature, but also sedimentary grains that have been rounded by erosion during transport by water, showing rivers and seas existed at the time.[24]

Cratons consist mostly of two alternating types of terranes. The first are so called greenstone belts, consisting of low grade metamorphosed sedimentary rocks. These "greenstones" are similar to the sediments today found in oceanic trenches, above subduction zones. For this reason, greenstones are sometimes seen as evidence for subduction during the Archaean. The second type are complexes of felsic magmatic rocks. These rocks are mostly tonalite, trondhjemite or granodiorite, types of rock similar in composition to granite (hence such terranes are called TTG-terranes). TTG-complexes are seen as the relicts of the first continental crust, formed by partial melting in basalt. The alternation between greenstone belts and TTG-complexes is interpreted as a tectonic situation in which small proto-continents were separated by a thorough network of subduction zones.

[edit]
Origin of life
 
The replicator in virtually all known life is deoxyribonucleic acid. DNA is far more complex than the original replicator and its replication systems are highly elaborate.
Main article: Abiogenesis

The details of the origin of life are unknown, but the broad principles have been established. There are two schools of thought about the origin of life. One suggests that organic components arrived on Earth from space (see “Panspermia”), while the other argues that they originated on Earth. Nevertheless, both schools propose similar mechanisms by which life initially arose.[25] If life arose on Earth, the timing of this event is highly speculative—perhaps it arose around 4 Ga.[26] In the energetic chemistry of early Earth, a molecule gained the ability to make copies of itself–a replicator. (More accurately, it promoted the chemical reactions which produced a copy of itself.) The replication was not always accurate: some copies were slightly different from their parent. If the change destroyed the copying ability of the molecule, the molecule did not produce any copies, and the line “died out”. On the other hand, a few rare changes might make the molecule replicate faster or better: those “strains” would become more numerous and “successful”. This created evolution. As choice raw materials (“food”) became depleted, strains which could exploit different materials, or perhaps halt the progress of other strains and steal their resources, became more numerous.[27]

The nature of the first replicator is unknown because its function was long since superseded by life’s current replicator, DNA. Several models have been proposed explaining how a replicator might have developed. Different replicators have been posited, including organic chemicals such as modern proteins, nucleic acids, phospholipids, crystals,[28] or even quantum systems.[29] There is currently no way to determine whether any of these models closely fits the origin of life on Earth. One of the older theories, and one which has been worked out in some detail, will serve as an example of how this might occur. The high energy from volcanoes, lightning, and ultraviolet radiation could help drive chemical reactions producing more complex molecules from simple compounds such as methane and ammonia.[30]:38 Among these were many of the relatively simple organic compounds that are the building blocks of life. As the amount of this “organic soup” increased, different molecules reacted with one another. Sometimes more complex molecules would result—perhaps clay provided a framework to collect and concentrate organic material.[30]:39 The presence of certain molecules could speed up a chemical reaction. All this continued for a very long time, with reactions occurring more or less at random, until by chance it produced a replicator molecule. In any case, at some point, the function of the replicator was superseded by DNA; all known life (except some viruses and prions) use DNA as their replicator, in an almost identical manner (see Genetic code).
 
A small section of a cell membrane. This modern cell membrane is far more sophisticated than the original simple phospholipid bilayer (the small blue spheres with two tails). Proteins and carbohydrates serve various functions in regulating the passage of material through the membrane and in reacting to the environment.

Modern life has its replicating material packaged inside a cellular membrane. It is easier to understand the origin of the cell membrane than the origin of the replicator, because a cell membrane is made of phospholipid molecules which often form a bilayer spontaneously when placed in water. Under certain conditions, many such spheres can be formed (see “The bubble theory”).[30]:40 The prevailing theory is that the membrane formed after the replicator, which perhaps by then was RNA (the RNA world hypothesis), along with its replicating apparatus and maybe other biomolecules. Initial protocells may have simply burst when they grew too large; the scattered contents may then have recolonized other “bubbles”. Proteins that stabilized the membrane, or that later assisted in an orderly division, would have promoted the proliferation of those cell lines. RNA is a likely candidate for an early replicator, because it can both store genetic information and catalyze reactions. At some point DNA took over the genetic storage role from RNA, and proteins known as enzymes took over the catalysis role, leaving RNA to transfer information and modulate the process. There is increasing belief that these early cells evolved in association with underwater volcanic vents known as black smokers[30]:42 or even hot, deep rocks.[31]

It is believed that of this multiplicity of protocells, only one survived. Current evidence suggests that the last universal common ancestor lived during the early Archean eon, perhaps roughly 3.5 Ga or earlier.[32][33] This “LUCA” cell is the ancestor of all life on Earth today. It was probably a prokaryote, possessing a cell membrane and probably ribosomes, but lacking a nucleus or membrane-bound organelles such as mitochondria or chloroplasts. Like all modern cells, it used DNA as its genetic code, RNA for information transfer and protein synthesis, and enzymes to catalyze reactions. Some scientists believe that instead of a single organism being the last universal common ancestor, there were populations of organisms exchanging genes in lateral gene transfer.[32]

[edit]
Proterozoic eon

[edit]
The oxygen revolution
Main article: Oxygen revolution
 
The harnessing of the sun’s energy led to several major changes in life on Earth.
 
Graph showing the estimated partial pressure of oxygen in the atmosphere (red) and concentration in the oceans and seas (green) through geologic time.
 
A banded iron formation from the 3.15 Ga Moories Group, Barberton Greenstone Belt, South Africa. Red layers represent the times when oxygen was available, gray layers were formed in anoxic circumstances.

It is likely that the initial cells were all heterotrophs, using surrounding organic molecules (including those from other cells) as raw material and an energy source.[34] As the food supply diminished, a new strategy evolved in some cells. Instead of relying on the diminishing amounts of free-existing organic molecules, these cells adopted sunlight as an energy source. Estimates vary, but by about 3 Ga[35], something similar to modern photosynthesis had probably developed. This made the sun’s energy available not only to autotrophs but also to the heterotrophs that consumed them. Photosynthesis used the plentiful carbon dioxide and water as raw materials and, with the energy of sunlight, produced energy-rich organic molecules (carbohydrates).

Moreover, oxygen was produced as a waste product of photosynthesis. At first it became bound up with limestone, iron, and other minerals. There is substantial proof of this in iron-oxide rich layers in geological strata that correspond with this time period. The reaction of the minerals with oxygen would have turned the oceans green. When most of the exposed readily-reacting minerals were oxidized, oxygen finally began to accumulate in the atmosphere. Though each cell only produced a minute amount of oxygen, the combined metabolism of many cells over a vast period of time transformed Earth’s atmosphere to its current state.[30]:50-51 Among the oldest examples of oxygen-producing lifeforms are fossil stromatolites. This was Earth’s third atmosphere.

Some of the oxygen was stimulated by incoming ultraviolet radiation to form ozone, which collected in a layer near the upper part of the atmosphere. The ozone layer absorbed, and still absorbs, a significant amount of the ultraviolet radiation that once had passed through the atmosphere. It allowed cells to colonize the surface of the ocean and ultimately the land:[36] without the ozone layer, ultraviolet radiation bombarding the surface would have caused unsustainable levels of mutation in exposed cells.

Photosynthesis had another, major, and world-changing impact. Oxygen was toxic; probably much life on Earth died out as its levels rose in what is known as the "Oxygen Catastrophe".[36] Resistant forms survived and thrived, and some developed the ability to use oxygen to enhance their metabolism and derive more energy from the same food.

[edit]
Snowball Earth and the origin of the ozone layer

An oxygen rich atmosphere had two important advantages for life. Organisms not using oxygen for their metabolism, such as anaerobe bacteria, base their metabolism on fermentation. The abundance of oxygen makes respiration possible, a much more effective energy source for life. The second advantage of an oxygen rich atmosphere is that oxygen reacts to ozone in the higher atmosphere, causing the origin of the Earth's ozone layer. The ozone layer protects the Earth's surface from ultraviolet radiation, which is harmful for life. Without the ozone layer, the development of more complex life later on would probably have been impossible.[37]

The natural evolution of the Sun made it gradually more luminous during the Archaean and Proterozoic eons. The Sun's luminocity increases 6% every billion years.[38] As a result, the Earth began to receive more heat from the Sun in the Proterozoic eon. However, the Earth did not get warmer. Instead, the geological record seems to suggest it cooled dramatically during the early Proterozoic. Glacial deposits found on all cratons show that about 2.3 Ga, the Earth underwent its first big ice age (the Makganyene ice age).[39] Some scientists suggest this and following Proterozoic ice ages were so severe that the planet was totally frozen over from the poles to the equator, a hypothesis called Snowball Earth. Not all geologists agree with this scenario and older, Archaean ice ages have been postulated, but the ice age 2.3 Ga is the first such event for which the evidence is universally accepted.

The ice age around 2.3 Ga could have been directly caused by the increased oxygen concentration in the atmosphere, which caused the decrease of methane (CH4) in the atmosphere. Methane is a strong greenhouse gas, but with oxygen it reacts to form CO2, a less effective greenhouse gas.[40] When free oxygen became available in the atmosphere, the concentration of methane could have decreased dramatically, enough to counter the effect of the increasing heat flow from the Sun.

[edit]
Proterozoic development of life
Main article: Endosymbiotic theory
 
Some of the pathways by which the various endosymbionts might have arisen.

Modern taxonomy classifies life into three domains. The time of the origin of these domains is speculative. The Bacteria domain probably first split off from the other forms of life (sometimes called Neomura), but this supposition is controversial. Soon after this, by 2 Ga,[41] the Neomura split into the Archaea and the Eukarya. Eukaryotic cells (Eukarya) are larger and more complex than prokaryotic cells (Bacteria and Archaea), and the origin of that complexity is only now coming to light.

Around this time, the first proto-mitochondrion was formed. A bacterial cell related to today’s Rickettsia[42] entered a larger prokaryotic cell. Perhaps the large cell attempted to ingest the smaller one but failed (maybe due to the evolution of prey defenses). Or, perhaps the smaller cell tried to parasitize the larger one. In any case, the smaller cell survived inside the larger cell. Using oxygen, it was able to metabolize the larger cell’s waste products and derive more energy. Some of this surplus energy was returned to the host. The smaller cell replicated inside the larger one. Soon, a stable symbiosis developed between the large cell and the smaller cells inside it. Over time, the host cell acquired some of the genes of the smaller cells, and the two kinds became dependent on each other: the larger cell could not survive without the energy produced by the smaller ones, and these in turn could not survive without the raw materials provided by the larger cell. The whole cell is now considered a single organism, and the smaller cells are classified as organelles called mitochondria.

A similar event occurred with photosynthetic cyanobacteria[43] entering larger heterotrophic cells and becoming chloroplasts.[30][44]:60-61 Probably as a result of these changes, a line of cells capable of photosynthesis split off from the other eukaryotes more than 1 billion years ago. There were probably several such inclusion events, as the figure at right suggests. Besides the well-established endosymbiotic theory of the cellular origin of mitochondria and chloroplasts, it has been suggested that cells gave rise to peroxisomes, spirochetes gave rise to cilia and flagella, and that perhaps a DNA virus gave rise to the cell nucleus,[45],[46] though none of these theories are generally accepted.[47]
File:Volvox aureus.jpg 
Volvox aureus is believed to be similar to the first multicellular plants.

Archaeans, bacteria, and eukaryotes continued to diversify and to become more sophisticated and better adapted to their environments. Each domain repeatedly split into multiple lineages, although little is known about the history of the archaea and bacteria. Around 1.1 Ga, the supercontinent Rodinia was assembling.[48] The plant, animal, and fungi lines had all split, though they still existed as solitary cells. Some of these lived in colonies, and gradually some division of labor began to take place; for instance, cells on the periphery might have started to assume different roles from those in the interior. Although the division between a colony with specialized cells and a multicellular organism is not always clear, around 1 billion years ago[49] the first multicellular plants emerged, probably green algae.[50] Possibly by around 900 million years ago[51] true multicellularity had also evolved in animals.

At first it probably somewhat resembled that of today’s sponges, where all cells were totipotent and a disrupted organism could reassemble itself.[52] As the division of labor became more complete in all lines of multicellular organisms, cells became more specialized and more dependent on each other; isolated cells would die.

[edit]
Rodinia and other supercontinents

When the theory of plate tectonics was developed around 1960, geologists began to reconstruct the movements and positions of the continents in the past. This appeared relatively easy until about 250 million years back, when all continents were united in what is called the "supercontinent" Pangaea. Before that time, reconstructions cannot rely on apparent similarities in coastlines or ages of oceanic crust, but instead solely rely on geologic observations and, more importantly, on paleomagnetic data.[53]

Throughout the history of the Earth, there have been times when the continental mass came together to form a supercontinent, followed by the break-up of the supercontinent and new continents moving apart again. This repetition of tectonic events is called a Wilson cycle. The further back in time, the scarcer and harder to interpret the data get. It is at least clear that, about 1000 to 830 million years ago, most continental mass was united in the supercontinent Rodinia.[54] It is very probable Rodinia was not the first supercontinent and a number of early Proterozoic supercontinents have been proposed. This means plate tectonic processes similar to today's must have been active during the Proterozoic.

After the break-up of Rodinia about 800 million years ago, it is possible the continents joined again around 550 million years ago. The hypothetical supercontinent is sometimes referred to as Pannotia or Vendia. The evidence for it is a phase of continental collision known as the Pan-African orogeny, which joined the continental masses of current-day Africa, South-America, Antarctica and Australia. It is very likely however, that the assemblage of continental masses was not complete, since a continent called Laurentia (roughly equal to current-day North America) had already started breaking off around 610 million years ago. It is at least certain that by the end of the Proterozoic eon, most of the continental mass lay united in a position around the south pole.[55]

[edit]
Late Proterozoic climate and life

Many scientists believe that a very severe ice age began around 770 million years ago, so severe that the surface of all the oceans completely froze (Snowball Earth). Eventually, after 20 million years, enough carbon dioxide escaped through volcanic outgassing that the resulting greenhouse effect raised global temperatures.[56] By around the same time, 750 million years ago,[57] Rodinia began to break up.

[edit]
Paleozoic era

[edit]
Colonization of land
 
For most of Earth’s history, there were no multicellular organisms on land. Parts of the surface may have vaguely resembled this view of Mars.[citation needed]

Oxygen accumulation from photosynthesis resulted in the formation of an ozone layer that absorbed much of Sun’s ultraviolet radiation, meaning unicellular organisms that reached land were less likely to die, and prokaryotes began to multiply and become better adapted to survival out of the water. Prokaryotes had probably colonized the land as early as 2.6 Ga[58] even before the origin of the eukaryotes. For a long time, the land remained barren of multicellular organisms. The supercontinent Pannotia formed around 600 Ma and then broke apart a short 50 million years later.[59] Fish, the earliest vertebrates, evolved in the oceans around 530 Ma.[60] A major extinction event occurred near the end of the Cambrian period,[61] which ended 488 Ma.[62]

Several hundred million years ago, plants (probably resembling algae) and fungi started growing at the edges of the water, and then out of it.[63] The oldest fossils of land fungi and plants date to 480–460 Ma, though molecular evidence suggests the fungi may have colonized the land as early as 1000 Ma and the plants 700 Ma.[64] Initially remaining close to the water’s edge, mutations and variations resulted in further colonization of this new environment. The timing of the first animals to leave the oceans is not precisely known: the oldest clear evidence is of arthropods on land around 450 Ma,[65] perhaps thriving and becoming better adapted due to the vast food source provided by the terrestrial plants. There is also some unconfirmed evidence that arthropods may have appeared on land as early as 530 Ma.[66]

At the end of the Ordovician period, 440 Ma, additional extinction events occurred, perhaps due to a concurrent ice age.[67] Around 380 to 375 Ma, the first tetrapods evolved from fish.[68] It is thought that perhaps fins evolved to become limbs which allowed the first tetrapods to lift their heads out of the water to breathe air. This would let them survive in oxygen-poor water or pursue small prey in shallow water.[68] They may have later ventured on land for brief periods. Eventually, some of them became so well adapted to terrestrial life that they spent their adult lives on land, although they hatched in the water and returned to lay their eggs. This was the origin of the amphibians. About 365 Ma, another period of extinction occurred, perhaps as a result of global cooling.[69] Plants evolved seeds, which dramatically accelerated their spread on land, around this time (by approximately 360 Ma).[70][71]
 
Pangaea, the most recent supercontinent, existed from 300 to 180 Ma. The outlines of the modern continents and other land masses are indicated on this map.

Some 20 million years later (340 Ma[72]), the amniotic egg evolved, which could be laid on land, giving a survival advantage to tetrapod embryos. This resulted in the divergence of amniotes from amphibians. Another 30 million years (310 Ma[73]) saw the divergence of the synapsids (including mammals) from the sauropsids (including birds and reptiles). Other groups of organisms continued to evolve and lines diverged—in fish, insects, bacteria, and so on—but less is known of the details. 300 million years ago, the most recent hypothesized supercontinent formed, called Pangaea.

The most severe extinction event to date took place 250 Ma, at the boundary of the Permian and Triassic periods; 95% of life on Earth died out,[74] possibly due to the Siberian Traps volcanic event. The discovery of the Wilkes Land crater in Antarctica may suggest a connection with the Permian-Triassic extinction, but the age of that crater is not known.[75] But life persevered, and around 230 Ma,[76] dinosaurs split off from their reptilian ancestors. An extinction event between the Triassic and Jurassic periods 200 Ma spared many of the dinosaurs,[77] and they soon became dominant among the vertebrates. Though some of the mammalian lines began to separate during this period, existing mammals were probably all small animals resembling shrews.[78]

By 180 Ma, Pangaea broke up into Laurasia and Gondwana. The boundary between avian and non-avian dinosaurs is not clear, but Archaeopteryx, traditionally considered one of the first birds, lived around 150 Ma.[79] The earliest evidence for the angiosperms evolving flowers is during the Cretaceous period, some 20 million years later (132 Ma).[80]

Competition with birds drove many pterosaurs to extinction and the dinosaurs were probably already in decline[81] when, 65 Ma, a 10-kilometre (6.2 mi) meteorite probably struck Earth just off the Yucatán Peninsula where the Chicxulub crater is today. This ejected vast quantities of particulate matter and vapor into the air that occluded sunlight, inhibiting photosynthesis. Most large animals, including the non-avian dinosaurs, became extinct,[82] marking the end of the Cretaceous period and Mesozoic era. Thereafter, in the Paleocene epoch, mammals rapidly diversified, grew larger, and became the dominant vertebrates. Perhaps a couple of million years later (around 63 Ma), the last common ancestor of primates lived.[83] By the late Eocene epoch, 34 Ma, some terrestrial mammals had returned to the oceans to become animals such as Basilosaurus which later gave rise to dolphins and whales.[84]

[edit]
Cenozoic era

[edit]
Human evolution
 
Australopithecus africanus, an early hominid.
Main article: Human evolution

A small African ape living around six million years ago was the last animal whose descendants would include both modern humans and their closest relatives, the bonobos, and chimpanzees.[85] Only two branches of its family tree have surviving descendants. Very soon after the split, for reasons that are still debated, apes in one branch developed the ability to walk upright.[86] Brain size increased rapidly, and by 2 Ma, the very first animals classified in the genus Homo had appeared.[87] Of course, the line between different species or even genera is rather arbitrary as organisms continuously change over generations. Around the same time, the other branch split into the ancestors of the common chimpanzee and the ancestors of the bonobo as evolution continued simultaneously in all life forms.[85]

The ability to control fire probably began in Homo erectus (or Homo ergaster), probably at least 790,000 years ago[88] but perhaps as early as 1.5 Ma.[89] In addition it has sometimes suggested that the use and discovery of controlled fire may even predate Homo erectus. Fire was possibly used by the early Lower Paleolithic (Oldowan) hominid Homo habilis and/or by robust australopithecines such as Paranthropus.[90]

It is more difficult to establish the origin of language; it is unclear whether Homo erectus could speak or if that capability had not begun until Homo sapiens.[91] As brain size increased, babies were born sooner, before their heads grew too large to pass through the pelvis. As a result, they exhibited more plasticity, and thus possessed an increased capacity to learn and required a longer period of dependence. Social skills became more complex, language became more advanced, and tools became more elaborate. This contributed to further cooperation and brain development.[92] Anatomically modern humans — Homo sapiens — are believed to have originated somewhere around 200,000 years ago or earlier in Africa; the oldest fossils date back to around 160,000 years ago.[93]

The first humans to show evidence of spirituality are the Neanderthals (usually classified as a separate species with no surviving descendants); they buried their dead, often apparently with food or tools.[94] However, evidence of more sophisticated beliefs, such as the early Cro-Magnon cave paintings (probably with magical or religious significance)[95] did not appear until some 32,000 years ago.[96] Cro-Magnons also left behind stone figurines such as Venus of Willendorf, probably also signifying religious belief.[95] By 11,000 years ago, Homo sapiens had reached the southern tip of South America, the last of the uninhabited continents (except for Antarctica, which remained undiscovered until 1820 AD).[97] Tool use and language continued to improve; interpersonal relationships became more complex.

[edit]
Civilization
Main article: History of the world
Further information: History of Africa, History of the Americas, History of Antarctica, and History of Eurasia
 
Vitruvian Man by Leonardo da Vinci epitomizes the advances in art and science seen during the Renaissance.

Throughout more than 90% of its history, Homo sapiens lived in small bands as nomadic hunter-gatherers.[98] As language became more complex, the ability to remember and transmit information resulted in a new sort of replicator: the meme.[99] Ideas could be rapidly exchanged and passed down the generations. Cultural evolution quickly outpaced biological evolution, and history proper began. Somewhere between 8500 and 7000 BC, humans in the Fertile Crescent in Middle East began the systematic husbandry of plants and animals: agriculture.[100] This spread to neighboring regions, and also developed independently elsewhere, until most Homo sapiens lived sedentary lives in permanent settlements as farmers. Not all societies abandoned nomadism, especially those in isolated areas of the globe poor in domesticable plant species, such as Australia.[101] However, among those civilizations that did adopt agriculture, the relative security and increased productivity provided by farming allowed the population to expand. Agriculture had a major impact; humans began to affect the environment as never before. Surplus food allowed a priestly or governing class to arise, followed by increasing division of labor. This led to Earth’s first civilization at Sumer in the Middle East, between 4000 and 3000 BC.[102] Additional civilizations quickly arose in ancient Egypt, at the Indus River valley and in China.

Starting around 3000 BC, Hinduism, one of the oldest religions still practiced today, began to take form.[103] Others soon followed. The invention of writing enabled complex societies to arise: record-keeping and libraries served as a storehouse of knowledge and increased the cultural transmission of information. Humans no longer had to spend all their time working for survival—curiosity and education drove the pursuit of knowledge and wisdom. Various disciplines, including science (in a primitive form), arose. New civilizations sprang up, traded with one another, and engaged in war for territory and resources: empires began to form. By around 500 BC, there were empires in the Middle East, Iran, India, China, and Greece, approximately on equal footing; at times one empire expanded, only to decline or be driven back later.[104]

In the fourteenth century, the Renaissance began in Italy with advances in religion, art, and science.[105] Starting around 1500, European civilization began to undergo changes leading to the scientific and industrial revolutions: that continent began to exert political and cultural dominance over human societies around the planet.[106] From 1914 to 1918 and 1939 to 1945, nations around the world were embroiled in world wars. Established following World War I, the League of Nations was a first step in establishing international institutions to resolve disputes peacefully; after its failure to prevent World War II and the subsequent end of the conflict it was replaced by the United Nations. In 1992, several European nations joined together in the European Union. As transportation and communication improved, the economies and political affairs of nations around the world have become increasingly intertwined. This globalization has often produced both discord and collaboration.

[edit]
Recent events
Main article: Modern era
See also: Modernity and Future
 
Four and a half billion years after the planet's formation, Earth’s life broke free of the biosphere. For the first time in history, Earth was viewed from space.

Change has continued at a rapid pace from the mid-1940s to today. Technological developments include nuclear weapons, computers, genetic engineering, and nanotechnology. Economic globalization spurred by advances in communication and transportation technology has influenced everyday life in many parts of the world. Cultural and institutional forms such as democracy, capitalism, and environmentalism have increased influence. Major concerns and problems such as disease, war, poverty, violent radicalism, and more recently, global warming, have risen as the world population increases.

In 1957, the Soviet Union launched the first artificial satellite into orbit and, soon afterward, Yuri Gagarin became the first human in space. Neil Armstrong, an American, was the first to set foot on another astronomical object, the Moon. Unmanned probes have been sent to all the major planets in the solar system, with some (such as Voyager) having left the solar system. The Soviet Union and the United States were the primary early leaders in space exploration in the 20th Century. Five space agencies, representing over fifteen countries,[107] have worked together to build the International Space Station. Aboard it, there has been a continuous human presence in space since 2000.[108]