Future of the Earth

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Conjectured illustration of the scorched Earth after the Sun has entered the red giant phase, 7 billion years from now.^[1]

The biological and geological future of the Earth can be extrapolated based upon the estimated effects of several long-term influences. These include the chemistry at the Earth's surface, the rate of cooling of the planet's interior, the gravitational interactions with other objects in the Solar System, and a steady increase in the Sun's luminosity. An uncertain factor in this extrapolation is the ongoing influence of technology introduced by humans, such as geoengineering,^[2] which could cause significant changes to the planet.^[3][4] The current biotic crisis^[5] is being caused by technology^[6] and the effects may last for up to five million years.^[7] In turn, technology may result in the extinction of humanity, leaving the planet to gradually return to a slower evolutionary pace resulting solely from long-term natural processes.^[8][9]

Over time intervals of hundreds of millions of years, random celestial events pose a global risk to the biosphere, which can result in mass extinctions. These include impacts by comets or asteroids with diameters of 5–10 km (3.1–6.2 mi) or more, and the possibility of a massive stellar explosion, called a supernova, within a 100-light-year radius of the Sun, called a Near-Earth supernova. Other large-scale geological events are more predictable. If the long-term effects of global warming are disregarded, Milankovitch theory predicts that the planet will continue to undergo glacial periods at least until the quaternary glaciation comes to an end. These periods are caused by eccentricity, axial tilt, and precession of the Earth's orbit.^[10] As part of the ongoing supercontinent cycle, plate tectonics will probably result in a supercontinent in 250–350 million years. Some time in the next 1.5–4.5 billion years, the axial tilt of the Earth may begin to undergo chaotic variations, with changes in the axial tilt of up to 90°.

During the next four billion years, the luminosity of the Sun will steadily increase, resulting in a rise in the solar radiation reaching the Earth. This will cause a higher rate of weathering of silicate minerals, which will cause a decrease in the level of carbon dioxide in the atmosphere. In about 600 million years, the level of CO
2 will fall below the level needed to sustain C₃ carbon fixation photosynthesis used by trees. Some plants use the C₄ carbon fixation method, allowing them to persist at CO
2 concentrations as low as 10 parts per million. However, the long-term trend is for plant life to die off altogether. The extinction of plants will be the demise of almost all animal life, since plants are the base of the food chain on Earth.^{[citation needed]}

In about 1.1 billion years, the solar luminosity will be 10% higher than at present. This will cause the atmosphere to become a "moist greenhouse", resulting in a runaway evaporation of the oceans. As a likely consequence, plate tectonics will come to an end.^[11] Following this event, the planet's magnetic dynamo may come to an end, causing the magnetosphere to decay and leading to an accelerated loss of volatiles from the outer atmosphere. Four billion years from now, the increase in the Earth's surface temperature will cause a runaway greenhouse effect. By that point, most if not all the life on the surface will be extinct.^[12][13] The most probable fate of the planet is absorption by the Sun in about 7.5 billion years, after the star has entered the red giant phase and expanded to cross the planet's current orbit.

Human influence

Humans now play a key role in the biosphere, with the large human population dominating many of Earth's ecosystems.^[3] This has resulted in a widespread, ongoing extinction of other species during the present geological epoch, now known as the Holocene extinction. The large-scale loss of species caused by human influence since the 1950s has been called a biotic crisis, with an estimated 10% of the total species lost as of 2007.^[6] At current rates, about 30% of species are at risk of extinction in the next hundred years.^[14] The Holocene extinction event is the result of habitat destruction, the widespread distribution of invasive species, hunting, and climate change.^[15][16] In the present day, human activity has had a significant impact on the surface of the planet. More than a third of the land surface has been modified by human actions, and humans use about 20% of global primary production.^[4] The concentration of carbon dioxide in the atmosphere has increased by close to 30% since the start of the Industrial Revolution.^[3]
The consequences of a persistent biotic crisis have been predicted to last for at least five million years.^[7] It could result in a decline in biodiversity and homogenization of biotas, accompanied by a proliferation of species that are opportunistic, such as pests and weeds. Novel species may also emerge; in particular taxa that prosper in human-dominated ecosystems may rapidly diversify into many new species. Microbes are likely to benefit from the increase in nutrient-enriched environmental niches. However, no new species of existing large vertebrates are likely to arise and food chains will probably be shortened.^[5][17]

There are multiple scenarios for known risks that can have a global impact on the planet. From the perspective of humanity, these can be subdivided into survivable risks and terminal risks. Risks that humanity pose to itself include the misuse of nanotechnology, a nuclear holocaust, warfare with a programmed superintelligence, a genetically engineered disease, or perhaps a disaster caused by a physics experiment. Similarly, several natural events may pose a doomsday threat, including a highly virulent disease, the impact of an asteroid or comet, runaway greenhouse effect, and resource depletion. There may also be the possibility of an infestation by an extraterrestrial lifeform.^[18] The actual odds of these scenarios are difficult if not impossible to deduce.^[8][9]

Should the human race become extinct, then the various features assembled by humanity will begin to decay. The largest structures have an estimated decay half-life of about 1,000 years. The last surviving structures would most likely be open pit mines, large landfills, major highways, wide canal cuts, and earth-fill flank dams. A few massive stone monuments like the pyramids at the Giza Necropolis or the sculptures at Mount Rushmore may still survive in some form after a million years.^[9]

Random events

The Barringer Meteorite Crater in Flagstaff, Arizona, showing evidence of the impact of celestial objects upon the Earth

As the Sun orbits the Milky Way, randomly moving stars may approach close enough to have a disruptive influence on the Solar System.^[19] A close stellar encounter may cause a significant reduction in the perihelion distances of comets in the Oort cloud—a spherical region of icy bodies orbiting within half a light year of the Sun.^[20] Such an encounter can trigger a 40-fold increase in the number of comets reaching the inner Solar System. Impacts from these comets can trigger a mass extinction of life on Earth. These disruptive encounters occur at an average of once every 45 million years.^[21] The mean time for the Sun to collide with another star in the solar neighborhood is approximately 3 × 10¹³ years, which is much longer than the estimated age of the Milky Way galaxy, at 1–2 × 10¹⁰ years. This can be taken as an indication of the low likelihood of such an event occurring during the lifetime of the Earth.^[22]

The energy release from the impact of an asteroid or comet with a diameter of 5–10 km (3.1–6.2 mi) or larger is sufficient to create a global environmental disaster and cause a statistically significant increase in the number of species extinctions. Among the deleterious effects resulting from a major impact event is a cloud of fine dust ejecta blanketing the planet, which lowers land temperatures by about 15 °C (27 °F) within a week and halts photosynthesis for several months. The mean time between major impacts is estimated to be at least 100 million years. During the last 540 million years, simulations demonstrated that such an impact rate is sufficient to cause 5–6 mass extinctions and 20–30 lower severity events. This matches the geologic record of significant extinctions during the Phanerozoic Eon. Such events can be expected to continue into the future.^[23]

A supernova is a cataclysmic explosion of a star. Within the Milky Way galaxy, supernova explosions occur on average once every 40 years.^[24] During the history of the Earth, multiple such events have likely occurred within a distance of 100 light years. Explosions inside this distance can contaminate the planet with radioisotopes and possibly impact the biosphere.^[25] Gamma rays emitted by a supernova react with nitrogen in the atmosphere, producing nitrous oxides. These molecules cause a depletion of the ozone layer that protects the surface from ultraviolet radiation from the Sun. An increase in UV-B radiation of only 10–30% is sufficient to cause a significant impact to life; particularly to the phytoplankton that form the base of the oceanic food chain. A supernova explosion at a distance of 26 light years will reduce the ozone column density by half. On average, a supernova explosion occurs within 32 light years once every few hundred million years, resulting in a depletion of the ozone layer lasting several centuries.^[26] Over the next two billion years, there will be about 20 supernova explosions and one gamma ray burst that will have a significant impact on the planet's biosphere.^[27]

The incremental effect of gravitational perturbations between the planets causes the inner Solar System as a whole to behave chaotically over long time periods. This does not significantly affect the stability of the Solar System over intervals of a few millions years or less, but over billions of years the orbits of the planets become unpredictable. Computer simulations of the Solar System's evolution over the next five billion years suggest that there is a small (less than 1%) chance that a collision could occur between Earth and either Mercury, Venus, or Mars.^[28][29] During the same interval, the odds that the Earth will be scattered out of the Solar System by a passing star are on the order of one part in 10⁵. In such a scenario, the oceans would freeze solid within several million years, leaving only a few pockets of liquid water about 14 km (8.7 mi) underground. There is a remote chance that the Earth will instead be captured by a passing binary star system, allowing the planet's biosphere to remain intact. The odds of this happening are about one chance in three million.^[30]

Orbit and rotation

The gravitational perturbations of the other planets in the Solar System combine to modify the orbit of the Earth and the orientation of its spin axis. These changes can influence the planetary climate.^{[10][31][32][33]}

Glaciation

Historically, there have been cyclical ice ages in which glacial sheets periodically covered the higher latitudes of the continents. Ice ages may occur because of changes in ocean circulation and continentality induced by plate tectonics.^[34] The Milankovitch theory predicts that glacial periods occur during ice ages because of astronomical factors in combination with climate feedback mechanisms. The primary astronomical drivers are a higher than normal orbital eccentricity, a low axial tilt (or obliquity), and the alignment of summer solstice with the aphelion.^[10] Each of these effects occur cyclically. For example, the eccentricity changes over time cycles of about 100,000 and 400,000 years, with the value ranging from less than 0.01 up to 0.05.^[35][36] This is equivalent to a change of the semiminor axis of the planet's orbit from 99.95% of the semimajor axis to 99.88%, respectively.^[37]

The Earth is passing through an ice age known as the quaternary glaciation, and is presently in the Holocene interglacial period. This period would normally be expected to end in about 25,000 years.^[33] However, the increased rate of carbon dioxide release into the atmosphere by humans may delay the onset of the next glacial period until at least 50,000–130,000 years from now. On the other hand, a global warming period of finite duration (based on the assumption that fossil fuel use will cease by the year 2200) will probably only impact the glacial period for about 5,000 years. Thus, a brief period of global warming induced through a few centuries worth of greenhouse gas emission would only have a limited impact in the long term.^[10]

Obliquity

The rotational offset of the tidal bulge exerts a net torque on the Moon, boosting it while slowing the Earth's rotation. This image is not to scale.

The tidal acceleration of the Moon slows the rotation rate of the Earth and increases the Earth-Moon distance. Friction effects—between the core and mantle and between the atmosphere and surface—can dissipate the Earth's rotational energy. These combined effects are expected to increase the length of the day by more than 1.5 hours over the next 250 million years, and to increase the obliquity by about a half degree. The distance to the Moon will increase by about 1.5 Earth radii during the same period.^[38]

Based on computer models, the presence of the Moon appears to stabilize the obliquity of the Earth, which may help the planet to avoid dramatic climate changes.^[39] This stability is achieved because the Moon increases the precession rate of the Earth's spin axis, thereby avoiding resonances between the precession of the spin and precession frequencies of the ascending node of the planet's orbit.^[40] (That is, the precession motion of the ecliptic.) However, as the semimajor axis of the Moon's orbit continues to increase in the future, this stabilizing effect will diminish. At some point perturbation effects will probably cause chaotic variations in the obliquity of the Earth, and the axial tilt may change by angles as high as 90° from the plane of the orbit. This is expected to occur within about 1.5–4.5 billion years, although the exact time is unknown.^[41]

A high obliquity would probably result in dramatic changes in the climate and may destroy the planet's habitability.^[32] When the axial tilt of the Earth reaches 54°, the equator will receive less radiation from the Sun than the poles. The planet could remain at an obliquity of 60° to 90° for periods as long as 10 million years.^[42]

Geodynamics

Pangaea was the last supercontinent to form before the present.

Tectonics-based events will continue to occur well into the future and the surface will be steadily reshaped by tectonic uplift, extrusions, and erosion. Mount Vesuvius can be expected to erupt about 40 times over the next 1,000 years. During the same period, about five to seven earthquakes of magnitude 8 or greater should occur along the San Andreas Fault, while about 50 magnitude 9 events may be expected worldwide. Mauna Loa should experience about 200 eruptions over the next 1,000 years, and the Old Faithful Geyser will likely cease to operate. The Niagara Falls will continue to retreat upstream, reaching Buffalo in about 30,000–50,000 years.^[9]

In 10,000 years, the post-glacial rebound of the Baltic Sea will have reduced the depth by about 90 m (300 ft). The Hudson Bay will decrease in depth by 100 m over the same period.^[29] After 100,000 years, the island of Hawaii will have shifted about 9 km (5.6 mi) to the northwest. The planet may be entering another glacial period by this time.^[9]

Continental drift

The theory of plate tectonics demonstrates that the continents of the Earth are moving across the surface at the rate of a few centimeters per year. This is expected to continue, causing the plates to relocate and collide. Continental drift is facilitated by two factors: the energy generation within the planet and the presence of a hydrosphere. With the loss of either of these, continental drift will come to a halt.^[43] The production of heat through radiogenic processes is sufficient to maintain mantle convection and plate subduction for at least the next 1.1 billion years.^[44]

At present, the continents of North and South America are moving westward from Africa and Europe. Researchers have produced several scenarios about how this will continue in the future.^[45] These geodynamic models can be distinguished by the subduction flux, whereby the oceanic crust moves under a continent. In the introversion model, the younger, interior, Atlantic ocean becomes preferentially subducted and the current migration of North and South America is reversed. In the extroversion model, the older, exterior, Pacific ocean remains preferentially subducted and North and South America migrate toward eastern Asia.^[46][47]

As the understanding of geodynamics improves, these models will be subject to revision. In 2008, for example, a computer simulation was used to predict that a reorganization of the mantle convection will occur over the next 100 million years, causing a supercontinent composed of Africa, Eurasia, Australia, Antarctica and South America to form around Antarctica.^[48]

Regardless of the outcome of the continental migration, the continued subduction process causes water to be transported to the mantle. After a billion years from the present, a geophysical model gives an estimate that 27% of the current ocean mass will have been subducted. If this process were to continue unmodified into the future, the subduction and release would reach a point of stability after 65% of the current ocean mass has been subducted.^[49]

Introversion

A rough approximation of Pangaea Ultima, one of the three models for a future supercontinent.

Christopher Scotese and his colleagues have mapped out the predicted motions several hundred million years into the future as part of the Paleomap Project.^[45] In their scenario, 50 million years from now the Mediterranean sea may vanish and the collision between Europe and Africa will create a long mountain range extending to the current location of the Persian Gulf. Australia will merge with Indonesia, and Baja California will slide northward along the coast. New subduction zones may appear off the eastern coast of North and South America, and mountain chains will form along those coastlines. To the south, the migration of Antarctica to the north will cause all of its ice sheets to melt. This, along with the melting of the Greenland ice sheets, will raise the average ocean level by 90 m (300 ft). The inland flooding of the continents will result in climate changes.^[45]

As this scenario continues, by 100 million years from the present the continental spreading will have reached its maximum extent and the continents will then begin to coalesce. In 250 million years, North America will collide with Africa while South America will wrap around the southern tip of Africa. The result will be the formation of a new supercontinent (sometimes called Pangaea Ultima), with the Pacific Ocean stretching across half the planet. The continent of Antarctica will reverse direction and return to the South Pole, building up a new ice cap.^[50]

Extroversion

The first scientist to extrapolate the current motions of the continents was Canadian geologist Paul F. Hoffman of Harvard University. In 1992, Hoffman predicted that the continents of North and South America would continue to advance across the Pacific Ocean, pivoting about Siberia until they begin to merge with Asia. He dubbed the resulting supercontinent, Amasia.^[51][52] Later, in the 1990s, Roy Livermore calculated a similar scenario. He predicted that Antarctica would start to migrate northward, and east Africa and Madagascar would move across the Indian Ocean to collide with Asia.^[53]

In an extroversion model, the closure of the Pacific Ocean would be complete in about 350 million years.^[54] This marks the completion of the current supercontinent cycle, wherein the continents split apart and then rejoin each other about every 400–500 million years.^[55] Once the supercontinent is built, plate tectonics may enter a period of inactivity as the rate of subduction drops by an order of magnitude. This period of stability could cause an increase in the mantle temperature at the rate of 30–100 K (30–100 °C) every 100 million years, which is the minimum lifetime of past supercontinents. As a consequence, volcanic activity may increase.^[47][54]

Supercontinent

The formation of a supercontinent can dramatically affect the environment. The collision of plates will result in mountain building, thereby shifting weather patterns. Sea levels may fall because of increased glaciation.^[56] The rate of surface weathering can rise, resulting in an increase in the rate that organic material is buried. Supercontinents can cause a drop in global temperatures and an increase in atmospheric oxygen. This, in turn, can affect the climate, further lowering temperatures.
All of these changes can result in more rapid biological evolution as new niches emerge.^[57]
The formation of a supercontinent insulates the mantle. The flow of heat will be concentrated, resulting in volcanism and the flooding of large areas with basalt. Rifts will form and the supercontinent will split up once more.^[58] The planet may then experience a warming period, as occurred during the Cretaceous period.^[57]

Solidification of the outer core

The iron-rich core region of the Earth is divided into a 1,220 km (760 mi) radius solid inner core and a 3,480 km (2,160 mi) radius liquid outer core.^[59] The rotation of the Earth creates convective eddies in the outer core region that cause it to function as a dynamo.^[60] This generates a magnetosphere about the Earth that deflects particles from the solar wind, which prevents significant erosion of the atmosphere from sputtering. As heat from the core is transferred outward toward the mantle, the net trend is for the inner boundary of the liquid outer core region to freeze, thereby releasing thermal energy and causing the solid inner core to grow.^[61] This iron crystallization process has been ongoing for about a billion years. In the modern era, the radius of the inner core is expanding at an average rate of roughly 0.5 mm (0.02 in) per year, at the expense of the outer core.^[62] Nearly all of the energy needed to power the dynamo is being supplied by this process of inner core formation.^[63]

The growth of the inner core may be expected to consume most of the outer core by some 3–4 billion years from now, resulting in a nearly solid core composed of iron and other heavy elements. The surviving liquid envelope will mainly consist of lighter elements that will undergo less mixing.^[64] Alternatively, if at some point plate tectonics comes to an end, the interior will cool less efficiently, which may end the growth of the inner core. In either case, this can result in the loss of the magnetic dynamo. Without a functioning dynamo, the magnetic field of the Earth will decay in a geologically short time period of roughly 10,000 years.^[65] The loss of the magnetosphere will cause an increase in erosion of light elements, particularly hydrogen, from the Earth's outer atmosphere into space, resulting in less favorable conditions for life.^[66]

Solar evolution

The energy generation of the Sun is based upon thermonuclear fusion of hydrogen into helium. This occurs in the core region of the star using the proton–proton chain reaction process. Because there is no convection in the solar core, the helium concentration builds up in that region without being distributed throughout the star. The temperature at the core of the Sun is too low for nuclear fusion of helium atoms through the triple-alpha process, so these atoms do not contribute to the net energy generation that is needed to maintain hydrostatic equilibrium of the Sun.^[67]
At present, nearly half the hydrogen at the core has been consumed, with the remainder of the atoms consisting primarily of helium. As the number of hydrogen atoms per unit mass decrease, so too does their energy output provided through nuclear fusion. This results in a decrease in pressure support, which causes the core to contract until the increased density and temperature bring the core pressure into equilibrium with the layers above. The higher temperature causes the remaining hydrogen to undergo fusion at a more rapid rate, thereby generating the energy needed to maintain the equilibrium.^[67]

Evolution of the Sun's luminosity, radius and effective temperature compared to the present Sun. After Ribas (2010)^[68]

The result of this process has been a steady increase in the energy output of the Sun. When the Sun first became a main sequence star, it radiated only 70% of the current luminosity. The luminosity has increased in a nearly linear fashion to the present, rising by 1% every 110 million years.^[69] Likewise, in three billion years the Sun is expected to be 33% more luminous. The hydrogen fuel at the core will finally be exhausted in 4.8 billion years, when the Sun will be 67% more luminous than at present. Thereafter the Sun will continue to burn hydrogen in a shell surrounding its core, until the increase in luminosity reaches 121% of the present value. This marks the end of the Sun's main sequence lifetime, and thereafter it will pass through the subgiant stage and evolve into a red giant.^[1]

Climate impact

Once the core hydrogen is exhausted in 5.4 billion years, the Sun will expand into a subgiant phase and slowly double in size over about half a billion years. It will then expand more rapidly over about half a billion years until it is over two hundred times larger than today. With the increased surface area of the Sun, the amount of energy emitted will increase. Unless Earth's orbital radius increases significantly, the global temperature of the Earth will climb because of the rising luminosity of the Sun. However, geoscientist James Kasting says the oceans may evaporate much earlier, in about a billion years.^[70]

The rate of weathering of silicate minerals will increase. This in turn will decrease the level of carbon dioxide in the atmosphere, as these weathering processes convert carbon dioxide into carbonate materials. Within the next 600 million years from the present, the concentration of CO
2 will fall below the critical threshold needed to sustain C₃ photosynthesis: about 50 parts per million. At this point, trees and forests in their current forms will no longer be able to survive,^[71] the last living trees being evergreen conifers.^[72] However, C₄ carbon fixation can continue at much lower concentrations, down to above 10 parts per million. Thus plants using C₄ photosynthesis may be able to survive for at least 0.8 billion years and possibly as long as 1.2 billion years from now, after which rising temperatures will make the biosphere unsustainable.^[73][74][75] Currently, C₄ plants represent about 5% of Earth's plant biomass and 1% of its known plant species.^[76] For example, about 50% of all grass species (Poaceae) use the C₄ photosynthetic pathway,^[77] as do many species in the herbaceous family Amaranthaceae.^[78]

When the levels of carbon dioxide fall to the limit where photosynthesis is barely sustainable, the proportion of carbon dioxide in the atmosphere is expected to oscillate up and down. This will allow land vegetation to flourish each time the level of carbon dioxide rises due to tectonic activity and animal life. However, the long term trend is for the plant life on land to die off altogether as most of the remaining carbon in the atmosphere becomes sequestered in the Earth.^[79] Some microbes are capable of photosynthesis at concentrations of CO
2 of a few parts per million, so these life forms would probably disappear only because of rising temperatures and the loss of the biosphere.^[73]

Plants—and, by extension, animals—could survive longer by evolving other strategies such as requiring less CO
2 for photosynthetic processes, becoming carnivorous, adapting to desiccation, or associating with fungi. These adaptations are likely to appear near the beginning of the moist greenhouse (see below).^[72]

The loss of plant life will also result in the eventual loss of oxygen as well as ozone due to chemical reactions in the atmosphere and volcanic eruptions, meaning less attenuation of DNA-damaging ultraviolet radiation,^[72] as well as the death of animals; the first animals to disappear would be large mammals and fish, followed by small mammals, fish and birds, amphibians, reptiles, and finally invertebrates.^[80]

In their work The Life and Death of Planet Earth, authors Peter D. Ward and Donald Brownlee have also argued that some form of animal life may continue even after most of the Earth's plant life has disappeared. Initially, they expect that some insects, lizards, birds and small mammals may persist, along with sea life. Without oxygen replenishment by plant life, however, they believe that the animals would probably die off from asphyxiation within a few million years. Even if sufficient oxygen were to remain in the atmosphere through the persistence of some form of photosynthesis, the steady rise in global temperature would result in a gradual loss of biodiversity. As temperatures continue to rise, the last animal life will inevitably be driven back toward the poles, and possibly even underground. Much of the surface would become a barren desert and life would primarily be found in the oceans;^[79] however, due also to a decrease of the amount or organic matter coming to the oceans from the land as well as oxygen in the water,^[72] life would disappear there too following a similar path to that on Earth's surface with invertebrates being the last living animals,^[80] and of them those that do not depend on living plants such as termites or those near hydrothermal vents such as worms of the genus Riftia^[72] As a result of these processes, multi-cellular lifeforms may be extinct in about 800 million years, and eukaryotes in 1.3 billion years from now, leaving only the prokaryotes.^[81]

Loss of oceans

By one billion years from now, about 27% of the modern ocean will have been subducted into the mantle. If this process were allowed to continue uninterrupted, it would reach an equilibrium state where 65% of the current surface reservoir would remain at the surface.^[82] Once the solar luminosity is 10% higher than its current value, the average global surface temperature will rise to 320 K (47 °C). The atmosphere will become a "moist greenhouse" leading to a runaway evaporation of the oceans.^[83][84] At this point, models of the Earth's future environment demonstrate that the stratosphere would contain increasing levels of water. These water molecules will be broken down through photodissociation by solar ultraviolet radiation, allowing hydrogen to escape the atmosphere. The net result would be a loss of the world's sea water by about 1.1 billion years from the present.^[85][86]

The atmosphere of Venus is in a "supergreenhouse" state.

In this ocean-free era, there will continue to be reservoirs at the surface as water is steadily released from the deep crust and mantle,^[49] where it is estimated there is an amount of water equivalent to several times that currently present in the Earth's oceans.^[87] Some water may be retained at the poles and there may be occasional rainstorms, but for the most part the planet would be a dry desert with large dunefields covering its equator, resembling how Saturn's largest moon Titan looks today.^[11] Even in these arid conditions, the planet may retain some microbial and possibly even multi-cellular life.^[84] Most of these microbes will be halophiles. However, the increasingly extreme conditions will likely lead to the extinction of the prokaryotes between 1.6 billion years^[81] and 2.8 billion years ^[80] from now, with the last of them living in residual ponds of water at high latitudes and heights or in caverns with trapped ice; underground life, however, could last longer.^[80] What happens next depends on the level of tectonic activity. A steady release of carbon dioxide by volcanic eruption could eventually cause the atmosphere to enter a "supergreenhouse" state like that of the planet Venus. But without surface water, plate tectonics would probably come to a halt and most of the carbonates would remain securely buried^[11] until the Sun became a red giant and its increased luminosity heated the rock to the point of releasing the carbon dioxide.^[87]

The loss of the oceans could be delayed until two billion years in the future if the total atmospheric pressure were to decline. A lower atmospheric pressure would reduce the greenhouse effect, thereby lowering the surface temperature. This could occur if natural processes were to remove the nitrogen from the atmosphere. Studies of organic sediments has shown that at least 100 kilopascals (0.99 atm) of nitrogen has been removed from the atmosphere over the past four billion years; enough to effectively double the current atmospheric pressure if it were to be released. This rate of removal would be sufficient to counter the effects of increasing solar luminosity for the next two billion years. However, beyond that point, unless most of Earth's surface water has been lost by that time, in which case the planet will stay in the same conditions until the starting of the red giant phase,^[84] the amount of water in the lower atmosphere will have risen to 40% and the runaway moist greenhouse will commence^[88] when the luminosity from the Sun reaches 35–40% more than its current value, 3–4 billion years from now.^[85] The atmosphere will heat up and the surface temperature will rise sufficiently to melt surface rock.^[86][84] However, most of the atmosphere will be retained until the Sun has entered the red giant stage.^[89]

Red giant stage

The size of the current Sun (now in the main sequence) compared to its estimated size during its red giant phase

Once the Sun changes from burning hydrogen at its core to burning hydrogen around its shell, the core will start to contract and the outer envelope will expand. The total luminosity will steadily increase over the following billion years until it reaches 2,730 times the Sun's current luminosity at the age of 12.167 billion years. During this phase the Sun will experience more rapid mass loss, with about 33% of its total mass shed with the solar wind. The loss of mass will mean that the orbits of the planets will expand. The orbital distance of the Earth will increase to at most 150% of its current value.^[69]

The most rapid part of the Sun's expansion into a red giant will occur during the final stages, when the Sun will be about 12 billion years old. It is likely to expand to swallow both Mercury and Venus, reaching a maximum radius of 1.2 AU (180,000,000 km). The Earth will interact tidally with the Sun's outer atmosphere, which would serve to decrease Earth's orbital radius. Drag from the chromosphere of the Sun would also reduce the Earth's orbit. These effects will act to counterbalance the effect of mass loss by the Sun, and the Earth will most likely be engulfed by the Sun.^[69] The ablation and vaporization caused by its fall on a decaying trajectory towards the Sun will remove Earth's crust and mantle, then finally destroy it after at most 200 years.^[90] Earth's sole legacy will be a very slight increase (0.01%) of the solar metallicity.^[91]

Before this happens, most of Earth's atmosphere will have been lost to space and its surface will consist of a magma ocean with floating continents of metals and metal oxides as well as icebergs of refractory materials, with its surface temperature reaching more than 2,400 K (2,130 °C).^[92]

The drag from the solar atmosphere may cause the orbit of the Moon to decay. Once the orbit of the Moon closes to a distance of 18,470 km (11,480 mi), it will cross the Earth's Roche limit. This means that tidal interaction with the Earth would break apart the Moon, turning it into a ring system. Most of the orbiting ring will then begin to decay, and the debris will impact the Earth. Hence, even if the Earth is not swallowed up by the Sun, the planet may be left moonless.^[93]

Another scenario is that Earth somehow survives being engulfed to the Sun, but that the ablation and vaporization mentioned above will strip both its crust and mantle leaving just its core.^[94]

Post red-giant stage

The Ring nebula, a planetary nebula similar to what the Sun will produce in 8 billion years.

After fusing helium in its core to carbon, the Sun will begin to collapse again, evolving into a compact white dwarf star after ejecting its outer atmosphere as a planetary nebula. Any planets that survive this process will continue to orbit this body, but will receive little thermal radiation and become frigid bodies. Over time intervals of around every 30 trillion years, the Sun will undergo a close encounter with another star. As a consequence, the orbits of their planets can become disrupted, potentially ejecting them from the system entirely.^[95]

Should the Earth have survived these calamities, over a time of about 10²⁰ (100 quintillion) years, the planet's orbit will steadily decay through emission of gravitational radiation, before finally being ripped apart by tidal forces at a distance of around 1-6 radii from the Sun.

History of the Earth

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The History of the Earth concerns the development of the planet Earth from its formation to the present day.^[1][2] Nearly all branches of natural science have contributed to the understanding of the main events of the Earth's past. The age of Earth is approximately one-third of the age of the universe. An immense amount of biological and geological change has occurred in that time span.

Earth formed around 4.54 billion (4.54×10⁹) years ago by accretion from the solar nebula. Volcanic outgassing probably created the primordial atmosphere, but it contained almost no oxygen and would have been toxic to humans and most modern life. Much of the Earth was molten because of extreme volcanism and frequent collisions with other bodies. One very large collision is thought to have been responsible for tilting the Earth at an angle and forming the Moon. Over time, the planet cooled and formed a solid crust, allowing liquid water to exist on the surface.

The first life forms appeared between 3.8 and 3.5 billion years ago. The earliest evidences for life on Earth are graphite found to be biogenic in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland^[3] and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia.^[4][5] Photosynthetic life appeared around 2 billion years ago, enriching the atmosphere with oxygen. Life remained mostly small and microscopic until about 580 million years ago, when complex multicellular life arose. During the Cambrian period it experienced a rapid diversification into most major phyla.

Geological change has been constantly occurring on our planet since the time of its formation and biological change since the first appearance of life. Species continuously evolve, taking on new forms, splitting into daughter species, or going extinct in response to an ever-changing planet. The process of plate tectonics has played a major role in the shaping of Earth's oceans and continents, as well as the life they harbor. The biosphere, in turn, has had a significant effect on the atmosphere and other abiotic conditions on the planet, such as the formation of the ozone layer, the proliferation of oxygen, and the creation of soil.

Geologic time scale

The history of the Earth is organized chronologically in a table known as the geologic time scale, which is split into intervals based on stratigraphic analysis.^[2][6] A full-time scale can be found at the main article.
The following four timelines show the geologic time scale. The first shows the entire time from the formation of the Earth to the present, but this compresses the most recent eon. Therefore the second scale shows the most recent eon with an expanded scale. The second scale compresses the most recent era, so the most recent era is expanded in the third scale. Since the Quaternary is a very short period with short epochs, it is further expanded in the fourth scale. The second, third, and fourth timelines are therefore each subsections of their preceding timeline as indicated by asterisks. The Holocene (the latest epoch) is too small to be shown clearly on the third timeline on the right, another reason for expanding the fourth scale. The Pleistocene (P) epoch. Q stands for the Quaternary period.




Millions of Years

Solar System formation

An artist's rendering of a protoplanetary disk

The standard model for the formation of the Solar System (including the Earth) is the solar nebula hypothesis.^[7] In this model, the Solar system formed from a large, rotating cloud of interstellar dust and gas called the solar nebula. It was composed of hydrogen and helium created shortly after the Big Bang 13.8 Ga (billion years ago) and heavier elements ejected by supernovae. About 4.5 Ga, the nebula began a contraction that may have been triggered by the shock wave of a nearby supernova.^[8] A shock wave would have also made the nebula rotate. As the cloud began to accelerate, its angular momentum, gravity and inertia flattened it into a protoplanetary disk perpendicular to its axis of rotation. Small perturbations due to collisions and the angular momentum of other large debris created the means by which kilometer-sized protoplanets began to form, orbiting the nebular center.^[9]

The center of the nebula, not having much angular momentum, collapsed rapidly, the compression heating it until nuclear fusion of hydrogen into helium began. After more contraction, a T Tauri star ignited and evolved into the Sun. Meanwhile, in the outer part of the nebula gravity caused matter to condense around density perturbations and dust particles, and the rest of the protoplanetary disk began separating into rings. In a process known as runaway accretion, successively larger fragments of dust and debris clumped together to form planets.^[9] Earth formed in this manner about 4.54 billion years ago (with an uncertainty of 1%)^{[10][11][12][13]} and was largely completed within 10–20 million years.^[14] 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. The same process is expected to produce accretion disks around virtually all newly forming stars in the universe, some of which yield planets.^[15]

The proto-Earth grew by accretion until its interior was hot enough to melt the heavy, siderophile metals. Having higher densities than the silicates, these metals sank. This so-called iron catastrophe resulted in the separation of a primitive mantle and a (metallic) core only 10 million years after the Earth began to form, producing the layered structure of Earth and setting up the formation of Earth's magnetic field.^[16] J. A. Jacobs ^[17] was the first to suggest that the inner core—a solid center distinct from the liquid outer core—is freezing and growing out of the liquid outer core due to the gradual cooling of Earth's interior (about 100 degrees Celsius per billion years^[18]).

Hadean and Archean Eons

The first eon in Earth's history, the Hadean, begins with the Earth's formation and is followed by the Archean eon at 3.8 Ga.^[2]:145 The oldest rocks found on Earth date to about 4.0 Ga, and the oldest detrital zircon crystals in rocks to about 4.4 Ga,^[19][20][21] soon after the formation of the Earth's crust and the Earth itself. The giant impact hypothesis for the Moon's formation states that shortly after formation of an initial crust, the proto-Earth was impacted by a smaller protoplanet, which ejected part of the mantle and crust into space and created the Moon.^[22][23][24]
From crater counts on other celestial bodies it is inferred that a period of intense meteorite impacts, called the Late Heavy Bombardment, began about 4.1 Ga, and concluded around 3.8 Ga, at the end of the Hadean.^[25] In addition, volcanism was severe due to the large heat flow and geothermal gradient.^[26] Nevertheless, detrital zircon crystals dated to 4.4 Ga show evidence of having undergone contact with liquid water, suggesting that the planet already had oceans or seas at that time.^[19]

By the beginning of the Archean, the Earth had cooled significantly. Most present life forms could not have survived in the Archean atmosphere, which lacked oxygen and an ozone layer. Nevertheless it is believed that primordial life began to evolve by the early Archean, with candidate fossils dated to around 3.5 Ga.^[27] Some scientists even speculate that life could have begun during the early Hadean, as far back as 4.4 Ga, surviving the possible Late Heavy Bombardment period in hydrothermal vents below the Earth's surface.^[28]

Formation of the Moon

Artist's impression of the enormous collision that probably formed the Moon

Earth's only natural satellite, the Moon, is larger relative to its planet than any other satellite in the solar system.^{[nb 1]} During the Apollo program, rocks from the Moon's surface were brought to Earth. Radiometric dating of these rocks has shown that the Moon is 4.53 ± .01 billion years old,^[31], formed at least 30 million years after the solar system.^[32] New evidence suggests the Moon formed even later, 4.48 ± 0.02 Ga, or 70–110 million years after the start of the Solar System.^[33]

Theories for the formation of the Moon must explain its late formation as well as the following facts. First, the Moon has a low density (3.3 times that of water, compared to 5.5 for the earth^[34]) and a small metallic core. Second, there is virtually no water or other volatiles on the moon. Third, the Earth and Moon have the same oxygen isotopic signature (relative abundance of the oxygen isotopes). Of the theories that have been proposed to account for these phenomena, only one is widely accepted: The giant impact hypothesis proposes that the Moon originated after a body the size of Mars (sometimes named Theia^[32]) struck the proto-Earth a glancing blow.^{[1]:256[35][36]}

The collision released about 100 million times more energy than the more recent Chicxulub impact that is believed to have caused the extinction of the dinosaurs. It was enough to vaporize some of the Earth's outer layers and melt both bodies.^[35][1]:256 A portion of the mantle material was ejected into orbit around the Earth. The giant impact hypothesis predicts that the Moon was depleted of metallic material,^[37] explaining its abnormal composition.^[38] 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.^[39]

First continents

Geologic map of North America, color-coded by age. The reds and pinks indicate rock from the Archean.

Mantle convection, the process that drives plate tectonics today, is a result of heat flow from the Earth's interior to the Earth's surface.^[40]:2 It involves the creation of rigid tectonic plates at mid-oceanic ridges. These plates are destroyed by subduction into the mantle at subduction zones. During the early Archean (about 3.0 Ga) the mantle was much hotter than today, probably around 1600 °C,^[41]:82 so convection in the mantle was faster. While a process similar to present day plate tectonics did occur, this would have gone faster too. It is likely that during the Hadean and Archean, subduction zones were more common, and therefore tectonic plates were smaller.^[1]:258

The initial crust, 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. However, it is thought that it was basaltic in composition, like today's oceanic crust, because little crustal differentiation had yet taken place.^[1]:258 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 end of the Hadean, about 4.0 Ga. What is left of these first small continents are called cratons. These pieces of late Hadean and early Archean crust form the cores around which today's continents grew.^[42]

The oldest rocks on Earth are found in the North American craton of Canada. They are tonalites from 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 then.^[43] Cratons consist primarily 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 Archean. The second type is a complex 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.^{[44]:Chapter 5}

Oceans and atmosphere

Graph showing range of estimated partial pressure of atmospheric oxygen through geologic time ^[45]

Earth is often described as having had three atmospheres. The first atmosphere, captured from the solar nebula, was composed of light (atmophile) elements from the solar nebula, mostly hydrogen and helium. A combination of the solar wind and Earth's heat would have driven off this atmosphere, as a result of which the atmosphere is now depleted of these elements compared to cosmic abundances.^[46] After the impact,^{[clarification needed]} the molten Earth released volatile gases; and later more gases were released by volcanoes, completing a second atmosphere rich in greenhouse gases but poor in oxygen. ^[1]:256 Finally, the third atmosphere, rich in oxygen, emerged when bacteria began to produce oxygen about 2.8 Ga.^{[47]:83–84,116–117}

In early models for the formation of the atmosphere and ocean, the second atmosphere was formed by outgassing of volatiles from the Earth's interior. Now it is considered likely that many of the volatiles were delivered during accretion by a process known as impact degassing in which incoming bodies vaporize on impact. The ocean and atmosphere would therefore have started to form even as the Earth formed.^[48] The new atmosphere probably contained water vapor, carbon dioxide, nitrogen, and smaller amounts of other gases.^[49]

Planetesimals at a distance of 1 astronomical unit (AU), the distance of the Earth from the Sun, probably did not contribute any water to the Earth because the solar nebula was too hot for ice to form and the hydration of rocks by water vapor would have taken too long.^[48][50] The water must have been supplied by meteorites from the outer asteroid belt and some large planetary embryos from beyond 2.5 AU.^[48][51] Comets may also have contributed. Though most comets are today in orbits farther away from the Sun than Neptune, computer simulations show they were originally far more common in the inner parts of the solar system.^[43]:130-132

As the planet cooled, clouds formed. Rain created the oceans. Recent evidence suggests the oceans may have begun forming as early as 4.4 Ga.^[19] By the start of the Archean eon they already covered the Earth. This early formation has been difficult to explain because of a problem known as the faint young Sun paradox. Stars are known to get brighter as they age, and at the time of its formation the Sun would have been emitting only 70% of its current power. Many models predict that the Earth would have been covered in ice.^[52][48] A likely solution is that there was enough carbon dioxide and methane to produce a greenhouse effect. The carbon dioxide would have been produced by volcanoes and the methane by early microbes. Another greenhouse gas, ammonia, would have been ejected by volcanos but quickly destroyed by ultraviolet radiation.^[47]:83

Origin of life

One of the reasons for interest in the early atmosphere and ocean is that they form the conditions under which life first arose. There are many models, but little consensus, on how life emerged from non-living chemicals; chemical systems that have been created in the laboratory still fall well short of the minimum complexity for a living organism.^[53][54]
The first step in the emergence of life may have been chemical reactions that produced many of the simpler organic compounds, including nucleobases and amino acids, that are the building blocks of life. An experiment in 1953 by Stanley Miller and Harold Urey showed that such molecules could form in an atmosphere of water, methane, ammonia and hydrogen with the aid of sparks to mimic the effect of lightning.^[55] Although the atmospheric composition was probably different from the composition used by Miller and Urey, later experiments with more realistic compositions also managed to synthesize organic molecules.^[56] Recent computer simulations have even shown that extraterrestrial organic molecules could have formed in the protoplanetary disk before the formation of the Earth.^[57]

The next stage of complexity could have been reached from at least three possible starting points: self-replication, an organism's ability to produce offspring that are very similar to itself; metabolism, its ability to feed and repair itself; and external cell membranes, which allow food to enter and waste products to leave, but exclude unwanted substances.^[58]

Replication first: RNA world

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.

Even the simplest members of the three modern domains of life use DNA to record their "recipes" and a complex array of RNA and protein molecules to "read" these instructions and use them for growth, maintenance and self-replication.

The discovery that a kind of RNA molecule called a ribozyme can catalyze both its own replication and the construction of proteins led to the hypothesis that earlier life-forms were based entirely on RNA.^[59] They could have formed an RNA world in which there were individuals but no species, as mutations and horizontal gene transfers would have meant that the offspring in each generation were quite likely to have different genomes from those that their parents started with.^[60] RNA would later have been replaced by DNA, which is more stable and therefore can build longer genomes, expanding the range of capabilities a single organism can have.^[61] Ribozymes remain as the main components of ribosomes, the "protein factories" of modern cells.^[62]

Although short, self-replicating RNA molecules have been artificially produced in laboratories,^[63] doubts have been raised about whether natural non-biological synthesis of RNA is possible.^[64][65][66] The earliest ribozymes may have been formed of simpler nucleic acids such as PNA, TNA or GNA, which would have been replaced later by RNA.^[67][68] Other pre-RNA replicators have been posited, including crystals^[69]:150 and even quantum systems.^[70]

In 2003 it was proposed that porous metal sulfide precipitates would assist RNA synthesis at about 100 °C (212 °F) and ocean-bottom pressures near hydrothermal vents. In this hypothesis, lipid membranes would be the last major cell components to appear and until they did the proto-cells would be confined to the pores.^[71]

Metabolism first: iron–sulfur world

Another long-standing hypothesis is that the first life was composed of protein molecules. Amino acids, the building blocks of proteins, are easily synthesized in plausible prebiotic conditions, as are small peptides (polymers of amino acids) that make good catalysts.^{[72]:295–297} A series of experiments starting in 1997 showed that amino acids and peptides could form in the presence of carbon monoxide and hydrogen sulfide with iron sulfide and nickel sulfide as catalysts. Most of the steps in their assembly required temperatures of about 100 °C (212 °F) and moderate pressures, although one stage required 250 °C (482 °F) and a pressure equivalent to that found under 7 kilometres (4.3 mi) of rock. Hence self-sustaining synthesis of proteins could have occurred near hydrothermal vents.^[73]
A difficulty with the metabolism-first scenario is finding a way for organisms to evolve. Without the ability to replicate as individuals, aggregates of molecules would have "compositional genomes" (counts of molecular species in the aggregate) as the target of natural selection. However, a recent model shows that such a system is unable to evolve in response to natural selection.^[74]

Membranes first: Lipid world

Cross-section through a liposome

It has been suggested that double-walled "bubbles" of lipids like those that form the external membranes of cells may have been an essential first step.^[75] Experiments that simulated the conditions of the early Earth have reported the formation of lipids, and these can spontaneously form liposomes, double-walled "bubbles", and then reproduce themselves. Although they are not intrinsically information-carriers as nucleic acids are, they would be subject to natural selection for longevity and reproduction. Nucleic acids such as RNA might then have formed more easily within the liposomes than they would have outside.^[76]

The clay theory

Some clays, notably montmorillonite, have properties that make them plausible accelerators for the emergence of an RNA world: they grow by self-replication of their crystalline pattern, are subject to an analog of natural selection (as the clay "species" that grows fastest in a particular environment rapidly becomes dominant), and can catalyze the formation of RNA molecules.^[77] Although this idea has not become the scientific consensus, it still has active supporters.^{[78]:150–158[69]}
Research in 2003 reported that montmorillonite could also accelerate the conversion of fatty acids into "bubbles", and that the bubbles could encapsulate RNA attached to the clay. Bubbles can then grow by absorbing additional lipids and dividing. The formation of the earliest cells may have been aided by similar processes.^[79]

A similar hypothesis presents self-replicating iron-rich clays as the progenitors of nucleotides, lipids and amino acids.^[80]

Last universal ancestor

It is believed that of this multiplicity of protocells, only one line survived. Current phylogenetic evidence suggests that the last universal ancestor (LUA) lived during the early Archean eon, perhaps 3.5 Ga or earlier.^[81][82] This LUA 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 by lateral gene transfer.^[81]

Proterozoic Eon

The Proterozoic eon lasted from 2.5 Ga to 542 Ma (million years ago).^[2]:130 In this time span, cratons grew into continents with modern sizes. The change to an oxygen-rich atmosphere was a crucial development. Life developed from prokaryotes into eukaryotes and multicellular forms. The Proterozoic saw a couple of severe ice ages called snowball Earths. After the last Snowball Earth about 600 Ma, the evolution of life on Earth accelerated. About 580 Ma, the Ediacara biota formed the prelude for the Cambrian Explosion.

Oxygen revolution

Lithified stromatolites on the shores of Lake Thetis, Western Australia. Archean stromatolites are the first direct fossil traces of life on Earth.

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.

The earliest cells absorbed energy and food from the environment around them. They used fermentation, the breakdown of more complex compounds into less complex compounds with less energy, and used the energy so liberated to grow and reproduce. Fermentation can only occur in an anaerobic (oxygen-free) environment. The evolution of photosynthesis made it possible for cells to manufacture their own food.^[83]:377

Most of the life that covers the surface of the Earth depends directly or indirectly on photosynthesis. The most common form, oxygenic photosynthesis, turns carbon dioxide, water and sunlight into food. It captures the energy of sunlight in energy-rich molecules such as ATP, which then provide the energy to make sugars. To supply the electrons in the circuit, hydrogen is stripped from water, leaving oxygen as a waste product.^[84] Some organisms, including purple bacteria and green sulfur bacteria, use an anoxygenic form of photosynthesis that use alternatives to hydrogen stripped from water as electron donors; examples are hydrogen sulfide, sulfur and iron. Such organisms are mainly restricted to extreme environments such as hot springs and hydrothermal vents.^{[83]:379–382[85]}

The simpler anoxygenic form arose about 3.8 Ga, not long after the appearance of life. The timing of oxygenic photosynthesis is more controversial; it had certainly appeared by about 2.4 Ga, but some researchers put it back as far as 3.2 Ga.^[84] The latter "probably increased global productivity by at least two or three orders of magnitude."^[86][87] Among the oldest remnants of oxygen-producing lifeforms are fossil stromatolites.^[86][87][45]

At first, the released oxygen was bound up with limestone, iron, and other minerals. The oxidized iron appears as red layers in geological strata called banded iron formations that formed in abundance during the Siderian period (between 2500 Ma and 2300 Ma).^[2]:133 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 time transformed Earth’s atmosphere to its current state. This was Earth’s third atmosphere.^{[88]:50–51[47]:83–84,116–117}

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 eventually the land: without the ozone layer, ultraviolet radiation bombarding land and sea would have caused unsustainable levels of mutation in exposed cells.^{[89][43]:219–220}

Photosynthesis had another major impact. Oxygen was toxic; much life on Earth probably died out as its levels rose in what is known as the oxygen catastrophe. Resistant forms survived and thrived, and some developed the ability to use oxygen to increase their metabolism and obtain more energy from the same food.^[89]

Snowball Earth

The natural evolution of the Sun made it progressively more luminous during the Archean and Proterozoic eons; the Sun's luminosity increases 6% every billion years.^[43]:165 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 in South Africa date back to 2.2 Ga, at which time, based on paleomagnetic evidence, they must have been located near the equator. Thus, this glaciation, known as the Makganyene glaciation, may have been global. 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.^[90]
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.^[43]:172 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.^[91]

Emergence of eukaryotes

Chloroplasts in the cells of a moss

Modern taxonomy classifies life into three domains. The time of the origin of these domains is uncertain. 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,^[92] 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 becoming known.

Around this time, the first proto-mitochondrion was formed. A bacterial cell related to today’s Rickettsia,^[93] which had evolved to metabolize oxygen, entered a larger prokaryotic cell, which lacked that capability. Perhaps the large cell attempted to digest the smaller one but failed (possibly due to the evolution of prey defenses). The smaller cell may have tried to parasitize the larger one. In any case, the smaller cell survived inside the larger cell. Using oxygen, it metabolized the larger cell’s waste products and derived more energy. Part of this excess 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.^[94]

A similar event occurred with photosynthetic cyanobacteria^[95] entering large heterotrophic cells and becoming chloroplasts.^{[88]:60–61[96]:536–539} 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. Besides the well-established endosymbiotic theory of the cellular origin of mitochondria and chloroplasts, there are theories that cells led to peroxisomes, spirochetes led to cilia and flagella, and that perhaps a DNA virus led to the cell nucleus,^[97],[98] though none of them are widely accepted.^[99]

Archaeans, bacteria, and eukaryotes continued to diversify and to become more complex 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.^[100][101] The plant, animal, and fungi lines had split, though they still existed as solitary cells. Some of these lived in colonies, and gradually a 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^[102] the first multicellular plants emerged, probably green algae.^[103] Possibly by around 900 Ma^[96]:488 true multicellularity had also evolved in animals.

At first it probably resembled today’s sponges, which have totipotent cells that allow a disrupted organism to reassemble itself.^[96]:483-487 As the division of labor was completed in all lines of multicellular organisms, cells became more specialized and more dependent on each other; isolated cells would die.

Supercontinents in the Proterozoic

A reconstruction of Pannotia (550 Ma).

Reconstructions of tectonic plate movement in the past 250 million years (the Cenozoic and Mesozoic eras) can be made reliably using fitting of continental margins, ocean floor magnetic anomalies and paleomagnetic poles. No ocean crust dates back further than that, so earlier reconstructions are more difficult. Paleomagnetic poles are supplemented by geologic evidence such as orogenic belts, which mark the edges of ancient plates, and past distributions of flora and fauna. The further back in time, the scarcer and harder to interpret the data get and the more diverse the reconstructions.^[104]:370

Throughout the history of the Earth, there have been times when continents collided and formed a supercontinent, which later broke up into new continents. About 1000 to 830 Ma, most continental mass was united in the supercontinent Rodinia.^{[104]:370[105]} Rodinia may have been preceded by Early-Middle Proterozoic continents called Nuna and Columbia.^{[104]:374[106][107]}

After the break-up of Rodinia about 800 Ma, the continents may have formed another short-lived supercontinent, Pannotia, around 550 Ma. The hypothetical supercontinent is sometimes referred to as Pannotia or Vendia.^{[108]:321–322} 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. The existence of Pannotia depends on the timing of the rifting between Gondwana (which included most of the landmass now in the Southern Hemisphere, as well as the Arabian Peninsula and the Indian subcontinent) and Laurentia (roughly equivalent to current-day North America).^[104]:374 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.^[109]

Late Proterozoic climate and life

A 580 million year old fossil of Spriggina floundensi, an animal from the Ediacaran period. Such life forms could have been ancestors to the many new forms that originated in the Cambrian Explosion.

The end of the Proterozoic saw at least two Snowball Earths, so severe that the surface of the oceans may have been completely frozen. This happened about 716.5 and 635 Ma, in the Cryogenian period.^[110] The intensity and mechanism of both glaciations are still under investigation and harder to explain than the early Proterozoic Snowball Earth.^[111] Most paleoclimatologists think the cold episodes were linked to the formation of the supercontinent Rodinia.^[112] Because Rodinia was centered on the equator, rates of chemical weathering increased and carbon dioxide (CO₂) was taken from the atmosphere. Because CO₂ is an important greenhouse gas, climates cooled globally.^{[citation needed]} In the same way, during the Snowball Earths most of the continental surface was covered with permafrost, which decreased chemical weathering again, leading to the end of the glaciations. An alternative hypothesis is that enough carbon dioxide escaped through volcanic outgassing that the resulting greenhouse effect raised global temperatures.^[112] Increased volcanic activity resulted from the break-up of Rodinia at about the same time.

The Cryogenian period was followed by the Ediacaran period, which was characterized by a rapid development of new multicellular lifeforms.^[113] Whether there is a connection between the end of the severe ice ages and the increase in diversity of life is not clear, but it does not seem coincidental. The new forms of life, called Ediacara biota, were larger and more diverse than ever. Though the taxonomy of most Ediacaran life forms is unclear, some were ancestors of groups of modern life.^[114] Important developments were the origin of muscular and neural cells. None of the Ediacaran fossils had hard body parts like skeletons. These first appear after the boundary between the Proterozoic and Phanerozoic eons or Ediacaran and Cambrian periods.

Phanerozoic Eon

The Phanerozoic is the current eon on Earth, which started approximately 542 million years ago. It consists of three eras: The Paleozoic, Mesozoic, and Cenozoic,^[6] and is the time when multi-cellular life greatly diversified into almost all of the organisms known today.^[115]

Paleozoic Era

The Paleozoic era (meaning: era of old life forms) was the first and longest era of the Phanerozoic eon, lasting from 542 to 251 Ma.^[6] During the Paleozoic, many modern groups of life came into existence. Life colonized the land, first plants, then animals. Life usually evolved slowly. At times, however, there are sudden radiations of new species or mass extinctions. These bursts of evolution were often caused by unexpected changes in the environment resulting from natural disasters such as volcanic activity, meteorite impacts or climate changes.
The continents formed at the break-up of Pannotia and Rodinia at the end of the Proterozoic would slowly move together again during the Paleozoic. This would eventually result in phases of mountain building that created the supercontinent Pangaea in the late Paleozoic.

Cambrian explosion

Trilobites first appeared during the Cambrian period and were among the most widespread and diverse groups of Paleozoic organisms.

The rate of the evolution of life as recorded by fossils accelerated in the Cambrian period (542–488 Ma).^[6] The sudden emergence of many new species, phyla, and forms in this period is called the Cambrian Explosion. The biological fomenting in the Cambrian Explosion was unpreceded before and since that time.^[43]:229 Whereas the Ediacaran life forms appear yet primitive and not easy to put in any modern group, at the end of the Cambrian most modern phyla were already present. The development of hard body parts such as shells, skeletons or exoskeletons in animals like molluscs, echinoderms, crinoids and arthropods (a well-known group of arthropods from the lower Paleozoic are the trilobites) made the preservation and fossilization of such life forms easier than those of their Proterozoic ancestors. For this reason, much more is known about life in and after the Cambrian than about that of older periods. Some of these Cambrian groups appear complex but are quite different from modern life; examples are Anomalocaris and Haikouichthys.

During the Cambrian, the first vertebrate animals, among them the first fishes, had appeared.^[96]:357 A creature that could have been the ancestor of the fishes, or was probably closely related to it, was Pikaia. It had a primitive notochord, a structure that could have developed into a vertebral column later. The first fishes with jaws (Gnathostomata) appeared during the next geological period, the Ordovician. The colonisation of new niches resulted in massive body sizes. In this way, fishes with increasing sizes evolved during the early Paleozoic, such as the titanic placoderm Dunkleosteus, which could grow 7 meters long.

The diversity of life forms did not increase greatly because of a series of mass extinctions that define widespread biostratigraphic units called biomeres.^[116] After each extinction pulse, the continental shelf regions were repopulated by similar life forms that may have been evolving slowly elsewhere.^[117] By the late Cambrian, the trilobites had reached their greatest diversity and dominated nearly all fossil assemblages.^[118]:34

Paleozoic tectonics, paleogeography and climate

Pangaea was a supercontinent that existed from about 300 to 180 Ma. The outlines of the modern continents and other landmasses are indicated on this map.

At the end of the Proterozoic, the supercontinent Pannotia had broken apart in the smaller continents Laurentia, Baltica, Siberia and Gondwana.^[119] During periods when continents move apart, more oceanic crust is formed by volcanic activity. Because young volcanic crust is relatively hotter and less dense than old oceanic crust, the ocean floors rise during such periods. This causes the sea level to rise. Therefore, in the first half of the Paleozoic, large areas of the continents were below sea level.
Early Paleozoic climates were warmer than today, but the end of the Ordovician saw a short ice age during which glaciers covered the south pole, where the huge continent Gondwana was situated.
Traces of glaciation from this period are only found on former Gondwana. During the Late Ordovician ice age, a few mass extinctions took place, in which many brachiopods, trilobites, Bryozoa and corals disappeared. These marine species could probably not contend with the decreasing temperature of the sea water.^[120] After the extinctions new species evolved, more diverse and better adapted. They would fill the niches left by the extinct species.

The continents Laurentia and Baltica collided between 450 and 400 Ma, during the Caledonian Orogeny, to form Laurussia (also known as Euramerica).^[121] Traces of the mountain belt this collision caused can be found in Scandinavia, Scotland, and the northern Appalachians. In the Devonian period (416–359 Ma)^[6] Gondwana and Siberia began to move towards Laurussia. The collision of Siberia with Laurussia caused the Uralian Orogeny, the collision of Gondwana with Laurussia is called the Variscan or Hercynian Orogeny in Europe or the Alleghenian Orogeny in North America. The latter phase took place during the Carboniferous period (359–299 Ma)^[6] and resulted in the formation of the last supercontinent, Pangaea.^[44]

Colonization of land

Artist's conception of Devonian flora

Oxygen accumulation from photosynthesis resulted in the formation of an ozone layer that absorbed much of the 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. Prokaryote lineages^[122] had probably colonized the land as early as 2.6 Ga^[123] 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.^[124] Fish, the earliest vertebrates, evolved in the oceans around 530 Ma.^[96]:354 A major extinction event occurred near the end of the Cambrian period,^[125] which ended 488 Ma.^[126]

Several hundred million years ago, plants (probably resembling algae) and fungi started growing at the edges of the water, and then out of it.^{[127]:138–140} 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.^[128] 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,^[129] perhaps thriving and becoming better adapted due to the vast food source provided by the terrestrial plants. There is also unconfirmed evidence that arthropods may have appeared on land as early as 530 Ma.^[130]

Evolution of tetrapods

Tiktaalik, a fish with limb-like fins and a predecessor of tetrapods. Reconstruction from fossils about 375 million years old.

At the end of the Ordovician period, 443 Ma,^[6] additional extinction events occurred, perhaps due to a concurrent ice age.^[120] Around 380 to 375 Ma, the first tetrapods evolved from fish.^[131] Fins evolved to become limbs that the first tetrapods used to lift their heads out of the water to breathe air. This would let them live in oxygen-poor water, or pursue small prey in shallow water.^[131] 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.^[132] Plants evolved seeds, which dramatically accelerated their spread on land, around this time (by approximately 360 Ma).^[133][134]

About 20 million years later (340 Ma^{[96]:293–296}), 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^{[96]:254–256}) 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.

Mesozoic Era

Dinosaurs were the dominant terrestrial vertebrates throughout most of the Mesozoic

The Mesozoic ("middle life") era lasted from 251 Ma to 66 Ma.^[6] It is subdivided into the Triassic, Jurassic, and Cretaceous periods. The era began with the Permian–Triassic extinction event, the most severe extinction event in the fossil record; 95% of the species on Earth died out.^[135] It ended with the Cretaceous–Paleogene extinction event that wiped out the dinosaurs. The Permian-Triassic event was possibly caused by some combination of the Siberian Traps volcanic event, an asteroid impact, methane hydrate gasification, sea level fluctuations, and a major anoxic event. Either the proposed Wilkes Land crater^[136] in Antarctica or Bedout structure off the northwest coast of Australia may indicate an impact connection with the Permian-Triassic extinction. But it remains uncertain whether either these or other proposed Permian-Triassic boundary craters are either real impact craters or even contemporaneous with the Permian-Triassic extinction event. Life persevered, and around 230 Ma, dinosaurs split off from their reptilian ancestors.^[137] The Triassic–Jurassic extinction event at 200 Ma spared many of the dinosaurs,^[6][138] and they soon became dominant among the vertebrates. Though some of the mammalian lines began to separate during this period, existing mammals were probably small animals resembling shrews.^[96]:169

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.^[139] The earliest evidence for the angiosperms evolving flowers is during the Cretaceous period, some 20 million years later (132 Ma).^[140] In 66 Ma, a 10-kilometre (6.2 mi) asteroid struck Earth just off the Yucatán Peninsula - somewhere in the south western tip of then Laurasia - 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,^[141] marking the end of the Cretaceous period and Mesozoic era.

Cenozoic Era

The Cenozoic era began at 66 Ma,^[6] and is subdivided into the Paleogene, Neogene, and Quaternary periods. Mammals and birds were able to survive the Cretaceous–Paleogene extinction event that killed off the dinosaurs and many other forms of life, and this is the era during which they diversified into their modern forms.

Diversification of mammals

Mammals have existed since the late Triassic, but prior to the Cretaceous–Paleogene extinction event they remained small. During the Cenozoic, mammals rapidly diversified to fill some of the niches that the dinosaurs and other extinct animals had left behind, branching out into many of the modern orders. With many marine reptiles extinct, some mammals began living in the oceans and became cetaceans. Others became felids and canids, swift and agile land predators. The drier global climate of the Cenozoic led to the expansion of grasslands and the evolution of grazing and hoofed mammals such as equids and bovids. Some arboreal mammals became the primates, of which one lineage would lead to modern humans.

Human evolution

A reconstruction of human history based on fossil data.^[142]

A small African ape living around 6 Ma was the last animal whose descendants would include both modern humans and their closest relatives, the chimpanzees.^{[96]:100–101} Only two branches of its family tree have surviving descendants. Very soon after the split, for reasons that are still unclear, apes in one branch developed the ability to walk upright.^[96]:95–99 Brain size increased rapidly, and by 2 Ma, the first animals classified in the genus Homo had appeared.^[127]:300 Of course, the line between different species or even genera is somewhat 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.^{[96]:100–101}

The ability to control fire probably began in Homo erectus (or Homo ergaster), probably at least 790,000 years ago^[143] but perhaps as early as 1.5 Ma.^[96]:67 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 or strong australopithecines such as Paranthropus.^[144]

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.^[96]:67 As brain size increased, babies were born earlier, 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 sophisticated, and tools became more elaborate. This contributed to further cooperation and intellectual development.^[145]:7 Modern humans (Homo sapiens) are believed to have originated around 200,000 years ago or earlier in Africa; the oldest fossils date back to around 160,000 years ago.^[146]

The first humans to show signs of spirituality are the Neanderthals (usually classified as a separate species with no surviving descendants); they buried their dead, often with no sign of food or tools.^[147]:17 However, evidence of more sophisticated beliefs, such as the early Cro-Magnon cave paintings (probably with magical or religious significance)^{[147]:17–19} did not appear until 32,000 years ago.^[148] Cro-Magnons also left behind stone figurines such as Venus of Willendorf, probably also signifying religious belief.^{[147]:17–19} 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).^[149] Tool use and communication continued to improve, and interpersonal relationships became more intricate.

Civilization

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.^[145]:8 As language became more complex, the ability to remember and communicate information resulted in a new replicator: the meme.^[150] Ideas could be exchanged quickly and passed down the generations. Cultural evolution quickly outpaced biological evolution, and history proper began. Between 8500 and 7000 BC, humans in the Fertile Crescent in Middle East began the systematic husbandry of plants and animals: agriculture.^[151] This spread to neighboring regions, and 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.^[152] However, among those civilizations that did adopt agriculture, the relative stability 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.^[145]:15 Additional civilizations quickly arose in ancient Egypt, at the Indus River valley and in China. 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 fought for territory and resources. Empires soon began to develop. By around 500 BC, there were advanced civilizations in the Middle East, Iran, India, China, and Greece, at times expanding, at times entering into decline.^[145]:3 In 221 BC, China became a single polity that would grow to spread its culture throughout eastern Asia, and it has remained the most populous nation in the world. The fundamentals of the Western world were largely shaped by the ancient Greco-Roman culture. The Roman Empire was Christianized by Emperor Constantine in the early fourth century and declined by the end of the fifth. Beginning with the seventh century, Christianization of Europe began. In 610, Islam was founded and quickly became the dominant religion in western Asia. In 1054 AD the Great Schism between the Roman Catholic Church and the Eastern Orthodox Church led to the prominent cultural differences between Western and Eastern Europe.

In the fourteenth century, the Renaissance began in Italy with advances in religion, art, and science.^{[145]:317–319} At that time the Christian Church as a political entity lost much of its power. In 1492, Christopher Columbus reached the Americas, initiating great changes to the new world. European civilization began to change beginning in 1500, leading to the scientific and industrial revolutions. That continent began to exert political and cultural dominance over human societies around the planet, a time known as the Colonial era (also see Age of Discovery).^{[145]:295–299} In the eighteenth century a cultural movement known as the Age of Enlightenment further shaped the mentality of Europe and contributed to its secularization. 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 settle disputes peacefully. After failing to prevent World War II, mankind's bloodiest conflict, it was replaced by the United Nations. After the war, many new states were formed, declaring or being granted independence in a period of decolonization. The United States and Soviet Union became the world's dominant superpowers for a time, and they held an often-violent rivalry known as the Cold War until the dissolution of the latter. In 1992, several European nations joined 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 conflict and cooperation.

Recent events

Astronaut Bruce McCandless II outside of the space shuttle Challenger in 1984

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 recently, human-caused climate change 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 known planets in the solar system, with some (such as Voyager) having left the solar system. The Soviet Union and the United States were the earliest leaders in space exploration in the 20th century. Five space agencies, representing over fifteen countries,^[153] have worked together to build the International Space Station. Aboard it, there has been a continuous human presence in space since 2000.^[154] The World Wide Web began in the 1990s, and since then has become an indispensable source of information in the developed world.