Introduction
The history of the Earth describes the most important events and fundamental stages in the development of the planet Earth from its formation 4.6 billion years ago to the present day. 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. Immense geological and biological changes have occurred during that time span.
The history of the Earth describes the most important events and fundamental stages in the development of the planet Earth from its formation 4.6 billion years ago to the present day. 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. Immense geological and biological changes have occurred during that time span.
Hadean and Archaean eons
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| Hadean eon called by Greeks. |
Starting with the Earth's formation by accretion from the solar nebula 4.54 billion years ago (4.54 Ga), the first eon in the Earth's history is called the Hadean. It lasted until the Archaean eon, which began 3.8 Ga. The oldest rocks found on Earth date to about 4.0 Ga, and the oldest detrital zircon crystals in some rocks have been dated to about 4.4 Ga, close to the formation of the Earth's crust and the Earth itself. Because not much material from this time is preserved, little is known about Hadean times, but scientists hypothesize at an estimated 4.53 Ga, 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.
During the Hadean, the Earth's surface was under a continuous bombardment by meteorites, and volcanism must have been severe due to the large heat flow and geothermal gradient. The detrital zircon crystals dated to 4.4 Ga show evidence of having undergone contact with liquid water, considered as proof that the planet already had oceans or seas at that time. 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.
By the beginning of the Archaean, the Earth had cooled significantly. It would have been impossible for most present day life forms to exist due to the composition of the Archaean atmosphere, which lacked oxygen and an ozone layer. Nevertheless it is believed that primordial life began to evolve by the early Archaean, with some possible fossil finds dated to around 3.5 Ga. Some researchers, however, 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.
Origin of Solar System
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| Origin of Solar System. |
The Solar System (including the Earth) formed from a large, rotating cloud of interstellar dust and gas called the solar nebula, orbiting the Milky Way's galactic center. It was composed of hydrogen and helium created shortly after the Big Bang 13.7 Ga and heavier elements ejected by supernovas. About 4.6 Ga, the solar nebula began to contract, possibly due to the shock wave of a nearby supernova. Such a shock wave would have also caused the nebula to rotate and gain angular momentum.
As the cloud began to accelerate its rotation, gravity and inertia flattened it into a protoplanetary disk oriented perpendicularly to its axis of rotation. Most of the mass concentrated in the middle and began to heat up, but small perturbations due to collisions and the angular momentum of other large debris created the means by which protoplanets up to several kilometres in length began to form, orbiting the nebular center.
The infall of material, increase in rotational speed and the crush of gravity created an enormous amount of kinetic energy at the center. Its inability to transfer that energy away through any other process at a rate capable of relieving the build-up resulted in the disk's center heating up. Ultimately, nuclear fusion of hydrogen into helium began, and eventually, after contraction, a T Tauri star ignited to create the Sun. Meanwhile, as gravity caused matter to condense around the previously perturbed objects outside the gravitational grasp of the new sun, dust particles and the rest of the protoplanetary disk began separating into rings. Successively larger fragments collided with one another and became larger objects, ultimately becoming protoplanets. These included one collection about 150 million kilometers from the center: Earth.
The planet formed about 4.54 billion years ago (within an uncertainty of 1%) and was largely completed within 10–20 million years. The solar wind of the newly formed T Tauri star cleared out most of the material in the disk that had not already condensed into larger bodies.
Computer simulations have shown that planets with distances equal to the terrestrial planets in our solar system can be created from a protoplanetary disk. The now widely accepted nebular hypothesis suggests that the same process, which gave rise to the solar system's planets, produces accretion disks around virtually all newly forming stars in the universe, some of which yield planets.
Earth's core and first atmosphere
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| Earth's Core |
The Proto-Earth grew by accretion, until the inner part of the protoplanet was hot enough to melt the heavy, siderophile metals. Such liquid metals, with now higher densities, began to sink to the Earth's center of mass. 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.
During the accretion of material to the protoplanet, a cloud of gaseous silica must have surrounded the Earth, to condense afterwards as solid rocks on the surface. What was left surrounding the planet was an early atmosphere of light (atmophile) elements from the solar nebula, mostly hydrogen and helium, but the solar wind and Earth's heat would have driven off this atmosphere.
This changed when Earth accreted to about 40% its present radius, and gravitational attraction retained an atmosphere which included water.
The giant impact hypothesis
The Earth's relatively large natural satellite, the Moon, is unique. During the Apollo program, rocks from the Moon's surface were brought to Earth. Radiometric dating of these rocks has shown the Moon to be 4527 ± 10 million years old, about 30 to 55 million years younger than other bodies in the solar system. Another notable feature is the relatively low density of the Moon, which must mean it does not have a large metallic core, like all other terrestrial bodies in the solar system.
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| The giant impact hypothesis |
The Moon has a bulk composition closely resembling the Earth's mantle and crust together, without the Earth's core. This has led to the giant impact hypothesis, the idea that the Moon was formed during a giant impact of the proto-Earth with another protoplanet by accretion of the material blown off the mantles of the proto-Earth and impactor.
The impactor, sometimes named Theia, is thought to have been a little smaller than the current planet Mars. It could have formed by accretion of matter about 150 million kilometres from the Sun and Earth, at their fourth or fifth Lagrangian point. Its orbit may have been stable at first, but destabilized as Theia's mass increased due to the accretion of matter. Theia oscillated in larger and larger orbits around the Lagrangian point until it finally collided with Earth about 4.533 Ga.
Models reveal that when an impactor this size struck the proto-Earth at a low angle and relatively low speed (8–20 km/sec), much material from the mantles and crusts of the proto-Earth and the impactor was ejected into space, where much of it stayed in orbit around the Earth. This material would eventually form the Moon. However, the metallic cores of the impactor would have sunk through the Earth's mantle to fuse with the Earth's core, depleting the Moon of metallic material. The giant impact hypothesis thus explains the Moon's abnormal composition. 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.
The radiometric ages show the Earth existed already for at least 10 million years before the impact, enough time to allow for differentiation of the Earth's primitive mantle and core. Then, when the impact occurred, only material from the mantle was ejected, leaving the Earth's core of heavy siderophile elements untouched.
The impact had some important consequences for the young Earth. It released an enormous amount of energy, causing both the Earth and Moon to be completely molten. Immediately after the impact, the Earth's mantle was vigorously convecting, the surface was a large magma ocean. The planet's first atmosphere must have been completely blown away by the impact. The impact is also thought to have changed Earth’s axis to produce the large 23.5° axial tilt that is responsible for Earth’s seasons (a simple, ideal model of the planets’ origins would have axial tilts of 0° with no recognizable seasons). It may also have sped up Earth’s rotation.
The First Continent
Mantle convection, the process that drives plate tectonics today, is a result of heat flow from the core to the Earth's surface. It involves the creation of rigid tectonic plates at mid-oceanic ridges. These plates are destroyed by subduction into the mantle at subduction zones. The inner Earth was warmer during the Hadean and Archaean eons, so convection in the mantle must have been faster. When a process similar to present day plate tectonics did occur, this would have gone faster too. Most geologists believe that during the Hadean and Archaean, subduction zones were more common, and therefore tectonic plates were smaller.
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| The First Continent on the Earth |
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. It is, however, assumed that this crust must have been basaltic in composition, like today's oceanic crust, because little crustal differentiation had yet taken place. The first larger pieces of continental crust, which is a product of differentiation of lighter elements during partial melting in the lower crust, appeared at the 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 Archaean crust form the cores around which today's continents grew.
The oldest rocks on Earth are found in the North American craton of Canada. They are tonalites 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.
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 Archaean. 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. The alternation between greenstone belts and TTG-complexes is interpreted as a tectonic situation in which small proto-continents were separated by a thorough network of subduction zones
Origin of Life
The details of the origin of life are unknown, but the basic principles have been established. There are two schools of thought about the origin of life. One suggests that organic components arrived on Earth from space (, while the other argues that they originated on Earth. Nevertheless, both schools suggest similar mechanisms by which life initially arose.
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| Origin of Life NASA Identifies Carbon-rich Molecules in Meteors. |
If life arose on Earth, the timing of this event is highly speculative—perhaps it arose around 4 Ga. It is possible that, as a result of repeated formation and destruction of oceans during that time period caused by high energy asteroid bombardment, life may have arisen and been extinguished more than once.
In the energetic chemistry of early Earth, a molecule gained the ability to make copies of itself — a replicator. (More accurately, it promoted the chemical reactions which produced a copy of itself.) The replication was not always accurate: some copies were slightly different from their parent.
If the change destroyed the copying ability of the molecule, the molecule did not produce any copies, and the line “died out”. On the other hand, a few rare changes might have made the molecule replicate faster or better: those “strains” would become more numerous and “successful”. This is an early example of evolution on abiotic material. The variations present in matter and molecules combined with the universal tendency for systems to move towards a lower energy state allowed for an early method of natural selection. As choice raw materials (“food”) became depleted, strains which could utilize different materials, or perhaps halt the development of other strains and steal their resources, became more numerous.
The nature of the first replicator is unknown because its function was long since superseded by life’s current replicator, DNA. Several models have been proposed explaining how a replicator might have developed. Different replicators have been posited, including organic chemicals such as modern proteins, nucleic acids, phospholipids, crystals, or even quantum systems. There is currently no way to determine whether any of these models closely fits the origin of life on Earth.
One of the older theories, one which has been worked out in some detail, will serve as an example of how this might occur. The high energy from volcanoes, lightning, and ultraviolet radiation could help drive chemical reactions producing more complex molecules from simple compounds such as methane and ammonia.Among these were many of the simpler organic compounds, including nucleobases and amino acids, which are the building blocks of life. As the amount and concentration of this “organic soup” increased, different molecules reacted with one another. Sometimes more complex molecules would result—perhaps clay provided a framework to collect and concentrate organic material.
Certain molecules could speed up a chemical reaction. All this continued for a long time, with reactions occurring at random, until by chance it produced a replicator molecule. In any case, at some point, the function of the replicator was superseded by DNA; all known life (except some viruses and prions) use DNA as their replicator, in an almost identical manner.
Modern life has its replicating material packaged inside a cellular membrane. It is easier to understand the origin of the cell membrane than the origin of the replicator, because a cell membrane is made of phospholipid molecules, which often form a bilayer spontaneously when placed in water. Under certain conditions, many such spheres can be formed.
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| Numerous theories for the Origin of Life |
The prevailing theory is that the membrane formed after the replicator, which perhaps by then was RNA (the RNA world hypothesis), along with its replicating apparatus and other biomolecules. Initial protocells may have simply burst when they grew too large; the scattered contents may then have recolonized other “bubbles”. Proteins that stabilized the membrane, or that later assisted in an orderly division, would have promoted the proliferation of those cell lines.
RNA is a likely candidate for an early replicator, because it can both store genetic information and catalyze reactions. At some point DNA took over the genetic storage role from RNA, and proteins known as enzymes took over the catalysis role, leaving RNA to transfer information, synthesize proteins and regulate the process. There is increasing belief that these early cells evolved in association with undersea volcanic vents known as black smokers deep rocks.
It is believed that of this multiplicity of protocells, only one line survived. Current phylogentic evidence suggests that the last universal common ancestor lived during the early Archean eon, perhaps roughly 3.5 Ga or earlier. This “LUCA” cell is the ancestor of all life on Earth today. It was probably a prokaryote, possessing a cell membrane and probably ribosomes, but lacking a nucleus or membrane-bound organelles such as mitochondria or chloroplasts.
Like all modern cells, it used DNA as its genetic code, RNA for information transfer and protein synthesis, and enzymes to catalyze reactions. Some scientists believe that instead of a single organism being the last universal common ancestor, there were populations of organisms exchanging genes in lateral gene transfer.
Proterozoic eon
The Proterozoic is the eon of Earth's history that lasted from 2.5 Ga to 542 Ma. In this time span, the cratons grew into continents with modern sizes. For the first time plate tectonics took place in a modern sense. 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 end of 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.
Origin of the ozone layer
An oxygen-rich atmosphere had two principal advantages for life. Organisms not using oxygen for their metabolism, such as anaerobe bacteria, base their metabolism on fermentation. The abundance of oxygen makes respiration possible, a much more effective energy source for life than fermentation. The second advantage of an oxygen-rich atmosphere is that oxygen forms ozone in the higher atmosphere, causing the emergence of the Earth's ozone layer. The ozone layer protects the Earth's surface from ultraviolet radiation, which is harmful for life. Without the ozone layer, the development of more complex life later on would probably have been impossible.
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| Ozone Layer |
The natural evolution of the Sun made it progressively more luminous during the Archaean and Proterozoic eons; the Sun's luminosity increases 6% every billion years. 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 all cratons show that about 2.3 Ga, the Earth underwent its first big ice age (the Makganyene ice age). Some scientists suggest this and following Proterozoic ice ages were so severe that the planet was totally frozen over from the poles to the equator, a hypothesis called Snowball Earth. Not all geologists agree with this scenario and older, Archaean ice ages have been postulated, but the ice age 2.3 Ga is the first such event for which the evidence is widely accepted.
The ice age around 2.3 Ga could have been directly caused by the increased oxygen concentration in the atmosphere, which caused the decrease of methane (CH4) in the atmosphere. Methane is a strong greenhouse gas, but with oxygen it reacts to form CO2, a less effective greenhouse gas. 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.
Oxygen
The first cells were likely heterotrophs, using surrounding organic molecules (including those from other cells) as raw material and an energy source. As the food supply diminished, a new strategy evolved in some cells. Instead of relying on the diminishing amounts of free-existing organic molecules, these cells adopted sunlight as an energy source.
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| The Oxygen revolution. |
Estimates vary, but by about 3 Ga, something similar to modern oxygenic photosynthesis had probably developed, which made the sun’s energy available not only to autotrophs but also to the heterotrophs that consumed them. This type of photosynthesis, which became by far the most common, used the abundant carbon dioxide and water as raw materials and, with the energy of sunlight, produced energy-rich organic molecules (carbohydrates).
Moreover, oxygen was released as a waste product of the photosynthesis. At first, it became bound up with limestone, iron, and other minerals. There is substantial proof of this in iron-oxide rich layers in geological strata that correspond with this period. The reaction of the minerals with oxygen would have turned the oceans green. When most of the exposed readily reacting minerals were oxidized, oxygen finally began to accumulate in the atmosphere.
Though each cell only produced a minute amount of oxygen, the combined metabolism of many cells over a vast time transformed Earth’s atmosphere to its current state. Among the oldest examples of oxygen-producing lifeforms are fossil stromatolites. This was Earth’s third atmosphere.
Some of the oxygen was stimulated by incoming ultraviolet radiation to form ozone, which collected in a layer near the upper part of the atmosphere. The ozone layer absorbed, and still absorbs, a significant amount of the ultraviolet radiation that once had passed through the atmosphere. It allowed cells to colonize the surface of the ocean and eventually the land: without the ozone layer, ultraviolet radiation bombarding land and sea would have caused unsustainable levels of mutation in exposed cells.
Photosynthesis had another, major, and world-changing impact. Oxygen was toxic; probably much life on Earth died out as its levels rose in what is known as the "oxygen catastrophe". 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.
