History of Earth

The history of Earth covers approximately 4.55×10⁹ years, from its formation out of the solar nebula to the present. This article presents a broad overview of this period, summarizing the leading scientific theories for each time period. Due to the difficulty of grasping very large amounts of time, an analogy to a clock will be used, with midnight beginning at the formation of Earth and the next midnight occuring now. Each second on this "clock" represents approximately 53,000 years, and the Big Bang and origin of the universe took place almost three days ago.

Origin

Artist's impression of a protoplanetary disc forming around a binary star system.

Main articles: Solar system § Origin and age of the solar system, and Planet § Planetary formation

The formation of Earth occurred as part of the formation of the solar system. It existed initially as a large rotating cloud of dust and gas, composed of hydrogen and helium produced in the Big Bang, as well as heavier elements produced in numerous supernovas from stars long gone. Then, about 4.6×10⁹ years ago, maybe fifteen minutes or half an hour before midnight, a nearby star probably exploded as a supernova. This sent a shock wave towards the solar nebula, causing it to contract. Due to inertia, as the cloud rotated, it became a flat disc perpendicular to its axis of rotation. Most of the mass concentrated in the middle and began to heat up. Meanwhile, the rest of the disc began to break up into rings, with gravity causing matter to condense around dust particles. Small fragments collided to become larger fragments, including one collection approximately 150 million kilometers from the center; this would later be named Earth by some of the life forms that arose on its surface. As the Sun condensed and heated, fusion initiated and the solar wind cleared out most of the material in the disc that had not condensed into larger bodies.

Moon

Animation of Theia forming in Earth's L₅ point and then drifting into impact. The animation progresses in one-year steps making Earth appear not to move. The view is of the south pole.

Main articles: Moon § Origin and history, and Giant impact hypothesis

The origin of the Moon is still uncertain, although considerable evidence exists for the giant impact hypothesis. Earth was probably not the only planet forming 150 million kilometers from the Sun. It is hypothesized that another collection occurred 150 million kilometers from from both the Sun and the Earth, at the fourth Lagrange point; this planet (named Theia is thought to have been smaller than the current Earth, probably about the size and mass of Mars. Its orbit may have initially stable, but destabilized as Earth increased its mass. It swung back and forth, relative to Earth, until finally, 4.553×10⁹ years ago (perhaps 12:33 a.m. on our clock), it collided at a low oblique angle. The low speed and angle were not enough to destroy Earth, but a large portion of its crust was ejected. Heavier elements from Theia sank to Earth's core, while the remaining material and ejecta condensed into a single body within a couple weeks; it would become a more uniform, spherical body probably within a year. The impact also probably knocked the 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° and no recognizable seasons). It may have also sped up Earth's rotation and been instrumental in the origins of Earth's plate tectonics.

Early days: Hadean eon

The early Earth was very different than the Earth of today. There were no oceans and no oxygen in the atmosphere. It was bombarded by planetoids and other material left over from the formation of the solar system; this, combined with heat from radioactive breakdown, residual heat, and heat from the pressure of contraction likely caused the Earth to be initially fully molten. Heavier elements sunk to the center, while lighter ones rose to the surface, giving rise to Earth's various layers (see Structure of the Earth). What atmosphere it had from the residual solar nebula was driven off by the solar wind and Earth's heat. The surface slowly cooled, forming the solid crust probably within 200 million years (around 1 a.m. on our clock). Steam escaped from the crust, and more gases were released by volcanos, giving rise to the atmosphere. Additional water was brought by meteorite and comet collisions. Earth cooled, clouds formed, and rain gave rise to the oceans by about 700 million years (around 3:45 a.m. on our clock). The new atmosphere, Earth's second atmosphere, probably contained ammonia, methane, water vapor, carbon dioxide, and nitrogen, as well as smaller amounts of other gases. Any free oxygen would have been bound by hydrogen or minerals on the surface. Volcanic activity was high, and without an ozone layer, large amounts of ultraviolet radiation rained on the surface.

Beginnings of life

Main article: Origin of life

The high energy from volcanos, lightning, and ultraviolet radiation helped drive chemical reactions producing more complex molecules from simple molecules like methane and ammonia. Among these were many of the simple organic compounds that are the building blocks of life. As the amount of this "organic soup" increased, different molecules would react with one another, essentially randomly. Sometimes more complex molecules would result—perhaps clay provided a framework to collect and concentrate organic material. Some of the molecules could help speed up a chemical reaction. This continued for a very long time, with reactions occurring more or less randomly, until by chance there arose a truly bizarre molecule: the replicator. This molecule had the property of promoting the chemical reactions which produced a copy of itself, and evolution was born. Although the nature of this molecule and the details of these events are unknown, the broad principles have been reasonably well established. The replicator continued making copies of itself, but occasionally (or often) a copy would have an error and not precisely match the original. If the change destroyed the copying ability of the molecule, there would be no more copies and the line would "die out". Some changes might make the molecule replicate faster or more efficiently, and those "strains" might become more numerous. As choice building blocks ("food") in the organic soup became depleted, strains which could use different materials or perhaps block other strains became more numerous.

See also

• Timeline of evolution
• Timeline of the Big Bang
• Geologic timescale

References

• http://www.nasa.gov/worldbook/earth_worldbook.html
• http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/text_plan_1.html
• http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/text_plan_2.html
• http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/text_plan_4.html