The Earth formed simultaneously with the other Solar System planets and the central Sun. Accretion of planetesimals produced a large body which assumed a spherical shape. Probably cool at the outset, this proto-Earth rapidly heated up, formed its metallic core within 100 million years, and was subjected to continuous impact bombardment by asteroids, comets, and meteorites. It may have had a molten exterior which quickly cooled to a crust. Very early in earth history, its Moon was produced from a glancing collision with another planetlike body. A second period of bombardment helped destroy the early crust. By about 3.8 billion years ago, rocks formed crusts of more silicic rocks embedded in a basaltic crustal layer that extended worldwide. Oceans were produced early, weathering attacked the crustal rocks to produce the first sedimentary rocks, and protocontinents began to form (from metamorphosis and melting of accumulated debris). These continental nuclei probably were moved about by processes akin to convection-driven plate tectonics. An early atmosphere consisted largely of nitrogen, with some carbon dioxide, ammonia, methane, and water. Those ingredients may have been converted to organic molecules which in turn organized into primitive one-celled bacteria about 3.85 b.y. ago. In time, living plant organisms developed the capability to photosynthesize solar energy, releasing oxygen as an end product, which gas gradually built up to present day levels, evolving more advanced life forms.
It's a small, small world...
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