During the long interval of the Precambrian Era (which includes approximately 90% of geologic time and includes three Eons: the Hadean, the Archean and the Proterozoic) the only inhabitants of the Earth were simple microscopic organisms, many of them comparable in size and complexity to modern-day bacteria.
The conditions under which these organisms lived differed greatly from those prevailing today, but the mechanisms of evolution were the same.
There was also an outgassing of molecules such as water, methane, ammonia, hydrogen, nitrogen, and carbon dioxide, which formed the early atmosphere of the Earth.
The initial steam atmosphere was made of water from comets and hydrated minerals from volcanic eruptions.
In addition, mineral grains called zircon from sedimentary rocks from west-central Australia have recently been reported with an age of about 4400 million years.
While we don't know exactly how the Earth was formed, we have some really good evidence to support the following hypothesis: The Earth began as part of the .
Most of the meteorite craters which are evident on Earth are much younger.
Life arose on Earth during the early Archaean, as indicated by the appearance of fossil bacteria in rocks thought to be about 3500 million years old.Prokaryotes, for example are the most successful and abundant organisms on Earth, in both numbers and biomass and still make up to 90% of the total weight of living things.For further information on each geological period or special life forms mentioned, there are links to other websites about this time.During this period the heavier molten iron sank to the middle of the newly forming Earth, to become the core.The lighter material rose to the surface, the lightest of all becoming the crust on the surface.The formation of the planets (including Earth), asteroids, meteors, comets and the central sun - formed through the tendency of matter to clump together, until finally there were substantial bodies, the planets and their moons, sweeping up all the 'left-over' bits in their orbits.