4,125 million years ago. Still in its infancy, planet Earth has cooled, has almost no free oxygen, and is a very different world from the one we know today. No life forms are yet present. 3,750 million years ago. 3,375 million years ago.
Simple life continues to radiate.
3000 million years ago. The first photosynthetic bacteria appear. All life is restricted to the sea, which provides a fairly constant environment and protection from the sun's ultraviolet rays. With photosynthesis, the levels of atmospheric oxygen begin to rise. 2,625 million years ago.