The Archaean Eon
When was it?
3.8 to 2.5 Billion years ago.
What was going on?
The surface of the earth finally stabilizes enough for life to begin to evolve.
What were the continents like?
The continents, as we know them, didn't exist yet; the surface of the earth had cooled off,
but was still much more geologically active than it is today. The continents are still
forming.
What are sediments from the Archaean like?
Archaean sediments appear in cratons -- very old, very stable parts of the crust. Also,
structures called stromatolites appear, which are very early fossils indicating threadlike
bacteria.
What kind of life existed?
Blue-green algae, and probably other simple bacteria and bacteria-like organisms; these
critters fall into the category called prokaryotes, which are single-celled organisms
lacking a defined nucleus and other infracellular organs. Similar forms of life still
exist today.