The Archean
![[ archean ]](archean.gif)
The Earth formed about 4600 million years ago. By at least 3800 million
years ago there was life on earth in the form of bacteria and cyanobacteria
or blue-green algae. The mounds near the beach are stromatolites made of
sediments trapped by mats of cyanobacteria.
Stromatolites produced oxygen as a by-product of photosynthesis. In approximately
2800 million years enough oxygen was produced
to make it possible for other life forms to evolve. These new organisms
grazed on and competed with stromatolites so that the once dominant forms
now only survive where conditions prevent "higher" organisms from
getting a foothold--for example, in highly saline sabhka environments.
Painting by Peter Sawyer, Smithsonian Institute.
