The Antiquity Of Life
The oldest known fossils date from about 3.5 billion years ago, and have been found in Western Australia. Called stromatolites, these domelike rocks are formed today by photosynthetic organisms (cyanobacteria) that live in shallow waters. Large communities of cyanobacteria form microbial mats that trap sediment, providing a sturdy foundation on which another layer of bacteria can grow.
The search for older fossils has been frustrated by the scarcity of pristine rocks that have not been altered by high temperatures and pressures. However, evidence from geological deposits in Greenland dated at 3.87 billion years ago may point to the presence of life at an even earlier time. Since our planet itself is approximately 4.6 billion years old, and several hundred million years were needed for the surface to cool to "hospitable" temperatures, the origin of life must have been quite rapid compared to the vast span of terrestrial history.