The oldest-known fossils in rock have been found in Australia. One set of fossils found in Western Australia is dated between 3.45 and 3.55 billion years old. They show evidence of layered mounds of limestone sediment called stromatolites, which were formed by primitive microorganisms similar to blue-green algae called cyanobacteria. Scientists know that stromatolites exist today. The fossils look amazingly like the stromatolites from the shallow waters off the coast of modern Australia.

There are other contenders for the oldest-known fossils. Tiny, simple cells have also been found in ancient cherts (crystalline-rich sedimentary rocks) from Australia, and there are similar ones in Africa. These cells are preserved by the silica from the chert, and appear to show a cell wall of some kind.