Stromatolites are some of the oldest fossils on Earth. They were created by single-celled microbes that grew and divided to form large, living mats of cells on the seafloor. These mats were then covered by mud and other sediments, causing them to fossilize. The oldest stromatolites are some 3.5 billion years old – six times as old as anything that is recognisable as an animal in the fossil record.
Because stromatolites are so simple, it is possible for very similar structures to form in rocks that do not contain any fossilized life forms. Eozoon is an example of a rock structure that was once thought to have been stromatolites. However, we now know that Eozoon was formed by the heating and compression of rocks deep within the Earth, in a process known as metamorphosis.
Because they are fossils of simple, single-celled organisms, stromatolites are the kind of fossils that we might look for when searching for evidence of life on other planets, like Mars.