What it was
A thin mat of microbes catches sand, then grows over it. More sand lands. More cells grow. Layer by layer, a stone mound rises from the water, built by a crowd too small to see one cell at a time.
That mound is a stromatolite (stro-mat-oh-lite), a layered structure built by microbes with sediment and minerals. It is no single giant creature. Cut through one and you can follow the thin layers left as the mat kept climbing toward the surface.
When and where
You can still stand beside growing stromatolites in shallow water in Australia. Some grow so slowly that a human lifetime adds only about the length of the last joint of your thumb. A mound can sit there for ages while its living surface builds upward.
Ancient examples turn up in rock. The Dresser Formation in Western Australia holds widely accepted traces of life about 3.48 billion years old. Its stromatolites are part of that evidence. The date does not mean scientists have ruled out all older life.
How we know
A mound's shape can fool you. Scientists check the minerals and look for carbon-based traces, asking whether the layers fit growth by living mats. Several clues need to agree. Rock can make some layered shapes without cells doing the building.
Still a mystery
Scientists still argue about some of the oldest claimed stromatolites. Heat and pressure can change buried rock until it looks like something grew there. Even a true microbial mound cannot name every cell that built it or prove they all made oxygen. The layers keep the work of a crowd, with many of the builders gone.