Stromatolites were one of the earliest multi-cellular life forms to appear on Earth. This type of stromatolite was made by cyanobacteria (blue-green algae) that formed sticky mats in shallow waters. Sediment in the water stuck to the mats and built up layers over time. Eventually these layers formed mounds we call stromatolites.
Cyanobacteria are photosynthetic and, like plants, produce oxygen. Oxygen produced by early stromatolites changed the Earth’s atmosphere. With the addition of oxygen, new life forms could become established.
Precambrian, 800 million years ago, Northern Territory, Australia