The earliest evidence of life on Earth comes from fossils discovered in Western Australia that date back to about 3.53.53, point, 5 billion years ago. These fossils are of structures known as stromatolites, which are, in many cases, formed by the growth of layer upon layer of single-celled microbes, such as cyanobacteria. (Stromatolites are also made by present-day microbes, not just prehistoric ones.)