The oldest known bacteria microfossil, stromatolites are structures made up of mat-like colonies of single-celled cyanobacteria. These microbial mats trap sediment, which becomes cemented by calcium carbonate secretions. The living mat grows upward and outward, forming layer upon layer over time. Its upward growth is limited by reaching tidal level. These cyanobacteria produced some of the first oxygen, which raised atmospheric levels and created the ozone layer. This blocked the Sun’s ultraviolet rays, making it safe and possible for life forms to evolve and colonize Earth. Our stromatolite fossils from Peru are estimated to be 2.3 billion years old, and those from Madagascar are known as kambaba jasper.