"A billion years or so after debris from a supernova started to coalesce into the rocky planet we now occupy, the possibility of lives like ours was set in motion by an as-yet-unidentified species of cyanobacteria. The cyanobacteria oxygenated an atmosphere otherwise poisonous to the forms of life that were to come. A good guess, according to some, is that these bacteria built stony habitations for themselves in wet environments. Today these now abandoned habitations are called stromatolites. Stromatolites, which accrete in shallow waters in the same way coral reefs do, are related to structures called thrombolites. Together, these two structures set a benchmark in Precambrian time."
Barry Lopez, Horizon
Thrombolites at Lake Clifton in Yalgorup National Park near Mandurah, Western Australia. Photograph by Ian Beattie.