The earliest eon of geological time, in which there is the first evidence of life on earth. It follows the Hadean eon of pregeological time and extends from the time of the earliest known rocks, roughly 3900 million years ago, to the beginning of the Proterozoic eon, about 2500 million years ago. Rock formations called stromatolites, dated at 3500 million years old or older, are among the oldest of all fossil remains. They are thought to have been produced by the activities of microbial mats of filamentous purple and green bacteria. These prokaryotes performed photosynthesis anaerobically, perhaps using hydrogen sulphide as an electron donor instead of water. Some of their descendants evolved the ability to use water as an electron donor, producing oxygen as a by-product, and eventually brought about the change in atmospheric conditions necessary for aerobic life.