Bacteria have existed from very early in the history of life on Earth. Bacteria fossils discovered in rocks date from at least the Devonian Period (419.2 million to 358.9 million years ago), and there are convincing arguments that bacteria have been present since early Precambrian time, about 3.5 billion years ago. Bacteria were widespread on Earth at least since the latter part of the Paleoproterozoic, roughly 1.8 billion years ago, when oxygen appeared in the atmosphere as a result of the action of the cyanobacteria. Bacteria have thus had plenty of time to adapt to their environments and to have given rise to numerous descendant forms.
The nature of the original predecessor involved in the origin of life is subject to considerable speculation. It has been suggested that the original cell might have used RNA as its genetic material, since investigations have shown that RNA molecules can have numerous catalytic functions. The Bacteria and Archaea diverged from their common precursor very early in this time period. The two types of prokaryotes tend to inhabit different types of environments and give rise to new species at different rates. Many Archaea prefer high-temperature niches. One major branch of the archaeal tree consists only of thermophilic species, and many of the methanogens in another major branch can grow at high temperatures. In contrast, no major eubacterial branch consists solely of thermophiles. Both Bacteria and Archaea contain members that are able to grow at very high temperatures, as well as other species that are able to grow at low temperatures. Another prominent difference is that bacteria have widely adapted to aerobic conditions, whereas many archaea are obligate anaerobes. No archaea are obligately photosynthetic. Perhaps the archaea are a more primitive type of organism with an impaired genetic response to changing environmental conditions. A limited ability to adapt to new situations could restrict the archaea to harsh environments, where there is less competition from other life-forms.
