The oldest known fossils are suspected to be bacteria, ancestors of the modern prokaryotes.
Prokaryotes were the first organisms when the atmosphere of the new Earth was anoxic (without oxygen) and there was extensive volcanic activity. Bacterium‐like filaments have been found in 3.5 billion‐year‐old rocks in western Australia, and bacterium‐like spheroids of the same age occur in South Africa. Fossil stromatolites are present in many ancient sedimentary rocks worldwide. Stromatolites are layered columns or mushroom‐shaped domes a few inches wide and a foot or more in height that formed when bacterial mats (composed primarily of cyanobacteria growing in primeval ponds) trapped sediments. Together sediment and bacteria solidified into rock over the ensuing eons. Stromatolites are forming today in many places in the same way by descendants of the same organisms.
Data from molecular sequencing of DNA has changed—and continues to change—the ideas concerning bacterial relationships. At this time there is no clear consensus among microbiologists concerning the lineages among the prokaryotes. With only an estimated 10percent of the bacteria named, and the majority of those identified not yet studied in detail, the task appears formidable. Stay tuned.