The Earliest Cells 1. From studies of microfossils, scientists have determined that the earliest cells resembled living bacteria in their physical characteristics. The ancient bacteria were small, single-celled and showed little evidence of internal organization. These early cells appeared about 3.5 billion years ago. C. Unusual Modern Bacteria 1.
Some bacteria living today resemble the ancient microorganisms.
Methane-producing bacteria grow only under anaerobic conditions. They convert CO2 and H2 into methane. The methane-producing bacteria differ in some of the fundamental biochemical processes of metabolism. Photosynthetic bacteria like plants and algae use pigments to capture light and transform it into chemical energy. The increase in oxygen led to the formation of a layer of ozone, which afforded protection from the sun's ultraviolet radiation.