Paleobiological Perspectives on Early Microbial Evolution

Microfossils, stromatolites, and chemical biosignatures indicate that Earth became a biological planet more than 3.5 billion years ago, making most of life''s history microbial. Proterozoic rocks preserve a rich record of cyanobacteria, including derived forms that differentiate multiple cell types. Stromatolites, in turn, show that microbial communities covered the seafloor from tidal flats to the base of the photic zone. The Archean record is more challenging to interpret, particularly on the question of cyanobacterial antiquity, which remains to be resolved. In the late Neoproterozoic Era, increasing oxygen and radiating eukaryotes altered the biosphere, with planktonic algae gaining ecological prominence in the water column, whereas seaweeds and, eventually, animals spread across shallow seafloors. From a microbial perspective, however, animals, algae, and, later, plants simply provided new opportunities for diversification, and, to this day, microbial metabolisms remain the only essential components of biogeochemical cycles.We live on a planet that records its own history, encrypted in the physical and chemical features of sedimentary rocks (Knoll 2003). Part of this history is biological; as appreciated by every child who has visited a natural history museum, bones and shells furnish a remarkable chronicle of animal evolution, complete with dinosaurs, trilobites, and other evocative taxa. The fossil record of animals extends nearly 600 million years into the past, but comparative biology makes it clear that diverse microorganisms populated our planet long before animals first evolved. The Earth itself is >4.5 billion years old, and the known sedimentary record begins with highly metamorphosed sedimentary rocks deposited ∼3.8 billion years ago. To what extent do Earth''s older sedimentary rocks provide a direct and informative record of our planet''s deep evolutionary history?