The evidence for earliest life on Earth comes not from fossils but from shifts in the carbon isotopes of preserved and altered carbonate minerals or rocks. In younger rocks, possible isolated bacterial fossils have been described. The metabolism of these early fossils has been uncertain, and some have suggested that they resemble cyanobacteria, implying at least some oxygenic photosynthesis and a rapid and early evolution of this biochemical pathway. One of the earliest indicators of more widespread life is in 3.4-billion-year-old rocks in Australia that contain abundant layered carbonaceous matter interpreted to be fossil microbial photsynthetic mats.
Tice and Lowe have examined the geochemistry of these early mats in order to decipher their origin and likely metabolism. The reduced oxidation state of iron and trace elements, notably cerium, indicates that the water column was highly anoxic. Tice and Lowe argue that uranium mobility was controlled by carbonate, not by oxygen as has been proposed. Together these data imply that the mats represent anoxygenic photosynthesis and a metabolism based on hydrogen gas as the source of electrons. — BH
Geology 34, 37 (2006).