Ocean-dwelling microorganisms have been around for at least 3.8 billion years. Exactly when they made their way onto terra firma, however, has remained unclear. So far the oldest undisputed evidence for terrestrial life comes from 1.2-billion-year-old fossils from Arizona. But research described today in the journal Nature may push that date back by more than 1.4 billion years. According to the report, organic matter has been detected in soil that dates to between 2.6 and 2.7 billion years old.
Pennsylvania State University geochemist Hiroshi Ohmoto and his colleagues found so-called reduced carbon in samples of ancient soil from Mpumalanga Province, South Africa. They determined how old the soil was based on nearby mineral deposits of known age. Then, after ruling out the possibility that the carbon came from graphite crystals that formed long ago, or that it represents liquid hydrocarbons such as petroleum that were deposited after soil formation, the team was left with a single explanation for its origin. The carbon, they say, probably came from ancient bacterial mats. What is more, the mats are almost certainly terrestrial, judging from their carbon isotope values, which are quite different from those of the organic carbon found in marine sedimentary rock.
As to what kinds of microbes formed these mats, the team is not sure. But they might well have been photosynthetic blue-green algae. The discovery of such early terrestrial organisms, the researchers conclude, "may then imply that an ozone shield developed in the atmosphere more than 2.6 [billion years] ago."