World's Oldest Rock
Then Furnes's team next turned their attention to "textbook examples" of some of the world's oldest rocks, he said. These pillow lava deposits are from the Barberton Greenstone Belt in South Africa, thought to represent thick deposits of oceanic crust laid down between 3,480,000 and 3,220,000 years ago.
"We studied the glassy parts of the rock and found little relics that look almost identical to what you see on the modern ocean floor," Muehlenbachs said (though the prehistoric tiny pits have fossilized and are filled with a mineral called titanite).
The team also detected the presence of so-called light carbon in these pits and furrows eaten out by ancient bacteria. That indicator of metabolic activity was not found deeper in the same rocks, where bacterial burrows were not found.
Most geologists searching for signs life in the oldest rocks have been looking at sedimentary deposits, not volcanic rock, Muehlenbachs said. A more established theory on the evolution of life holds that organisms first developed in warm, mineral-rich tidal pools, where sedimentary deposits might accumulate, he said.
"Our data comes from a geological setting that has not been extensively explored in the search for early life on Earth," writes Muehlenbachs and his co-authors. However, "there is increasing evidence that early life may have been connected to volcanic environments, such as the deep-sea hydrothermal vents," they said.
Four years ago a study in the journal Nature reported bacteria-size pits and markings in 3.2-billion-year-old sulfide deposits, likely formed around superheated, sulfurous, underwater hot springs, known as black smokers or hydrothermal vents.
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