3.5-Billion-Year-Old Lava Yields Signs of Early Life
for National Geographic News
|April 22, 2004|
Earth's earliest organisms may have liked it hot, according to scientists. New fossils bolster claims that early life may have been linked to volcanic environments.
Tiny bacteria-size filaments and tubes have been found in 3.5-billion-year-old lava deposits from South Africa. They are strong evidence that ancient microbes ate their way into the glassy rock as it cooled deep on the ocean floor, researchers write in tomorrow's edition of the journal Science.
"This is by far the best evidence I've seen for extremely early life on Earth," commented Martin Fisk, oceanographer at Oregon State University in Corvallis. The style of the pits and fossilized tubes in these ancient pillow lavas are identical in appearance to the marks left behind by bacteria in lava that cools today on the ocean floor, Fisk said. "Pillow lava" is the term used for groups of rounded masses of lava formed underwater.
There are a number of other pieces of evidence of life collected from extremely old rocks, but these have been vigorously debated. Possible examples of the earliest bacterial fossils are from 3.5-billion-year-old rocks from Western Australia. These examples, though, are not universally accepted as valid.
In 1996 scientists argued that they had found chemical evidence of life in 3.8-billion-year-old rocks from western Greenland. The evidence came from pockets of a type of carbon, known as light carbon, which living organisms today accumulate.
Now geologist Harald Furnes of the University of Bergen in Norway and colleagues in North America and South Africa may have found the strongest evidence so far that our planet was teeming with primordial life 3.5 billion years agothough perhaps not in the most obvious places.
Scientists already knew that some kinds of bacteria are able to munch their way into man-made glass, dissolving it to extract nutrients. Then in the early 1990s Furnes and his team found evidence from deep-ocean drilling projects that bacteria also dig their way into naturally formed glassy rocks.
These rocks form in the top few hundred meters of the ocean floor, said Furnes's co-author, geochemist Karlis Muehlenbachs of the University of Alberta in Edmonton, Canada. "Basalt lava chills instantly into glass when it comes into contact with cold water," Muehlenbachs said.
To back up their claims, the scientists used an x-ray technique and found traces of organic carbon, phosphorous, and nitrogenchemical signatures of lifein the tiny bacterial burrows but not in the surrounding rock. Furthermore, researchers have been unable to reproduce the same markings using chemical processes alone.
"These marks provide unmistakable evidence that organisms live in these [modern] rocks," agreed Oregon State's Fisk, who was not involved in the new study.
The next logical step was to look for the same evidence of life in older deposits, said Muehlenbachs, and the scientists have subsequently found similar markings in 100-million-year-old volcanic rock from Cyprus, and 430-million-year-old rocks from Norway.
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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