By Paul Rincon
BBC News Online science staff
Geologists have discovered microscopic burrows where some of Earth's earliest lifeforms bored their way into volcanic glass 3.5 billion years ago.
The microbes broke down volcanic glass to extract nutrients
The tubes, from rocks in South Africa's Barberton Greenstone Belt, retain traces of organic carbon left behind by the microorganisms, the authors say.
The microbes etched their way into rocks that formed as lava oozed out across a sea floor in Archaean times.
An international team published details of the work in the journal Science.
Harald Furnes, of the University of Bergen, Norway, and his colleagues found tubular structures in the glassy rind on so-called pillow lavas.
Researchers have previously identified similar structures called microtubules in present-day pillow lavas, where they were interpreted as the trails left by rock-munching microorganisms.
Modern examples have been shown to contain nucleic acids and increased traces of carbon and nitrogen, the key elements of life.
The team found ancient microtubules in the Barberton rock which are on average about four micrometres in width and 50 micrometres in length (a micrometre is a millionth of a metre).
In the inner walls of these microtubules, the geologists found traces of carbon, which the authors claim is organic.
Digging a hole
"The volcanic rock itself has very little carbon. It's concentrated in these little specks inside the tubules due to life processes," co-author Professor Karlis Muelenbachs, of the University of Alberta in Edmonton, Canada, told BBC News Online.
But any fossil evidence of the microbes themselves has long since disappeared: "They're good and cooked," Professor Muelenbachs explained.
The rocks were altered after they were formed but the burrows were preserved
The rock has not lain unaltered since Archaean times: "It was pushed down into the Earth and cooked at well over 300 degrees (Celsius) for millions of years."
As the rocks were squeezed and heated, they "metamorphosed". Volcanic glass transformed into a mineral called chlorite. But this was a subtle conversion, say the researchers, which preserved original structures such as the microtubules.
The team claims observations of the fine structure of the burrows show they were overgrown by chlorite after being pushed beneath the Earth.
This is a key piece of evidence which the researchers rely on to show that the tubules are much older than the metamorphism which transformed the Archaean rock. It rules out the suggestion that the tubules could have been made more recently.
"It's like the London Underground," said Dr Roger Buick, associate professor of astrobiology at the University of Washington in Seattle, US. "The holes were dug 100 years ago, but they were made in 50 million-year-old rocks. Dating the rocks in London doesn't tell you the age of the London Underground."
In 1999, Danish geologist Minik Rosing presented evidence of a chemical signature for primitive lifeforms in 3.7-billion-year-old rocks from Isua, Greenland.