"Our evidence is among the oldest evidence for life found so far," said Hubert Staudigel, a research geophysicist at the Scripps Institution of Oceanography at the University of California, San Diego.

Writing in Friday's issue of the journal Science, Staudigel and colleagues said they found tell-tale tubes in "pillow lava" in South Africa's Barberton Greenstone Belt, which was formed underwater in the oceanic crust but is now above ground.

"This area within the oceanic crust is a favorable place for the origin of life," Staudigel said. "It offers relatively easy access to seawater and volcanic environments such as deep-sea hydrothermal systems - including a wide range of catalysts that are required in the origin of life."

So far, no one has found indisputable remains of the very earliest life. The main problem is that most of these ancient rocks have been through geologic processes of heating, pressure and folding that would destroy any evidence.

In 1996 researchers found rocks in Greenland that they dated to 3.85 billion years ago containing what they believe were traces of bacteria. In 1999 a team found remains of 2.7 billion-year-old algae in Australian shale.

In Friday's report, the researchers said they found carbon along the inside of the tubes that may represent organic material left behind by microbes.

At the time, "there were no plants or animals to eat," Staudigel said. "So to make a living these microbes adapted to eating volcanic rock. That's all there was."

Archaea still exist. They make up many of the so-called extremophiles - organisms found in extreme environments such as undersea vents, hot sulfur springs and Antarctica.