It was the first underground movement in our planet's history: Primitive bacteria that lived 2.75 billion years ago built themselves caves to live in, according to a new study. Today, the traces they left behind are stoking hopes that similar life forms could exist on Mars.
Early Earth was a rough place to live — there was no oxygen in the atmosphere, or ozone layer to protect the surface from harmful ultraviolet radiation. Most life at the time protected itself by living in the oceans, or excreting thin films of material that acted as biological shields.
The earliest evidence of subterranean life was 1.5 billion years ago, until now. Birger Rasmussen of Curtin University of Technology in Bentley, Australia, and a team of researchers have found what they believe is evidence that bacteria nearly twice as old lived on the roofs of tiny hollows in lake and river sediments.
Just a centimeter or so tall and a couple of millimeters across, the 'caves' aren't much to look at. But they formed in a curious fashion — bacteria grew in air-tight sheets, which inflated like balloons as they trapped methane gas seeping up through the sediments.
Methane could have been food for the bacteria, or it could have simply opened space for water to flow through the cavity, carrying sulfur and other nutrients with it.
Over time the bacteria built up layer upon layer of material on the cave roofs, descending deeper into the sediment and forming the laminate fossils Rasmussen's team discovered.
"Cavity dwelling would've been a good way of escaping harsh radiation at the surface," Rasmussen said. "The cavities were protected and probably had water seeping through."
The team's finding, published this month in the journal Geology, strengthens a longstanding hypothesis that similar life forms may be hiding out on Mars. Today the Martian surface is desolate, and probably too harsh to support life — though much colder, it's similar in many ways to the young Earth. And newly uncovered hints of water provide hope that life underground is still hanging on.
"By 2.75 billion years ago, you've got life on land and in the sea, so life in a cave is not that shocking," Robert Rye of the University of Southern California said. "On the other hand, it's nice to know that we have evidence of life in a potentially good analogue environment for underground voids on Mars."