Fossil find points to life on Earth 3.7 bn years ago

Sydney (AFP) - Life on Earth is even older than we thought, Australian scientists said Thursday as they unveiled fossils dating back a staggering 3.7 billion years.

The tiny structures -- called stromatolites -- were found in ancient rock along the edge of Greenland's ice cap, and were 220 million years older than the previous record holders.

They show that life emerged fairly shortly -- in geological terms -- after Earth was formed some 4.5 billion years ago, said lead researcher Allen Nutman of the University of Wollongong.

And, he added, they offer hope that very basic life may at one point have existed on Mars.

"This discovery represents a new benchmark for the oldest preserved evidence of life on Earth," Professor Martin Julian Van Kranendonk, a geology expert at the University of New South Wales and study co-author, said in a statement.

The structure and geochemistry of the rock in which they were found provided clues to a biological origin for the microfossils, he said, which in turn "points to a rapid emergence of life on Earth".

The one-to-four centimetre (0.4-1.6 inch) high Isua stromatolites were exposed after the melting of a snow patch in the Isua Greenstone Belt of Greenland.

Stromatolites are formed when microorganisms, such as certain kinds of bacteria, trap bits of sediment together in layers. These layers build up over time to create solid rock.

These rocks themselves were never alive, but their existence suggest that the very simple single-cell organisms that made them were present on Earth hundreds of millions of years earlier than previously thought, said the team.

- Life on Mars? -

Another scientist was more skeptical.

Structures that look just like stromatolites can form without the presence of any living organism, Abigail Allwood of the California Institute of Technology wrote in a comment on the study.

"The interpretation of stromatolite-like structures has been notoriously difficult in Earth's oldest rocks," she wrote, and predicted the study findings would "spark controversy".

"The case for a biological origin of the Greenland structures is limited by the information available in the tiny outcrop," she argued.

But Vickie Bennett from the Australian National University, who also worked on the study, said the research "turns the study of planetary habitability on its head".

"Rather than speculating about potential early environments, for the first time we have rocks that we know record the conditions and environments that sustained early life," she said.

The discovery could help the hunt for life on Mars, considered the most likely location for microbial life-forms among other planets in the Solar System.

The Red Planet is believed to have once run with water and had an atmosphere, which together with warmth, could provide the right conditions for bacterial life.

"The significance for Mars is that 3,700 million years ago, Mars was probably still wet and probably still had oceans and so on, so if life develops so quickly on Earth to be able to form things like stromatolites -- it might be more easy to detect signs of life on Mars," Nutman told AFP.

"Instead of looking at just the chemical signature, we might be able to see things like stromatolites in images (from Mars) sent back to Earth."

The earliest evidence of life on Earth ahead of the Greenland discovery came from near-3.5 million-year-old stromatolites found in western Australia in 2006.

The new findings were published in the journal Nature.