A paradigm-shifting hypothesis has pushed back the time for the emergence of microbial life on Earth by 580 million years, suggesting that life began not in the sea but on land.

The new model is based on stromatolites  – round, multi-layered mineral structures that range from the size of golf balls to weather balloons and represent the oldest evidence that there were living organisms on Earth 3.5 billion years ago.

The team from the University of California-Santa Cruz and University of New South Wales in Sydney scoured the forbidding landscape of the Pilbara region of Western Australia looking for clues to how ancient microbes could have produced the abundant stromatolites that were discovered there in the 1970s. Scientists who believed life began in the ocean thought these mineral formations had formed in shallow, salty seawater, just like living stromatolites in the World Heritage-listed area of Shark Bay, which is a two-day drive from the Pilbara. But Tara Djokic, PhD student at University of New South Wales Sydney, discovered that the stromatolites had not formed in salt water but instead in conditions more like the hot springs of Yellowstone.

"What she (Djokic) showed was that the oldest fossil evidence for life was in fresh water. It's a logical continuation of life beginning in a freshwater environment," said David Deamer, an astrobiologist from UC Santa Cruz.

Djokic's discovery -- together with Martin Van Kranendonk, director of the Australian Centre for Astrobiology -- is described in the August issue of journal
Scientific American.

In Deamer's vision, ancient Earth consisted of a huge ocean spotted with volcanic land masses.

Rain would fall on the land, creating pools of fresh water that would be heated by geothermal energy and then cooled by runoff.

Some of the key building blocks of life, created during the formation of our solar system, would have fallen to Earth and gathered in these pools, becoming concentrated enough to form more complex organic compounds. The edges of the pools would go through periods of wetting and drying as water levels rose and fell. During these periods of wet and dry, lipid membranes would first help stitch together the organic compounds called polymers and then form compartments that encapsulated different sets of these polymers.

The membranes would act like incubators for the functions of life.

Deamer and his team believe the first life emerged from the natural production of vast numbers of such membrane-encased "protocells".

If life began on land, then Mars, which was found to have 3.65-billion-year-old hot spring deposits similar to those found in the Pilbara region of Australia, might be a good place to look, they noted.