Australasian Science: Australia's authority on science since 1938

Ancient Pilbara Microbes Push Back Date for First Life

By Stephen Luntz

Microbial ecosystems have been found in the oldest suitable rocks on the planet, 3.5 billion-year-old sedimentary sequences in the Pilbara.

The objects are microbially induced sedimentary structures (MISS) where combinations of bacteria flourish. They are able to respond to the conditions around them, growing towards the light and nutrients if a layer of sediment is dumped on top, and trapping and binding sand grains when in danger of being ground down by erosion.

MISS differ from the more famous stromotalites by being formed in sandy environments rather than carbonate-rich circumstances.

MISS are one of the forms of life being sought by Mars Rovers, and the discovery was published in Astrobiology.

Although older sedimentary rocks are known in Greenland, these have been subject to considerable deformation and A/Prof Wacey of the University of NSW Centre for Exploration Targeting says the Pilbara, along with parts of South Africa, is the only place where such ancient fossils might be found.

Objects that are thought to be stromatolites of similar age have also been observed in the Pilbara, but Wacey says these look so different from modern stromatolites that the identification has been questioned. The ancient MISS that Wacey found are very similar to those that exist today.

Consequently, the finding of MISS that are 300 million years older than the previous confirmed record pushes back the time when such systems are known to have existed to the earliest point for which records are possible. “Earlier rocks may be found,” says Wacey, but in their absence the finding still suggests that life may have begun even sooner after the planet’s formation than previously thought.

Little is known about the conditions in which these MISS thrived. “We know the oceans at the time were warmer, there was very little land and no oxygen,” says Wacey. “Today MISS form in tidal flats and sandy environments with lots of evaporation. However, they are restricted to these because they have lots of competition from animals and other bacteria. Back then they were the only thing around and may have flourished more widely.”