The fossilized remains of microbes that lived beside the sea in the earliest chapter of life on Earth have been discovered in a slab of rock in Western Australia.
Researchers found the tiny fossils in rock formations that date to 3.4 billion years ago, making them strong candidates to be the oldest microbes found. Some clung to grains of sand that had gathered on one of the first known stretches of beach on Earth.
The findings paint a picture of life arising when the first landmasses began to emerge in fragmentary fashion from the oceans, much like the Caribbean or Indonesian islands today.
At the time, volcanic eruptions spewed gas and lava, while a blanket of thick cloud grayed the skies. The moon — much closer then than it is today — pulled the oceans into vast tidal surges. There was no breathable oxygen.
“To us it would have seemed like a hellish place to live,” said Martin Brasier at Oxford University, who co-authored a report on the fossils in the journal Nature Geoscience. “To early life, this was paradise. A true Eden.”
Brasier worked with a team led by David Wacey at the University of Western Australia, who discovered the fossils in the region’s Strelley Pool formation, one of the oldest outcrops of sedimentary rock on Earth.
High magnification images showed the fossils were spherical, oval and tubular, much like modern microbes, and were of similar size, between 0.01mm and 0.02mm across.
Researchers who study the origin of life on Earth typically draw on several strands of evidence to support their findings. Apart from the size and shape of the fossilized microbes, Wacey points to carbon and nitrogen in their cell walls, a hallmark of all living things today.
Further evidence comes from the cells forming chains and clusters, and clinging to grains of sand. Inside some fossils were exquisitely fine structures that appear as microbes grow and divide. Some of the microbes are likely to have fed off pyrite, a sulfur-rich iron compound, and produced sulphate as a waste product. Others used this sulphate and produced hydrogen sulphide, the gas that smells like rotten eggs.
“The new evidence from our research points to earliest life being sulfur-based, living off and metabolizing compounds containing sulfur rather than oxygen for energy and growth,” Wacey said.
Unraveling the nature of the world’s oldest organisms will help scientists write the first chapter of life on Earth and aid the search for life elsewhere. Future missions to Mars, for example, might focus on ancient beaches and river sands.
“It is vital to know what the most simple life on our planet looked like and how to unambiguously identify it, if we are to have any chance of identifying life elsewhere,” Wacey said.