Hard evidence has finally been obtained to indicate that 3.5 billion-year-old rocks in Australia contain fossils of the oldest known microorganisms.
Raphael Baumgartner at the University of New South Wales in Australia and his colleagues looked at rocks in the Pilbara region of Western Australia. This area contains some of the oldest preserved rocks on Earth, such as the Dresser Formation, which is 3.48 billion years old.
The formation appears to contain layered structures called stromatolites. These form when microbes grow into thin layers, each of which is covered in sediment before another layer of microbes forms on top. However, many researchers have argued that the Pilbara rock structures are not stromatolites and could have formed without life being present.
Baumgartner and his colleagues drilled into the rocks to extract samples. They found many layers that looked like stromatolites, containing “exceptionally preserved organic matter”, including strands of the sort seen when microbes form slimy layers called biofilms. Multiple chemical analyses indicated that this matter came from living organisms.
“We have found smoking gun evidence for some of the earliest life on Earth,” says Baumgartner. “There are no convincing organic matter or microbial remains older than ours.”
There are many claims of older fossils or chemical traces of life, some dating to over 4 billion years ago, but none are widely accepted.
The organic matter that Baumgartner and his colleagues found was mostly trapped inside a mineral called pyrite, or fool’s gold, which is based on iron and sulphur. “The pyrite is extraordinary,” says Baumgartner. Because the microbes are so well-preserved, it must have formed quickly – perhaps even while they were alive.
Some modern microbes live off sulphur and produce pyrite as a waste product. The Dresser Formation microbes may have done the same, says Baumgartner.
“They’ve done a good job,” says Lindsay Hays, deputy program scientist for the NASA astrobiology initiative in Washington, DC. She says she can’t say for certain that the study’s conclusions are correct, but the fact that it is based on multiple techniques makes it more reliable.