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In search of the first life on Earth, it can be difficult to know if you are looking at a real fossil or folds in the rock itself. These doubts have long overshadowed the discovery in the 1980s of fossils dating back 3.5 billion years in the Australian desert. Now scientists think they have finally solved the problem.
In ancient fossilized microbe formations called stromatolites, found in the fossil site of the Dresser Formation of the Pilbara region, researchers eventually detected traces of organic matter.
"This is an exciting discovery, and for the first time we are able to show the world that these stromatolites are irrefutable evidence of the very first life on Earth," said geologist Raphael Baumgartner of the University of New South. Wales (UNSW) in Australia. .
You may remember the time when scientists claimed to have found fossils dating back 3.7 billion years in Greenland. Subsequent research determined that these fossils were simply old rocks and that the crown had been returned to the Pilbara fossils.
However, even though everyone was pretty sure Pilbara's fossils were the real deal, they had not been proven conclusively. They had the shape and structure of microbial stromatolites, but no evidence of organic matter proves it.
There is not only a tiara and a belt on which one can read "The oldest fossils". This deeply concerns one of the fundamental questions concerning our very existence: when and how did life develop on this blue marble?
Baumgartner and his team went digging. Not literally, though; they analyzed samples of previously drilled cores in subterranean depths below the point where rocks could have been affected by weather conditions.
This means that these samples have been much better preserved than those of the surface; in their document, the team stated that the conservation was "exceptional".
(Baumgartner et al., Geology, 2019)
The researchers analyzed the samples in thin slices using several techniques, including scanning electron microscopy and transmission electron microscopy; energy dispersive X – ray spectroscopy and Raman spectroscopy; mass spectrometry of secondary ions at the nanoscale; and stable analysis of carbon isotopes.
If this sounds exaggerated, well, it's not really. If any of these leads yielded a positive result and the rest did not, it would mean a much drier field for drawing a conclusion. But things looked good.
The team's analyzes revealed that stromatolites consist mainly of a mineral called pyrite, riddled with nanoscopic pores. And in the pyrite are inclusions of nitrogenous organic matter, as well as strands and filaments of organic matter which are very similar to the remains of biofilms formed by microbial colonies.
"The organic matter we found preserved in stromatolite pyrite is exciting, we are looking at cohesive filaments and exceptionally preserved filaments that are typically microbial biofilm residues," said Baumgartner.
"I was quite surprised, we never expected to find this level of evidence before starting this project."
Previously, another team of UNSW researchers had discovered evidence of 3.38 billion year old microbes in hot spring deposits in the Pilbara. Since these deposits are about the same age as the Mars crust, it is thought that they could tell us how to find potential fossils on Mars – all the more so as it is proven that the red planet had hot springs too.
NASA has been investigating the Pilbara in an attempt to learn what geological signatures might indicate the presence of stromatolites.
"Understanding where life could have emerged is really important to understanding our ancestry," Baumgartner said. "And from there, it could help us understand where life could have happened – for example, where it was launched on other planets."
The search was published in Geology.
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