3.5 billion-year-old Australian fossils are indeed oldest ever found

In the search for the earliest life on Earth, it can be hard to tell whether you’re looking at an actual fossil or crinkles in the rock itself. Such doubts have long shadowed the 1980s discovery of 3.5 billion-year-old fossils in the Australian desert, thought to be the oldest fossils ever found. Now, scientists think they have finally put the matter to bed.

In ancient fossilized rock formations called stromatolites, found in the Dresser Formation fossil site of the Pilbara region of western Australia, researchers have finally detected traces of organic matter, and describe their findings in a new study published by the journal Geology.

“This is an exciting discovery — for the first time, we’re able to show the world that these stromatolites are definitive evidence for the earliest life on Earth,” geologist from the University of New South Wales and leader author of the new study, Raphael Baumgartner, said in a press release.

Confirming the fossil evidence

In 2016, scientists claimed to have found 3.7 billion-year-old fossils in Greenland, which unseated the Pilbara fossils’ record. Later research determined that those Greenland fossils were just plain old rocks, and the crown was returned to the Australian fossils.

But, although researchers were fairly confident that the Pilbara fossils were the real deal, it hadn’t actually been conclusively proven that they contained signs of life. They had the shape and structure of microbial stromatolites, but no evidence of organic matter to back it up.

There’s more riding on this than a tiara and a sash reading “Oldest fossils.” It’s deeply relevant to one of the fundamental questions about our very existence: When and how did life develop on Earth?

So Baumgartner and his team went digging. Not literally, though; they analysed previously drilled core samples from deep underground, below where the rocks could have been affected by weather.

This means these samples were much better preserved than those from the surface; the researchers described the samples’ preservation as “exceptional.”

The researchers analyzed the samples in thin slices using multiple techniques, including scanning electron microscopy and scanning transmission electron microscopy; energy dispersive X-ray spectroscopy and Raman spectroscopy; nano-scale secondary ion mass spectrometry; and stable carbon isotope analysis.

If that seems like overkill, well, it’s not really. If one of those lines of inquiry showed a positive result and the rest didn’t, it would mean much shakier ground for drawing a conclusion. But things looked good across the board.

Definitive evidence of organic life

The team’s analyses revealed that the stromatolites are predominantly made up of a mineral called pyrite, riddled with nanoscopic pores. And the pyrite includes nitrogen-bearing organic material, as well as strands and filaments of organic matter that closely resemble the remnants of biofilms formed by microbe colonies.

“The organic matter that we found preserved within pyrite of the stromatolites is exciting — we’re looking at exceptionally preserved coherent filaments and strands that are typically remains of microbial biofilms,” Baumgartner said.

“I was pretty surprised — we never expected to find this level of evidence before I started this project.”

Previously, a different team of University of New South Wales researchers found evidence of 3.48 billion-year-old microbes in hot spring deposits in the Pilbara. Because those deposits are about the same age as the crust of Mars, it’s thought that they could tell us how to find potential fossils on Mars — especially since there’s evidence the red planet once had hot springs too.

Indeed, NASA has been investigating the Pilbara region to try to pinpoint possible geological signatures that indicate the presence of stromatolites.

“Understanding where life could have emerged is really important in order to understand our ancestry,” Baumgartner said. “And from there, it could help us understand where else life could have occurred — for example, where it was kick-started on other planets.”