3.7 Billion-Year-Old Fossils May Actually Be Just Rocks

3.7 Billion-Year-Old Fossils May Actually Be Just Rocks

Several years ago, a group of researchers revealed a new fossil discovery suggesting that life on Earth started even earlier than we thought. The 3.7 billion-year-old structures found in Greenland certainly looked like they had a biological origin, but a new study casts doubt on the original claims. A different team says the structures identified in Greenland are most likely just rocks. Technically, fossils are rocks, but the question is whether they used to be alive.

The original team uncovered unusual cone-like structures in the Isua supracrustal belt in Greenland, dating to 3.7 billion years in the past. At the time, the researchers concluded that these shapes were most likely “stromatolites,” small clusters of single-cell organisms and biofilms that fossilized. If true, that would push back the earliest evidence for life on Earth by hundreds of millions of years. The next oldest fossils are from the Strelley Pool Formation in Australia, which date from 3.45 billion years ago.

With fossils this old, there’s not much you can do to differentiate them from surrounding rock on a chemical level — there’s no DNA or protein to confirm without a doubt that this structure used to be alive. However, the morphology of the fossils can tell you a lot. The Greenland find does look like newer fossils of single-celled organisms, and the rare-Earth element composition didn’t rule out life.

The new analysis led by NASA astrobiologist Abigail Allwood looked at the 3D shape of the rock features rather than just the exposed surface. Allwood was involved with the discovery of the Australian stromatolites that are the currently oldest accepted evidence of life, and she’s unconvinced by the Greenland find.

Samples identified in the original study as fossils. Via Nature.
Samples identified in the original study as fossils. Via Nature.

On the end, they do look like biological stromatolites. However, the latest study says that’s just a cross section of elongated triangular tubes. In addition, some of the structures were upside down, which doesn’t make sense if they were growing upward from an ancient seabed, as the previous team had surmised. That is definitely not something we would expect from fossils in of this age. Allwood also ran the samples through a prototype instrument called PIXL that will go to Mars on the 2020 rover. This test showed the rocks don’t have the distinctive lamination layer pattern you’d expect from microorganism fossils.

The team puts forward a number of geological explanations for the structures that don’t involve living organisms. Essentially, sedimentary rocks could have been warped by extreme pressure to take on a tube shape. The original researchers strenuously object to the new analysis of their work, saying the analysis was conducted on the wrong samples. This is science. Someone is either going to confirm the original study or the new one. We’ll just have to wait and see.

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