Rocks seem like pretty unchanging time capsules, but not even stone is set in stone. A few years ago researchers studying stromatolites in Greenland dated them to 3.7 billion years old, making them the oldest evidence of life on Earth. However, a new study has now taken another look at the rocks and found that they probably weren't biological in origin.
Stromatolites are some of the most striking and stable evidence for early lifeforms on Earth. They were never alive themselves but instead were formed as colonies of microbes slowly stacked layers of sediment on top of each other, leaving behind rocky formations that can form column or cone shapes. It's kind of like if beings from the distant future found our skyscrapers but not our bones.
In 2016, a team of researchers examined stromatolite-like structures, measuring between 1 and 4 cm (0.4 and 1.6 in) high, from an ancient shallow sea in what is now Greenland. They concluded that the structures "display all of the features used in younger rocks to argue for a biological origin," and given that these rocks were some 3.7 billion years old, they argued that these were the earliest evidence for life on Earth.
But now an independent team from Caltech has analyzed the Greenland rocks and found that they were more likely formed through geological processes instead. When the Caltech researchers looked at them from the front and sides, they found that one side showed signs of being compressed while the opposite side had been extended – evidence that they had been pushed into that shape by surrounding rock. When observed in three dimensions, the team says, they're not cone-shaped but more like ridges.
Backing that up, the region of Greenland the structures were found in is known to be extremely geologically active, with high temperatures and pressure likely deforming the rock into the observed shapes over millions or billions of years. In addition, the structures have none of the internal layers nor chemical signatures associated with microbe activity.
With these "fossils" called into question, the title for earliest evidence of life falls back onto stromatolites from northwestern Australia, which were described in 2006 and date back to 3.45 billion years ago. Other unconfirmed contenders include rocks from a 3.5-billion-year-old hot spring in the same region of Australia, and highly-contentious structures found in Quebec that could be as old as 4.3 billion years.
The new research was published in the journal Nature.
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