Abigail Allwood
The oldest fossils in the world might not be anything of the kind. Instead they may simply be deformed rocks, reopening the question of when life began to leave its mark in the fossil record.
However, even if the fossils are not real, other evidence still suggests that life began early in Earth’s history.
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For decades, palaeontologists have been on the hunt for ever-older fossils. The oldest accepted fossils are those from Strelley Pool in the Pilbara region of western Australia. They are stromatolites: preserved mats of microorganisms sandwiched between layers of sediment. The fossils are 3.4 billion years old.
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But some research teams claim to have found older fossils. In 2016 Allen Nutman at the University of Wollongong in Australia and his colleagues made one of the most prominent claims. They described stromatolites from rocks in Greenland that are 3.7 billion years old.
Layers of microbes
Most rocks that old have at some point been carried deep into the Earth, where heat and pressure destroy fossils. But Nutman’s team found a patch of rocks that appears to have passed through this “metamorphism” relatively unharmed: it seemed to still partially preserve fossils.
They are cone-shaped objects that resemble the Strelley Pool stromatolites, albeit less well preserved. Nutman claimed to have found subtle traces of the pancake-layering found in stromatolites, although it is not visible in photos. They also contain a rock called low-temperature dolomite, which is often laid down in the presence of microorganisms. Chemical traces suggested the objects formed in a shallow sea.
Abigail Allwood
However, the results were immediately controversial. In particular, the cone-shaped structures do not contain fossils of the actual microorganisms that build stromatolites. That is not surprising, given how much the rocks have been heated, but it does pose a problem.
Now Abigail Allwood at the Jet Propulsion Laboratory in California and her colleagues have visited the Greenland site and reanalysed the rocks. They say they are not stromatolites at all.
Allwood thought it was an odd coincidence that the rock face had broken open straight through the middle of all the cone-shaped stromatolites. So she cut 10 centimetres into the rock and found that they are long ridges “like chewing gum stretched out”, with a roughly triangular cross-section. Such structures are often formed in deformed rocks, she says.
The chemical analysis also doesn’t stand up, says co-author Joel Hurowitz of Stony Brook University in New York. The “stromatolites” contain carbonate, supposedly drawn out of seawater by the microbes. But Hurowitz found it was concentrated in the outer layers. “There was some carbonate-bearing fluid that basically soaked into these rocks after they had formed,” he says.
Criticisms rejected
Nutman rejects these criticisms. He says that if the objects were produced by deformation, they would not all have flat bottoms – but they do, and this suggests they are stromatolites.
Nutman also criticises Allwood’s team for focusing on one patch of rock, which he says is poorly preserved. “This is a classic comparing apples and oranges scenario, leading to the inevitable outcome that ours and their observations do not exactly match,” he says. However, Allwood points out that the same rocks were the centrepiece of the original paper.
Regardless of the outcome of the debate, evidence is accumulating from genetic studies reconstructing the family tree of all living things that life was present 3.7 billion years ago, or earlier. According to a study published in August, the Last Universal Common Ancestor – from which all modern life is descended – lived at least 3.9 billion years ago.
Journal reference: Nature, DOI: 10.1038/s41586-018-0610-4
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