On 2 March 2017, the journal Nature published a paper describing what its authors argue could be the earliest evidence of life on Earth:
Here we describe putative fossilized microorganisms that are at least 3,770 million and possibly 4,280 million years old in ferruginous sedimentary rocks, interpreted as seafloor-hydrothermal vent-related precipitates, from the Nuvvuagittuq belt in Quebec, Canada.
The findings, however, have received an “unusual” amount of criticism for something published in one of science’s most elite journals, the Sydney Morning Herald reported. Numerous experts in the fields of paleobiology and geochemistry have argued that available evidence is insufficient support the researchers’ claims.
These researchers — an international team comprising scientists from the UK, the United States, Canada, and Australia — suggest that microscopic filaments 10 times thinner than human hair found in ancient Canadian rocks are analogous to the structures created by modern day bacteria that live around deep sea hydrothermal vents. In a press release, the paper’s lead author, Matthew Dodd, stated:
Our discovery supports the idea that life emerged from hot, seafloor vents shortly after planet Earth formed. This speedy appearance of life on Earth fits with other evidence of recently discovered 3,700 million year old sedimentary mounds that were shaped by microorganisms.
The kind of bacteria Dodd and his colleagues argue these rocks record, broadly, are a class of bacterial chemoautotrophs that derive energy not from the sun (like a photosynthetic organism) but from the modification of inorganic chemicals: in this case the transformation of a solid form of iron (ferric iron) to a soluble form (ferrous iron).
In this process, bacteria facilitate the transfer of an electron from the solid iron to another chemical (ideally to an oxygen molecule, but nitrate can also be used less efficiently), providing energy for cellular metabolism and leaving dissolved ferrous iron behind. The authors argue this mechanism imparts a clear signature in the geologic record:
A consequence of this reaction is the oxidation of carbonaceous material to carbonate and the reduction of insoluble ferric iron into soluble ferrous iron, followed by precipitation of iron-bearing carbonate, ferrous silicates, and magnetite.
In other words, the authors don’t have direct evidence of the bacteria’s presence, but instead rely on collections of minerals and structures interpreted to be byproducts of this metabolism, as described in their press release:
The haematite structures have the same characteristic branching of iron-oxidising bacteria found near other hydrothermal vents today and were found alongside graphite and minerals like apatite and carbonate which are found in biological matter including bones and teeth and are frequently associated with fossils.
Critiques of this work, however, focus on multiple aspects of the authors’ conclusions. In an e-mail, Roger Summons, a professor geobiology at MIT and an expert in the chemical signatures of early life, told us:
I don’t think this report meets any of the stringent requirements for addressing a question of this importance. They have not convincingly demonstrated biogenicity or the true age of the objects. Other commentators have addressed the deformation history of the rocks which makes their value questionable.
The three issues raised by Summons — lack of proof of biological origin, uncertainty about the age of the rock or the features preserved, and chemical alterations that have occurred to the rock after it was formed — have been echoed by numerous experts.
In a feature on the paper’s findings published in National Geographic, University of Colorado at Boulder geologist Nigel Kelly said that he has doubts about the age range the authors have assigned to their samples:
“These rocks have a long and complicated history […] and so deciphering an unambiguous age and origin of these features will always be complicated,” [Kelly said].
[…] Scientists such as Kelly maintain that in such complex rocks, determining the relative ages of features is difficult, and assigning such an ancient age to the fossils is not accurate. Instead, he says, it’s possible the structures containing those fossils (if they’re real) are younger than 2.7 billion years.
Speaking to Australia’s ABC News, University of New South Wales early life expert Martin Van Kranendonk also spoke of the challenges that accompany such old rock, and said he was not convinced the material was biologic in nature:
“It’s very possible that there was life at that time — I think it’s almost certainly the case — but as we delve deeper in the rock record, each interpretation gets more contentious, but here I think the science doesn’t hold,” said Professor Van Kranendonk.
He argued these rocks would have been subjected to enormous temperature and pressure changes, which could have a range of effects on the minerals contained within them.
Malcolm Walter, the director of the Australian Center for Astrobiology, described the results as “not believable” in an interview with the Sydney Morning Herald, suggesting it would be hard to discern any actionable information from the rocks:
Professor Walter said these rocks are extremely altered and have been cooked deep in the crust at more than 500 degrees. This, he said, destroys their original structures.
“In these sorts of studies you usually start with the most well-preserved rocks. These rocks are unusually badly preserved,” he said.
In Summons’ view, however, the above-referenced issues (which are the common critiques of any claim of extremely ancient fossils) are not even the biggest problem. The issue, he says, is that iron oxidizing bacteria require the presence of dissolved oxygen in the water, which flies in the face of what is currently known about Earth history:
The real question unanswered in this paper is ‘where does the oxygen come from 3.8 billion years ago’? They are claiming iron oxidizing bacteria as the likely identity of these objects. 3.8 Billion years ago there was no oxygen in the atmosphere let alone at the sea floor. To the best of our knowledge the deep ocean did not become oxygenated until at least 800 million years ago and more likely 500 million years ago. This is why modern hydrothermal systems are inappropriate analogues for these old rocks.
Speaking to the BBC, co-author Dominic Papineau acknowledged this point:
Dr Papineau does concede though that the idea of metabolising micro-organisms using oxygen so soon after the Earth’s formation will surprise many geologists.
“They would not consider that there were organisms breathing oxygen at this time. It brings back the production of oxygen on the Earth’s surface, albeit by tiny amounts, to the beginning of the sedimentary record,” he said.
In response to these criticisms, co-author Franco Pirajno said:
I’m not surprised at the criticisms […]. But you have to put the whole picture together. […] No.1, [the research shows] the structures were formed in a submarine environment; two, there are thermal springs; and three, we have these tubular features. What else could they be?
If someone comes up with a convincing answer to that question, the controversy regarding this find will likely end. Absent that, it is more probable this study will continue to be debated for some time to come.