Scientists have found what they claim are the oldest-known fossils on Earth, embedded in Canadian rocks that are at least 3.7 billion years old.
If they are right, the discovery suggests that life on Earth began within 200 million to 500 million years of the birth of the planet 4.5 billion years ago.
He added that even at this early date, the organisms appear to be quite complex in their design.
"That means we're not even at the origin of life yet," he said.
The discovery was made in rocks collected from the Nuvvuagittuq supracrustal belt in Québec.
In 2004, scientists discovered that the oldest rocks from this area date back nearly 3.8 billion years, and they may go back as far as 4.3 billion years.
"There are less than five other places on the planet with rocks as old as these," Dodd said.
Dominic Papineau, a geochemist and astrobiologist also at University College London and a co-author of the paper, collected palm-sized rock samples from the site in 2008 and brought them back to the lab for further study.
The team sliced the rocks into sections about 30 microns in width — thin enough to see light shining through them. With the aid of a microscope, the researchers were able to map the minerals in each slice.
After that, they scanned the cut side of the rocks with a laser, which gave them an even finer view of the chemistry inside.
Previous work had already revealed that the iron formations in the Nuvvuagittuq belt had seawater-like chemical signatures. Other clues suggested they formed near hydrothermal vents far beneath the ocean surface.
In the new study, funded in part by NASA and the UK Engineering and Physical Sciences Research Council, the authors identified microscopic tubes and filaments embedded in the rocks, including those attached to clumps of iron. In their view, the filaments look remarkably similar to microfossils in much younger rocks found near hydrothermal vents in Norway and off the coast of California.
Dodd, Papineau and their colleagues also point out that the filaments from the Nuvvuagittuq belt rocks are similar in size and shape to microbial filaments found today in low-temperature ocean vents that are known to be associated with iron-metabolizing bacteria.
"What we see is a rather consistent continuum in the shapes of these organisms, with twisting filaments attached to iron clumps," Dodd said. "That suggests these organisms would have had a similar lifestyle to iron-oxidizing bacteria today, and a similar way of deriving energy."
Dodd knew these conclusions would be controversial — and indeed, they are — so he and his co-authors looked for further evidence to bolster their case that what they found truly represents traces of ancient life.
For example, they determined that the graphite they discovered alongside the fossils had an isotopic fingerprint consistent with biological processing. They also found circular carbonate patterns that have been seen in other iron sediments known to have microbial activity.
Other clues that the tubes and filaments were made by biological processes included the presence of granules inside the rocks that contain carbonaceous material. Similar granules are often associated with fossils in younger rocks, the authors wrote.
In addition, the researchers said they considered all the known and likely scenarios that might have formed these structures without the aid of a living organism.
"None of them can unite all the findings under one explanation," Dodd said. "The only thing that can do that is the biological explanation."
Some experts disagree.
One of them is Martin Van Kranendonk, an early-life researcher and head of the School of Biological, Earth and Environmental Sciences at the University of New South Wales in Australia.
"These fall in the category of dubiofossils at best," he said, " but I would say they are not fossils."
He noted that the study authors themselves said the features they described are consistent with fossils that have a biological origin, but not unambiguously so.
"The reality is, we have no idea of how the structures formed or why," he said.
Abigail Allwood, a geologist at NASA's Jet Propulsion Laboratory in La Cañada Flintridge, agreed with Van Kranendonk's assessment.
"I'm skeptical about the interpretations in the paper," she said.
So it seems the authors have more work ahead of them if they're going to convince their peers that these ancient fossils are the real deal.
However, if they are correct, the implications are thrilling.
Just six months ago, another team of scientists found evidence of life that formed in shallow pools of water at roughly the same period. If both findings are validated, that means life was flourishing in a variety of environments when the planet was only about one-tenth as old as it is today.
"The fact that these organisms emerged so quickly suggests to us that life may not be so complicated a process as we think," Dodd said.
He added that if life sprung up so quickly here on Earth, there is a good chance we will find it in other places as well — especially Mars.
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