Claims of ancient fossils are always contentious. Rocks as old as the ones in the new study rarely survive the weathering, erosion, subduction and deformation of our geologically active Earth. Any signs of life in the rocks that do survive are difficult to distinguish, let alone prove. Other researchers in the field expressed skepticism about whether the structures were really fossils, and whether the rocks that contain them are as old as the study authors say.

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But the scientists behind the new finding believe their analysis should hold up to scrutiny. In addition to structures that look like fossil microbes, the rocks contain a cocktail of chemical compounds they say is almost certainly the result of biological processes.

If their results are confirmed, they will boost a belief that organisms arose very early in the history of Earth — and may find it just as easy to evolve on worlds beyond our own.

“The process to kick-start life may not need a significant length of time or special chemistry, but could actually be a relatively simple process to get started,” said Matthew Dodd, a biogeochemist at University College London and the lead author of the paper. “It has big implications for whether life is abundant or not in the universe.”

The microfossils were discovered in rocks from the Nuvvuagittuq (nuh-vu-ah-gi-took) belt in northeastern Canada. This strip of iron-rich jasper now cuts across the eastern shore of Hudson Bay, but it was once a hydrothermal vent on the ocean floor. Billions of years ago, Dodd and his colleagues say, ancient microbes flourished around those vents, taking advantage of their chaotic chemistry to generate fuel.

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When the microbes died, iron in the water was deposited on their decaying bodies, replacing cellular structures with stone. The rocks that contained them were buried, heated, squashed, and then forced upward to form the part of North America where they now sit. Depending on the dating method used, the material could be as old as 3.77 billion years — or as stunningly ancient as 4.28 billion years.

When Dodd's UCL colleague Dominic Papineau visited the Nuvvuagittuq belt in 2008, he knew immediately he would have to bring some samples back to his lab. Once back in London, he and Dodd peered at very thin slices of the rocks, first with an optical microscope, then with a laser-based device called a Raman microscope.

The optical observations revealed complex fossil structures encased in hematite, a mineral that would have formed as iron in the seawater interacted with the microbe's decaying organic matter. John Slack, a co-author and emeritus scientist at the U.S. Geological Survey who studies jaspers from ancient hydrothermal vents, said the fossils look just like the ones he sees in younger rocks, and around modern-day vents.

The Raman analysis, which measures the vibrations of atoms as they are struck by a laser to figure out what molecules a material contains, showed that the rocks contain carbonate, apatite and magnetite — minerals that often form in the presence of organic matter.

Graphite in the rocks also contained telltale signs of life. The mineral disproportionately contained the isotope carbon-12, a form of the atom in which the nucleus has six protons and six neutrons. This form of carbon is preferred for biological processes and is considered an isotopic signature of life.

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“We can think of alternative explanations for each of these singular observations,” said Dodd, “but why all of these features occur together can really only be explained by one thing, which is a biological interpretation.”

Not everyone is so convinced. Tanja Bosak, a geobiologist at MIT, said that the authors of the Nature paper are missing some key evidence for their claims. For one thing, she felt that the authors didn't include sufficient images of the site where they found the fossils, or a detailed explanation of their geologic setting.

“This is the very first thing we tell our students, to look at the context and report the context and interpret the context carefully,” she said. “Because if the context isn’t right, then everything else you do doesn't matter.”

Bosak was also skeptical of the stated minimum age for the fossils. The Nuvvuagittuq belt is composed of metamorphic rock — stone that's been heated, squeezed and deformed by processes deep within the Earth. It is also bisected by veins of igneous, or volcanic, rock that intruded into the sediments during this process. Dodd and his colleagues got their estimates of the formation's youngest age by dating zircon crystals in the igneous intrusions (the logic being that those intrusions had to have formed after the fossils did). Their oldest estimate — 4.28 billion years old — comes from a more contested dating method that measures the decay of the element samarium into neodymium.

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Other researchers said this fell into the category of "extraordinary claims” that "require extraordinary evidence.” Abigail Allwood, an astrobiologist at NASA's Jet Propulsion Laboratory, said the researchers did "a remarkable job” analyzing the structures contained within their samples. But there just was just not enough data to definitely say they are evidence of life.

"That's geology — geology hands you whatever it hands you, and you deal with that,” she said. "The problem with these rocks after they've been metamorphosed is that evidence of environment in and of itself is hard to find. Let alone studying how populations and supposed organisms reacted to that environment.”

Kenneth Williford, another astrobiologist at JPL, was more optimistic about the potential for these findings to be confirmed. He noted that the team behind the paper has a long track record working in the Nuvvuagittuq area, and that the features they describe match those of younger, undisputed fossils.

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"It’s a very exciting set of observations carefully made,” Williford wrote in an email. "… They may indeed have found something truly remarkable.”

Findings like these are subject to intense scrutiny because they have potentially far-reaching implications for the study of early organisms on Earth and other planets. The oldest universally accepted evidence of life on Earth is dated to about 3.4 billion to 3.5 billion years ago. The new paper proposes pushing that date back by nearly 300 million years.

“That's a long time,” Bosak said. Just think: In the most recent 300 million years of history, the Earth has seen three mass extinctions, a reshuffling of continents, the rise and fall of dinosaurs and the evolution of humankind.

An earlier start date for the history of life also means that organisms were evolving at a time when Earth would have been quite hostile. Between 4 billion and 3.8 billion years ago, the planet was subjected to what's called the “Late Heavy Bombardment,” a time when asteroids and comets flew through the solar system and barraged every body they struck. If microbes were able to thrive in this chaotic time, that would imply that life can take hold even under the worst of circumstances.

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Significantly, the Nature paper comes just six months after researchers working in Greenland reported finding ancient stromatolites in 3.7 billion-year-old rocks. Those conical structures are usually produced by photosynthetic bacteria living in shallow seas. If both papers are confirmed, Slack explained, that means life not only existed early in Earth's history, but also was diverse enough to include both chemosynthetic and photosynthetic bacteria.

If organisms found it so easy to thrive here on Earth, why not elsewhere? Could it be, in the words of Nobel laureate Christian de Duve, a “cosmic imperative?”

That's what Dodd would like to know. He noted recent research suggesting that 3.77 billion years ago, when the fossils formed, Mars was warm and had oceans on its surface.

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“It means we could expect to find evidence of life on Mars at this time,” Dodd said. And if we don't — “that suggests that life is a result of some fluke or phenomenon on Earth.”

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Whatever consensus is reached on this paper, it will undoubtedly be helpful in the search for life on other worlds. Williford of JPL noted that hematite filaments like the ones discovered by Dodd are exactly what astrobiologists imagine they might find as they look for fossils on Mars.

And Allwood said that studies of ancient Earth life can be a "proving ground” for the techniques required to identify alien organisms.

"When something like this comes out in the literature, and it is met with a lot of skepticism, and researchers say, 'What are the alternative explanations? Can something like this can be produced non-biologically? ... Every time we do that the science gets better,” she said. "So if we find something beyond Earth, we’re in a better position to understand it.”

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Correction: A previous version of this story mischaracterized the isotope Carbon-12. It has six protons and six neutrons in its nucleus.