Amid the crags and scrublands of Northern Quebec, a stone's throw from the shores of Hudson Bay, scientists have stumbled upon what could be the earliest known chapter in the story of life on Earth.

In a find that is certain to spark debate among experts, a team of Canadian and international researchers has determined that tiny filament-like structures found in rock samples from an ancient outcrop are the mineral remains of bacteria that lived more than 3.77 billion years ago.

If the interpretation is correct it means the curious formation where the rocks were found – known to geologists as the Nuvvuagittuq belt – bears the first direct signs of the microbial life that inhabited our planet billions of years before the emergence of multicellular plants and animals.

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"This will definitely come as a big surprise to the scientific community, no doubt about it," said Dominic Papineau, a geochemist at University College London in the U.K., who led the work.

The surprise is not that life existed at such a remote time. Scientists have long suspected that ancestral micro-organisms emerged soon after Earth formed a stable surface with an ocean and atmosphere. Carbon isotopes measured in ancient rocks also offer an indirect clue that something was alive back then.

But picking out preserved traces of that initial flowering of life is another matter. A similar claim of microfossils in 3.5-billion-year-old rocks from Western Australia has long been disputed. Last year, a recent find in Greenland pushed the clock further back to 3.7 billion years ago. Now the Canadian find, published Wednesday in the journal Nature, reaches back even further – possibly much further depending on precisely how old the Nuvvuagittuq rocks and the features they carry turn out to be.

Today those rocks are found in exposed glacier-polished outcrops that protrude from the lichens and other vegetation of the Northern Quebec tundra. When they formed they were part of an ancient ocean crust atop simmering volcanic activity that powered hydrothermal eruptions and oozed lava onto the seafloor.

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"We think this is where life started and thrived, in these kinds of environments," said Jonathan O'Neil, a University of Ottawa geologist and team member who has studied the unique belt since it was first spotted in a regional survey more than a decade ago.

Dr. O'Neil's earlier work suggests the belt could be up to 4.28 billion years old, making it the most venerable fragment of Earth's crust found anywhere. However, this figure is not universally accepted. The type of rock that the belt is made of – a layered blend of basalt and chert – can only be dated by using a method that is less accurate than the one used to date a section of continental crust in the Northwest Territories thought to be about as old.

During a 2008 expedition, Dr. Papineau collected samples from a burst of brick-coloured rock in the belt that he suspected to be the remains of a hydrothermal vent. It was in these rocks that the team later found filaments of the mineral hematite along with other structures that resemble those produced by iron-loving bacteria found around hydrothermal systems today.

In their publication, the team argues that the filaments are similar in age to the rock formation itself – a key step for making the case that they're the earliest evidence of life to date.

Bradley De Gregorio, a geologist with the U.S. Naval Research Laboratory in Washington, said the reasoning was persuasive. "It would lead me to think that these features they're finding actually do match the age of the rock," he said.

Boswell Wing, a University of Colorado geobiologist who has also studied the Nuvvuagittuq belt, was not convinced. He said the team would need to do more to rule out other explanations for the location where the samples were gathered, and the possibility that it formed at a later time relative to the belt.

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"These rocks have been cooked, they've been squeezed, they've been cooled down, then cooked and squeezed again. The picture is a complicated one," he said.

Dr. Wing added that the most important thing about the discovery is that it would draw attention to the area's geological significance and encourage other scientists to explore the Quebec site.

"It is a fantastic resource. Anyone who's been there can recognize that right away," he said.

Graphic: Trish McAlaster