The study of life’s history on Earth is an ongoing effort to follow the thread further and further back in time. As amazing and fascinating as a few billion years’ worth of fossils are, we still ultimately want that Holy Grail that illuminates the origins of life on Earth. Unfortunately, that goal collides with the fact that the earliest evidence stands the least chance of being preserved through the eons for us to find.

Each new find that purports to supplant the earliest known signs of life is virtually guaranteed to be controversial, subject to poking and prodding from skeptical scientists uncertain that every competing (non-living) explanation can be ruled out. Last August, for example, a paper claimed to show the remnants of stromatolites (small mounds built by communities of shallow water microorganisms) in 3.7 billion-year-old rocks in Greenland. Similar fossils from Australia that come in at about 3.5 billion years old are generally accepted as legit, but anything older is still subject to scientific debate.

Now, a new study led by University College London PhD student Matthew Dodd describes evidence of what the researchers believe to be seafloor bacteria that lived at least 3.7 billion years ago.

That evidence comes from rocks that are part of Quebec’s Nuvvuagittuq belt, which contain some of the oldest rocks on the planet. They comprised an ancient seafloor made of volcanic rock, but there are also layers of iron minerals that precipitated out of the seawater. Those are believed to have formed near hydrothermal vents that gushed super-hot water laden with minerals. The Nuvvuagittuq rocks have proven hard to date, but they are known to be at least 3.77 billion years old, and could even be as old as 4.28 billion years—very early indeed, considering that our planet formed only a little over 4.5 billion years ago.

This is all further complicated by the fact that the Nuvvuagittuq rocks were metamorphosed under high temperature and pressure about 2.7 billion years ago. But within some jasper in the hydrothermal vent layers, the researchers found tiny tubes and filaments filled with hematite, an iron oxide mineral. The size and shape of these structures was fairly consistent (especially considering the metamorphic torture they’ve endured), and some pairs of tubes were connected by small “knobs.”

All this, the researchers say, is consistent with iron-oxidizing bacteria that we find at hydrothermal vents today, as well as slightly less ancient fossil examples. These bacteria build iron-oxide tubes around themselves and extend filaments out into their surroundings.

Around these familiar-looking structures, there are some even tinier bits of carbon-rich material here and there. The ratio of different carbon isotopes that are present is one good way to tell biological carbon from other sources, as organisms have a preference for lighter isotopes. In this case, the carbon that can be found in the rock does fall within the biological range—“although not unambiguously," the researchers write.

There are also some blobs ringed with magnetite, which have been seen in other rocks recognized to contain these sorts of fossils. Several minerals can be found within the blobs, which also contain bits of carbon-rich material. The researchers think that this set of minerals reflects reactions between organic matter and the iron precipitating from the hydrothermal vents—supporting a biological origin.

If the researchers are right about what they’ve found, this would be the oldest direct evidence of life we’ve ever seen. What’s more, if the recent discovery of stromatolites in Greenland also turns out to be the real deal, we would be able to say that there were at least two very different types of microorganisms living in very different parts of the ocean by 3.7 billion years ago.

This new paper is particularly interesting because it involves hydrothermal vent environments, which have been proposed as a logical place where life on Earth could have had its origins. Any evidence supporting that idea also makes hydrothermal vents on other worlds look more and more like priority targets in the search for life beyond our home planet.

Of course, it’s best not to get too carried away in daydreams until we’re more certain about what the researchers have found in these Canadian rocks. But the University of Western Australia’s David Wacey, who was not involved in this new study, told Ars that the multiple lines of evidence presented make a pretty strong case. “The morphological comparisons with younger vent-dwelling organisms are quite striking and it is rather difficult to envisage how the microstructures shown [in the paper] could be formed in a purely abiotic scenario.

“There will, no doubt, be arguments, and it may be many years before a consensus is reached, but this is how science progresses," Wacey added. "I think it will be particularly important to understand more about the regional geology of the area and perhaps try to better constrain the age of these potential organisms.

“The other exciting aspect of this report is that these potential microfossils are from a rock unit not previously studied for early life,” he continued. The Nuvvuagittuq rocks might even be older than the rocks researchers have been looking at in Greenland, South Africa, and Australia, and they represent a different marine environment, too. “This new study once again gets us thinking about a potential hydrothermal cradle of life.”

Nature, 2017. DOI: 10.1038/nature21377 (About DOIs).