ANCESTRAL DAYS

Everything you need to know about the discovery of the oldest evidence of life on Earth

Scientists have just written the first page of the book Life on Earth.

A group of researchers led by Dominic Papineau of University College London announced that they’ve found microfossils that could be 4.3 billion years old, which is a mere 200 million years after the planet was formed.

Where did they find them?

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A rock sample of hematitic chert (an iron- and silica-rich rock), which contains tubular and filamentous microfossils, was once a part of hydrothermal vents. (Dominic Papineau)

In a remote part of Quebec in Canada, Papineau’s team found a piece of rock containing microfossils that they believe was once a part of a hydrothermal vent, which are giant structures spewing hot water and chemicals. These vents were created because of fissures in the seafloor, opening tiny cuts directly into the Earth’s mantle, a hot liquid layer that exists between the crust and the solid iron core. Scientists have long believed that the most likely place for life to have born Earth would be near hydrothermal vents.

Although the rock was once at the bottom of the ocean, it is now on the surface because of how Earth’s tectonic plates move. Over millions of years, rocks that were once deep in hydrothermal vents can in some cases slowly be brought up to the Earth’s surface. Papineau’s team believes their sample fits that description—evidence to support the claim is the dark-green volcanic rock seen in top right of the image.

What do the microfossils look like?

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A hematite filament attached to a clump of iron in the lower right, similar to microbes found in today’s hydrothermal vent environments. (M.Dodd)

The microfossils contain structures that are remarkably similar to the bacteria found in hydrothermal vents today. These bacteria like to form filaments, as can be seen in the picture above.

Are scientists sure these were bacteria and not something else?

The tree of life on Earth consists of three branches: archaea, bacteria, and eukaryotes (which includes humans). We know that the last branch of life, which is the most complex, was created much later. So the fossils could be either bacteria or archaea.

The chemicals that would’ve formed the microbe in the rock have long since degraded, which means we can’t be 100% certain that what we’re seeing were once bacteria. Still, archaea that exist today are rod-shaped or circular, Crispin Little, a co-author of the study and a researcher that University of Leeds, explains. That makes it more likely the microfossils were bacteria. There’s also a chance that the microfossils are something else: an early ancestor of both archaea and bacteria.

Are scientists sure it’s evidence of life, and not just some minerals?

“It’s a very tricky question to answer,” Little says. “But I’m 90% certain.”

One way scientists ensure a claim has a high chance of being true is to try and find evidence that would disprove the claim. In this case, for instance, if they found evidence that any of the carbon in the fossil is not of biological origin, it would lower the likelihood that the rock holds any evidence of life.

Life as we understand it prefers carbon-12, an isotope of carbon with six protons and six neutrons. If the ratio of carbon-12 to carbon-13 (another natural isotope, with six protons and seven neutrons) in the microfossil is quite different from that found in non-biological minerals, it is strong evidence that the sample found is of biological origin. And that’s what scientists found in this case.

How certain are scientists that this is the oldest evidence of life?

To age the evidence, scientists use a method called radioactive dating. This involves calculating the ratio of a slowly decaying radioactive material, like uranium to its final decay product (for uranium, that’s lead), in the sample. From that ratio, scientists can derive the age of the sample.

It’s the best method we have, but it has inherent uncertainties. That’s why the study says the microfossils could be between 3.8 billion and 4.3 billion years old.

But even at the lower end, the microfossils would beat the previous record for the oldest evidence of life.

What was the previous record for the oldest evidence of life?

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Stromatolites, with two slices cut out by researchers. (Allen Nutman)

In April 2016, researchers in Greenland found structures called stromatolites, which are layered mounds of sediment thought to be created by microbes living in shallow water. These structures are thought to be 3.7 billion years old, and they held the record for the earliest evidence of life—until now.

Will scientists find even older evidence of life on Earth?

“It’s possible,” Little says. “But the chances are slim.”

Very few rocks older than 3.8 billion years exist today. And even among those that exist, over such a long time most evidence of life would’ve undergone degradation beyond recognition. Papineau, Little, and their team were lucky to find one of the few that remain and are protected from severe degradation by their unique geological features.

What are the implications of the discovery?

The most intriguing implication is that the discovery could radically change how we search for life on Mars. Read Quartz’s story explaining how.

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