Nature

New discovery points to life on Earth nearly 4 billion years ago


Time to add even more candles to life’s birthday cake – around another 150 million of them, to be precise.

Rocks from northern Canada have shown signs that life was doing its thing about 3.95 billion years ago, setting a new record for fossils while showing biology was more eager to get started on Earth than we had previously suspected.

 

Researchers from the University of Tokyo made the discovery by analysing the compositions of carbon isotopes in sedimentary rocks from the region of Labrador in Canada’s north-east.

The kinds of rocks they analysed date to a period of Earth’s history called the Eoarchean, a time between 4 and 3.6 billion years ago when the crust was still new and the atmosphere was heavy and relatively free of oxygen.

As you might expect, there aren’t many places on the planet’s surface that rocks left over from Eoarchean can still be found. Most have been melted beyond recognition, churned back into the mantle, or weathered into dust.

Of those that do remain, few are good candidates for finding signs of ancient chemistry.

One exception to this rule is a strip of rock in Greenland called the Isua Greenstone Belt. Samples of this rock belt point at the chemical signature of life at least 3.7 billion years ago.

The problem is these same clues hadn’t been uncovered in rocks taken from similar Eoarchean sites.

 

Unlike dinosaur teeth or impressions of ancient leaves, ancient life doesn’t leave behind much detail to mark its presence. We’re not talking the impressions of slimes or outlines of primitive bacteria.

Instead, these suspected fingerprints of early biochemistry are in the form of graphite and carbonate.

By heating the material and analysing the carbon isotopes they contain, researchers can determine whether they are biogenic – representing the fossilised remains of early cells – or are the result of some geochemical process.

Finding biogenic graphite in some ancient rocks but not others has been a cause to pause.

Fortunately, we can now get back on the case, as researchers have found the graphite in 54 metasedimentary Canadian samples from the same period is in fact the product of living systems.

Not only that, the rocks they’re found in are older than the Isua specimens by 150 million years, suggesting life was busy rearranging carbon atoms a mere half a billion years after Earth settled into shape.

 

They paid close attention to the consistency between the crystallisation temperatures of the graphite and the temperatures that heated the sedimentary rock, ruling out contamination at some later date.

Given the ‘remains’ are little more than chemical shadows of graphite and carbonate, they don’t tell us a whole lot about the nature of the organisms that left them behind. At least not on their own.

But they do fit into a bigger picture of how life might have evolved here, while suggesting the hostile conditions of our new-born planet did little to impede the march of life.

That bodes well for our search for living systems on other bodies in our Solar System and beyond.

How ancient life both on Earth and elsewhere functions is still a perplexing mystery.

 

One theory is that it began largely as competing strands of RNA that folded and recombined until some pockets of nucleic acid soup could co-opt other useful chemical processes.

Others think metabolism-like processes were well underway early in Earth’s history, and replicating nucleic acids joined later.

Earlier this year, Australian researchers found hints of life in ancient hot spring deposits dating back to 3.48 billion years ago, posing questions about whether life might have had its starts in less oceanic environments.

It’s even possible that today’s biosphere stuttered into existence after a series of extinctions and rapid restarts.

For all of the questions that remain, we can be fairly confident that the chemistry of life has been affecting our planet’s development practically from the very beginning.

Life among the stars seems more inevitable than ever. 

This research was published in Nature.

 



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