Elizabeth Finkel
Elizabeth Finkel is editor-at-large of Cosmos.
For decades, researchers have bickered about claims to have found the oldest life on Earth – but a new analysis seems to have solved the problem, and with a surprise result.
Arguments often revolve around whether tiny squiggles, blobs and tubes found in ancient rocks are fossilised lifeforms or just weird minerals.
Now, a study published in the journal Nature Ecology and Evolution leapfrogs these squabbles. Rather than arguing about forms, a team from the University of Bristol in the UK, led by phylogeneticist Davide Pisane, has taken a big data approach. Using 29 DNA codes that are common to species across the tree of life, they pinpoint the emergence of the last universal common ancestor (LUCA) at an extraordinarily early 4.5 billion years ago – just an eye-blink after our planet formed.
“In general they have a great paper,” says William Martin, a microbiologist who specialises in early life research at Heinrich-Heine-University in Dusseldorf, Germany. “For the origin of life, I think everyone agrees that there had to be liquid water, and that would constrain the date of LUCA to 4.23 billion years; their number is quite close.”
In 1992, William Schopf at the University of California, Los Angeles, in the US, was brave enough to assert that microscopic chain structures found in rocks in the Pilbara region of Western Australia were fossilised bacteria. He’s been defending the claim against criticism ever since. His experience is hardly unique explains paleogeologist Malcom Walter, former director of the Australian Centre for Astrobiology at the University of New South Wales. “Microbial fossils provide fertile grounds for debate.”
It’s not just microscopic blobs that must endure scepticism. Boulder-sized structures called stromatolites also come in for vigorous debate over their biological origins. Living stromatolites, like those found in Shark Bay, Western Australia, are indubitably formed by cyanobacteria which grow in matted layers in shallow waters, and trap sediments. When they fossilise, they appear as domed structures with fine strata visible inside. But similar structures can also be formed by purely geological means, so on their own they don’t clinch the case for life.
To build a case that stromatolites or microstructures have a biological origin, researchers have to gather more evidence.
One favoured approach is to look at chemical isotopes of carbon and sulfur. Life prefers its carbon “light”, so rocks that that show a reduction in the ratio of heavy carbon (known as C13) to light (C12) suggest the hand of biology. When it comes to sulfur, some microbes, especially those that live in volcanic vents, use this atom for their metabolism instead of oxygen. This chemistry is also recorded in changes in the ratio of sulfur isotopes. But analysing carbon or sulfur requires grinding up the rocks. So even if lifelike signatures are found, it doesn’t prove the microscopic structures were responsible. Meteorites, which showered the early earth, can also deliver rocks with unusual isotope ratios.
More recent spectroscopic techniques can target a beam directly at the putative microfossils to detect organic molecules. Finding that a microscopic blob is also rich in organic molecules bolsters its biological credentials.
Researchers also pay close attention to the history of the rocks that the putative evidence is found in. If the rocks have been extremely deformed through pressure and heating, it’s unlikely that any structures are preserved fossils.
Last year several publications used collections of such data to mount a case for early life candidates from around the globe.
Western Australia’s Pilbara craton – one of only two regions of the Earth’s ancient crust that has resisted major alterations for 3.5 billion years – is the site of several possible early life claims, all within a region spanning about five kilometres.
The “Strelley Pool” deposit, a one-time shallow seabed not unlike the Great Barrier Reef, provides the strongest suite of evidence for ancient life, says Walter. Some of the 3.43-billion-year-old rocks show stromatolites, while under the microscope others have revealed startling images of spherical and lentil-shaped microfossils.
“You can see the evidence of life,” says Pisane.
In 2017, spectroscopy showed these fossils were also enriched in nitrogen- and oxygen-rich organic molecules.
The case for ancient life in other Pilbara rocks is not as clear. Walter acknowledges that this includes claims by his own team, which has offered evidence for stromatolites and microfossils in rocks of the 3.48-billion-year-old Dresser formation. Not far from Strelley Pool, it represents a former hot spring region inside a volcanic caldera that Walter describes as “something like Yellowstone”.
Schopf’s claims for microfossils in 3.5-billion-year-old Pilbara rocks in the Mt Ada region continue to be controversial, despite a recent paper in Proceedings of the National Academy of Sciences last year that co-located organic compounds with the structures.
Equally controversial is a claim published last year in Nature for rocks aged 3.77 to 4.28 billion years old in the Nuvvuagittuq belt in Quebec, Canada. Matthew Dodd and colleagues from University College, London, UK, reported the existence of microscopic haematite tubes here, and propose they were formed by lifeforms living in undersea volcanic vents. The biggest criticism of these claims, says Walter, is that these rocks have experienced so much heat and pressure, it’s hard to see how they could have preserved the imprints of fossils.
“These are highly altered rocks,” he notes.
Sitting on slightly firmer ground is a claim for West Greenland’s 3.7-billion-year-old Isua greenstone belt. Some of these rocks are “less altered”, says Walter. In 2016, putative stromatolites were found there.
But finding a few dome-shaped imprints in rocks won’t close a case. More evidence came last year when a paper in the journal Nature from Tue Hassenkam and colleagues at the University of Copenhagen, Denmark. The scientists used infrared spectroscopy to identify complex organic compounds protected deep within the armour of crystal garnets in these same rocks.
Evidence of light carbon isotopes trapped in crystalline armour also comes from zircon crystals in the Jack Hills, south of the Pilbara. If you buy this argument, then life was already around 4.1 billion years ago – the age of these zircons.
But not everybody does.
To leapfrog the arguments, Pisane and his team turned to DNA.
During evolution, the letters of DNA change randomly at a constant rate, rather like the shifting cards on a poker machine. So, a gene that codes for blood haemoglobin is expected to show a small number of changes between humans and chimpanzees; and far more changes between humans and dogs. The total number of changes can be used as a “molecular clock”, to estimate the time elapsed since any two species diverged from a common ancestor.
The modern version of the technique, known as the “the relaxed molecular clock”, takes into account that the mechanism actually runs at different speeds depending on the function of the DNA, the generation time of the species. and the environment in which it lives.
Pisane used a relaxed clock, 29 sample genes that are present in all known species, and some hefty computing power to carry out his birth date calculation for LUCA. The genes code for ribosomal proteins, part of the universal mechanism for translating DNA into proteins.
Like any clock, this one also needed calibration. One hard calibration point was the collision between Earth and the Mars-sized planet, Theia, that created the moon about 4.52 billion years ago. This collision would have sterilised the world, setting the zero time point.
For the latest time point at which life emerged, Pisane used the date from Strelley pool: 3.4 billion years.
And for the latest time point to set the emergence of advanced cellular life, Pisane chose large, complex fossil cells called acritarchs from the 1.6191 billion-year-old Ga Changcheng Formation in North China.
The clock delivered the date for the emergence of LUCA at 4.5 billion years – barely 20 million years after the sterilisation of the planet. It’s a date, Pisane admits, that needs to be treated “with some level of uncertainty”. But the team is very confident that LUCA emerged before 3.9 billion years ago.
That places the emergence of life even before asteroids stopped pummelling the earth 3.8 billion years ago – a period known as “the late heavy bombardment”.
Indeed, a paper in March this year finds that life could have hung on below the surface of the Earth’s crust.
The Pisane team is also confident of two other significant time dates predicted by their clock concerning the emergence of the two main groups of bacteria by 3.4 billion years ago.
If the clock is right, then LUCA must have been a hardy creature, able to withstand asteroid bombardments, toasty temperatures and near-zero oxygen.
What was LUCA like? Probably a methanogen, suspects Martin. These are organisms that split hydrogen for energy and produce methane as a by-product. The modern day descendants of methanogens – archaebacteria – still thrive in hot volcanic springs and undersea volcanic hydrothermal vents, parts of the planet that resemble the way it was four billion years ago.
So what might these findings mean for the crescendo of discoveries from other planets in the past few months, including water and organic molecules on Mars, and organic compounds in the volcanic water plumes ejected from Saturn’s moon Enceladus?
For Martin, the message is, “Life has an opportunity on planets like our early Earth. If the hydrothermal vent theories are right, all we need are rocks and water and carbon dioxide and heat.”
