Geologists have discovered in Greenland evidence for ancient life in rocks that are 3.7 billion years old. The find, if confirmed, would make these fossils the oldest on Earth and may change scientific understanding of the origins of life.
Experts are likely to debate whether the structures described in the new report were formed biologically or through natural processes. If biological, the great age of the fossils complicates the task of reconstructing the evolution of life from the chemicals naturally present on the early Earth. It leaves comparatively little time for evolution to have occurred and puts the process close to a time when Earth was being bombarded by destructive asteroids.
The fossils were discovered four years ago but not publicized while the geologists, a team led by Allen P. Nutman of the University of Wollongong in Australia, checked out their find.
“Of course one felt very excited, but we’re not the rushing types and we took our time,” Dr. Nutman said. “We kept it secret because we wanted to present it in the most robust way we could manage.”
The fossils were part of an outcrop of ancient rock that had lost its usual snow cover. The rock layer forming the outcrop, known to geologists as the Isua supracrustal belt, lies on the southwest coast of Greenland and is some 3.9 to 3.7 billion years old.
Researchers earlier had claimed that Isua rocks had a chemical composition indicative of life, but critics said this mix of chemicals could have arisen through natural processes.
The new fossils, described on Wednesday in the journal Nature, are the first visible structures found in the Isua rocks. They are thought to be stromatolites, layers of sediment packed together by microbial communities living in shallow water.
They are some 220 million years more ancient than the oldest previously known fossils, also stromatolites. Those are 3.48 billion years old and were discovered in the Pilbara Craton of Western Australia.
The new report “provides the oldest direct evidence of microbial life,” said Gerald Joyce, an expert on the origin of life at the Scripps Research Institute in La Jolla, Calif.
Dr. Nutman said there was a “diminishing probability” that older fossils will ever be found. Rocks from this period are very rare. Those that survive have been cooked to such high heats by geological processes like mountain-forming that evidence of fossils and sedimentary layers is destroyed. Most of the Isua rocks have been cooked in this way.
Only the small outcrop now discovered, measuring some 98 by 230 feet, avoided intense heating. Through this chance escape, the little patch throws a narrow shaft of light onto events close to the dawn of life on Earth.
The rocks contain several small cones, the tallest just over an inch and half. The cones have a layered structure that resembles the bands of sediment gathered and cemented together by the microbial communities that form today’s stromatolites.
Because of the importance of the claim made by Dr. Nutman and his colleagues, considerable discussion can be expected before their interpretation of the structures as evidence of life is generally accepted.
Certain features “are fairly credible hallmarks of microbial activity,” Abigail C. Allwood of Caltech’s Jet Propulsion Laboratory wrote in a commentary accompanying the Nature article. They have a few features that make them “interesting and possibly biological,” she added in an email.
Another expert in the early Earth’s environment, Tanja Bosak of M.I.T., said the structures do resemble modern stromatolites but their origin “will be hotly debated,” given there is no sign of certain features that might bolster the case for biological origin, such as crinkling in the layers of sediment.
Several different species of microbes are involved in stromatolite creation. The Isua structures, if indeed stromatolites, would represent fairly evolved organisms.
Dr. Nutman argues that life must therefore have originated even earlier, probably in the late Hadean stage of Earth’s history, which lasted from 4.65 billion years ago — when the planet formed from debris in orbit around the sun — to 4 billion years ago.
But the Hadean was so called because of the hellish conditions thought to have prevailed, including cataclysmic meteorite impacts that boiled the oceans into steam and turned Earth’s surface into molten lava. The largest of these impacts, at 4.5 billion years ago, tore a piece from Earth that became the moon.
It is difficult to see how life could have begun under such circumstances. But some geologists now favor a milder version of the Hadean, with the rain of asteroids quickly tapering off after the moon was formed.
The early sun was much weaker then, and the threat to life, in this view, would not have been molten lava but frozen oceans, a calamity that may have been averted by a surge of greenhouse gases.
“The largest Hadean impacts may have been severe, but not planet-sterilizing. Once life had developed to the point that it could spread across the planet, it would have been hard to wipe out,” said Jack Szostak, an expert on life origins at Harvard Medical School.
But the chemistry of life favors an origin on land, not the deep ocean, he said.
Still, even when the Hadean ended, a final rain of large asteroids descended on Earth at the beginning of the ensuing Archaean stage, possibly set loose when the giant planets Saturn, Uranus and Neptune drifted out into the Kuiper belt of asteroids.
This cataclysm, known as the Late Heavy Bombardment, hit Earth between 3.9 and 3.8 billion years ago.
Dr. Nutman believes life could have survived through the end of the Hadean and the bombardment. Some geologists, he notes, now think the asteroid impacts were spread over time, lessening their effects.
“The Late Heavy Bombardment is becoming less heavy as the years go by,” he said.
But others believe the bombardment was no light peppering. Evidence of these ancient craters has vanished from Earth but is still evident in the pockmarked face of the moon. And for every crater on the moon, 20 would be expected to have been made on Earth.
The moon has two craters more than 600 miles across that were created during the Late Heavy Bombardment. Some 40 craters this size may have been gouged out of our planet in the same interval, said William F. Bottke, an asteroid expert at the Southwest Research Institute in Boulder, Colo.
By comparison, the asteroid that wiped out the dinosaurs 66 million years ago left a crater only 110 miles in diameter.
If life on Earth did not begin until after the Late Heavy Bombardment, then it had a mere 100 million years in which to evolve to the quite advanced stage seen in the new fossils.
If so, Dr. Allwood wrote, then “life is not a fussy, reluctant and unlikely thing.” It will emerge whenever there’s an opportunity.
But the argument that life seems to have evolved very early and quickly, so therefore is inherently likely, can be turned around, Dr. Joyce said. “You could ask why, if life were such a probable event, we don’t have evidence of multiple origins,” he said.
In fact, with trivial variations, there is only one genetic code for all known forms of life, pointing to a single origin.
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