A team of Australian geologists announced their discovery in a paper titled “Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures,” published Wednesday in Nature.
They made their find in July 2012 while doing field research in Isua, a region of Greenland so remote that they had to travel there by helicopter. The site is known for having some of the oldest rocks on Earth, in what is known as the Isua supracrustal belt. Allen Nutman, a University of Wollongong geologist who has studied the rocks there since 1980, said one day he and his colleagues were working at the site when they spied some outcroppings they'd never seen before. The formations had been exposed where the snow pack had melted — the result, Nutman said, of the global warming that is so pronounced in Greenland or of low levels of snowfall the previous winter.
They examined the outcropping and immediately saw something intriguing: conical structures, just one to four centimeters (less than two inches) high. They look like fossilized microbial mats — basically, pillows of slime — known as stromatolites, which are formed today by bacterial communities living in shallow water.
“We all said, 'This is amazing. These look like stromatolites,' " Nutman told The Washington Post.
Subsequent laboratory analysis established that the formation is 3.7 billion years old, and turned up additional chemical signatures consistent with a biological origin for the conical structures, Nutman said. The scientists determined the age of the rocks through radiometric dating, measuring the abundance of elements created by the steady decay of uranium.
Fossilized stromatolites nearly 3.5 billion years old have previously been found in Western Australia. Those fossils have until now been the oldest widely accepted evidence for life on Earth. Some researchers have cited signatures of life from an even earlier time, including in the Isua formation in Greenland, but typically these assertions have involved biologically friendly molecules rather than actual fossils. Moreover, very old rocks — older than 3 billion years — are exceedingly rare, because the Earth's surface has been eroded over time and recycled through plate tectonics.
Claims about evidence for ancient life have invariably been controversial. The multiple lines of evidence for the Greenland stromatolites "are not as clear cut as you’d ideally want for such an extraordinary claim," cautioned Abigail Allwood, a geologist at the NASA Jet Propulsion Laboratory who has studied fossil stromatolites.
"They might really be biological but it's hard to absolutely refute the possibility that they formed by localized mineral precipitation from seawater. If we found these on Mars, would we plant a flag and declare that we had found life on Mars? I think not, but we would definitely get very excited and continue looking around for more information," she said.
“We expect there will be some robust debate. That’s what science is all about. There will be people surely who will put forward alternative hypotheses. But we think we’ve covered all those alternatives," Nutman said.
The Australians' claim of Greenland stromatolites is "plausible and likely correct," said J. William Schopf, a pioneering paleobiologist at the University of California at Los Angeles who was not involved in the discovery.
Schopf, who in 1993 reported the discovery of 3.465-billion-year-old microfossils in Western Australia, said he expects scientists to find more Greenland stromatolites as the warming atmosphere continues to melt the huge ice sheet covering the world's largest island.
The Australian researchers do not contend that these stromatolites represent the first examples of life on the planet. Rather, these would have to be the descendants of the earlier life forms. Microbes capable of performing photosynthesis and forming communities are relatively sophisticated organisms. They presumably had less-sophisticated ancestors that lived more than 4 billion years ago, the Nature paper states.
“Stromatolites are really complex, so you have to have a lot of evolution from when life started to when stromatolites appeared in the fossil record. So life either had to start earlier, or evolution is more rapid than you might expect," said Sara Walker, an astrobiologist at Arizona State University who was not involved in the new study.
Earth, along with the other planets in our solar system, formed about 4.5 billion years ago from a cloud of dust and gas swirling around the embryonic sun. For hundreds of millions of years, ours was a harsh, molten world, heavily bombarded by debris. At one point, a Mars-sized object slammed into the Earth and blasted into space the material that eventually cohered into the moon.
Schopf said that when he first began working in paleobiology half a century ago, the leaders of the field believed that life began on Earth only about a billion years ago. Discoveries kept pushing the start date for life further into the past.
"The origin of life, at least on a planet like ours, is a lot faster, and you think a lot easier than anyone had imagined. To the extent that that is true, life ought to be abundant in the universe — because there are lots of Earth-like planets out there," Schopf said.
This remains a matter of conjecture and philosophy, because no one has discovered a sign of extraterrestrial life. The early dates for the origin of life on Earth also could suggest that complex, multi-cellular life (including, perhaps, an intelligent and technological species) typically arises on a planet only after a very long period of evolution and diversification.
No one knows how life began on Earth. Charles Darwin hypothesized that life emerged in a “warm little pond,” but other researchers imagine that it emerged around a deep-sea hydrothermal vent, or even came to Earth from space, perhaps after sparking into existence on Mars, or even in some other, distant planetary system.
The Greenland discovery comes just a week after the announcement that astronomers have detected evidence of a “habitable” planet orbiting Proxima Centauri, a red dwarf star that is the sun’s closest stellar neighbor. But little is known about that planet other than its mass and temperature, which suggest that, if it has an atmosphere, water could remain liquid at the surface.
Closer to home, Mars remains the top target for the search for extraterrestrial life, and the Greenland discovery, if it holds up, suggests that the investigation on Mars should include a hunt for fossilized stromatolites.
“It means that there is a heightened interest in the search for life on Mars," Nutman said Wednesday. "Three thousand seven hundred million years ago, Mars was wet. If life had managed to evolve to produce structures like stromatolites by 3,700 million years ago on Earth, there is an increased probability — certainly not a certainty — that the same type of process might have happened on Mars before it dried out.”