Scientists will gather in a remote and snowy part of southwestern Greenland next summer to try to determine if rocks from 3.7 billion years ago contain some of the oldest fossils of life on Earth — with implications for the search for evidence of life on Mars.
Tiny, triangular structures found in these rocks have been a source of controversy, with some scientists now saying they are not evidence of early life on Earth. The scientists who first reported that they were fossilized evidence of life are defending their claims.
In a paper published online Oct. 17 in the journal Nature, planetary scientist Abigail Allwood and colleagues, who examined the ancient rocks in Greenland, reported that purely geological processes could explain the triangular rock formations — and that while they might still be formed by microbial life, there was not enough evidence to show definitely that they were. [In Images: The Oldest Fossils on Earth]
The scientific dispute has direct implications for the search for life on other planets in our solar system, especially on Mars.
Allwood, who works for NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, is the principal investigator building an instrument called PIXL, based on x-ray fluorescence of rock samples, which will be used by the Mars 2020 rover to search for fossilized evidence of life in Martian rocks.
Allwood’s team used the PIXL instrument to test rock samples from Greenland and reported that concentrations of titanium, potassium, and carbonate chemicals in the rocks did not show that the triangular structures had been formed by microbial life. [The 7 Most Mars-Like Places on Earth]
Scientists think that about 3.7 billion years ago, the environment on Mars was very similar to the environment on Earth at that time, and that early forms of life might have evolved at the same time on both planets — although the very cold and dry Martian environment is now thought to be lifeless.
Scientists needed to learn how to accurately interpret results from the PIXL instrument before deploying it to look for life on the Red Planet, Allwood told Live Science: "We want to try to figure out how to do this stuff before we go to Mars.”
The Isua Greenstone Belt in southwestern Greenlandis thought by geologists to contain some of the oldest exposed rocks on Earth. Until recently, much of the area was covered by snow — only in the last few years has the snow melted enough for scientists to examine many of the rocks.
In 2016, geochemist Allen Nutman of Wollongong University in Australia and his colleagues reported in a paper published in the journal Nature that triangular structures seen on the surface of some of the Isua rocks were cross-sections of cone-shaped stromatolites — tiny, fossilized structures built by microbial colonies on the floors of bodies of water.
Nutman told Live Science in August 2016 that the concentrations of titanium and potassium inside the triangular structures were different than in the rock outside the structures — a possible chemical “biosignature”. His team also reported that the concentrations of carbonate chemicals suggested the microbial colonies were drawing carbonates out of the surrounding seawater.
Until the discovery in Greenland, the earliest known stromatolites had been found at Strelley Pool in the Pilbara region of Western Australia and dated to about 3.4 billion years ago. Nutman's find, if verified, would push back the earliest evidence for the appearance of life on Earth by about 300,000 years.
Allwood visited the same rocks in the Isua region of Greenland this summer and took samples for laboratory testing with the PIXL instrument, which will be attached to the rover to carry out X-ray fluorescence analysis on rock on Mars.
She said that after examining the rocks and studying the geochemistry of the rock samples, she did not believe that the triangular structures were fossilized evidence of very early microbial life: "I think the evidence is very much not in favor of that interpretation," she said.
As well as finding that the supposedly conical structures actually formed long triangular ridges within the rock, Allwood said that the geochemistry of the structures was the same as in some other patches of weathered rock nearby that did not appear to contain any stromatolites.
"I think the onus is on [Nutman] to prove them to be biological, and I think we've definitely shown that there is a much more plausible abiological explanation than a biological one," Allwood said.
Return to Greenland
Nutman is writing a rebuttal of Allwood's research for publication in the journal Nature. He told Live Science in an email that he thinks Allwood and her colleagues spent insufficient time examining the rocks in Greenland — a helicopter trip lasting one day — and did not examine the rock samples that were used for his original study.
Because Allwood and her colleague had analyzed different rock samples than the ones his team had analyzed, they had inevitably found that their observations did not exactly match those of his team. "This is a classic comparing apples and oranges scenario," he wrote.
Nutman said that his team had found the structures could not be purely geological in origin, and that their shape and geochemistry indicated they were created by "extremely rare stromatolites in the Isua rocks, preserved in a tiny relict of a 3,700-million-year-old shallow sea environment."
Allwood, meanwhile, is organizing a field trip next summer to the disputed rocks' location in Greenland, to which Nutman and other scientists will be invited.
"In the middle of next year, we are going to go out to the field again, and we'll take a bunch of experts to have a look and say, yay or nay — what do we think?” Allwood said.
The dispute over the Greenland rocks means that the oldest established fossils of life on Earth may again be the stromatolites found at Strelley Pool in Western Australia.
Allwood first reported the discovery of those fossils in a research paper published in the journal Nature in 2006, and she also wrote an article for Nature on Nutman's discoveryin 2016. But she said she was careful not to come off as "turf defending" over the contending ancient stromatolite finds. "I was very cautious to give the benefit of the doubt, and I certainly didn't jump to any conclusions," she said.
Another recent study has suggested that an outcropping of primitive ocean crust in Quebec in Canada could contain fossils that are even older — between 3.77 billion and 4.29 billion years old — but that study also needs more scientific scrutiny, Allwood said.
Meanwhile, she hopes that the experience of retesting the rocks in Greenland will help the search for the fossil remains of life on Mars, if it exists. "PIXL is being used both to help determine how to prove a biosignature as well as to disprove a biosignature." She added, "That's a helpful thing, anyway."
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Originally published on Live Science.