ARI SHAPIRO, HOST:
A couple of years ago, scientists announced that they had found the oldest evidence of life in rocks that are 3.7 billion years old. A separate group of researchers found that hard to believe. They think that the rocks might just be old rocks. NPR's Nell Greenfieldboyce reports.
NELL GREENFIELDBOYCE, BYLINE: When and how life emerged on our planet is a mystery. But some fossils are incredibly old.
JOEL HUROWITZ: The most widely agreed-upon evidence for the earliest fossil life on Earth is found out in Western Australia.
GREENFIELDBOYCE: Joel Hurowitz is a geochemist at Stony Brook University in New York. He says the rocks in Australia date back about 3 1/2 billion years. They contain cone-shaped structures created by masses of microbes growing up from a seafloor.
HUROWITZ: We're actually seeing sort of the total community that has basically been turned into a bunch of minerals.
GREENFIELDBOYCE: Back in 2016, researchers revealed they'd found similar mineral structures in Greenland. That was a real shocker because the rocks in Greenland were even more ancient, 200 million years older. And they'd been more cooked and tortured by geologic processes that should have destroyed any signs of life.
HUROWITZ: You know, my thought was - boy, someone should follow up on that (laughter).
GREENFIELDBOYCE: Hurowitz and some colleagues did follow up. And now, in the journal Nature, they say there is no way these things are fossils. A deeper look into the rock shows that the structures aren't really shaped like the cones that microbes would make.
HUROWITZ: Instead, what they look like are - for lack of a better word, they kind of look like Toblerone bars. They're stretched out ridges that extend deeply into the rock.
GREENFIELDBOYCE: The scientists also studied the rock's chemical composition. Abigail Allwood is an astrobiologist at NASA's Jet Propulsion Lab. She says they used a prototype of an instrument that NASA is planning to send to Mars to look for signs of ancient life. She says the chemistry of the Greenland rocks did not support the idea that microbes had left their mark.
ABIGAIL ALLWOOD: It is disappointing. And it's also disappointing from the perspective of what it means for the search for life on Mars.
GREENFIELDBOYCE: She says, if life could have gained a foothold so soon after the Earth formed, that would have suggested that life could arise and take hold quickly anywhere it's given half a chance.
Now, the researchers who made the original claim about the Greenland rocks are vigorously disputing this new analysis. They still maintain that they have found the world's oldest fossils. So far, relatively few scientists have studied these rocks. That means it's a pretty sure bet that this won't be the last word. Nell Greenfieldboyce, NPR News.
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