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When did people first arrive in South America? It's a matter of some debate, but curious marks cut into an ancient shelled mammal suggest that humans may have been roaming the continent thousands of years earlier than scientists once thought. Here's science reporter Ari Daniel.
ARI DANIEL, BYLINE: The Reconquista River flows through the western outskirts of Buenos Aires. Back in 2016, a bulldozer had dug up the riverbank to expand the canal. Shortly after finishing that work, one of Miguel Delgado's colleagues, a paleontologist, went for a stroll there.
MIGUEL DELGADO: Walking across the riverbanks found the exposed fossil bones.
DANIEL: A handful of small, fossilized bones that belong to an ancient armadillo-like mammal - something called a glyptodont.
DELGADO: This animal was heavily armored.
DANIEL: Delgado is an archaeologist at the National University of La Plata in Argentina. Anyway, there his colleague was, mid-walk, having just discovered these bones.
DELGADO: It was an accident.
DANIEL: But a very fortuitous accident because, when he looked closer, he saw something unexpected.
DELGADO: Different kind of marks in the bones.
DANIEL: These marks could have been chipped into the bones by rocks or other bones scraping across them, or maybe rodents or carnivores biting or scratching them, or perhaps prehistoric humans doing something to them. To figure out which, Delgado and his colleagues excavated a portion of the site and unearthed various fossilized bones of the glyptodont, including pieces of the hard outer shell...
DELGADO: The tail, vertebrae and the pelvis.
DANIEL: Back at the lab, they analyzed the specimens and measured the cut marks in detail, rendering them as 3D models. The results, says Delgado, were unmistakable.
DELGADO: We realized that, actually, the shape of that marks are quite similar to cut marks made experimentally by humans.
DANIEL: In other words, Delgado believes the V-shaped cut marks were inflicted on this animal when it was butchered with stone tools by ancient humans.
DELGADO: The most important evidence is the location of the marks themselves - in parts of the bones with denser flesh.
DANIEL: You would want to cut the animal where there's a lot of flesh, to cut into it to eat it?
DELGADO: Yeah. Yeah. This is the idea.
DANIEL: This isn't the first fossilized glyptodont to turn up with these kinds of marks, but it's certainly among the oldest. When the team dated the fossils, they found this animal lived some 21,000 years ago. So if humans were responsible for the cut marks, they must have been around then, too.
DELGADO: So it's one of the oldest evidence of human presence here in South America.
DANIEL: This would have been towards the end of the Pleistocene, when all sorts of large animals tromped across a harsh and frigid landscape.
DELGADO: There are giant sloths, mastodons, also saber-tooth.
DANIEL: All of which shared the earth with humans until some 10,000 years ago. The findings are published in the journal PLOS One. Alia Lesnek is a geologist at CUNY Queens College who wasn't involved in the study.
ALIA LESNEK: I think this is a really exciting step forward.
DANIEL: But she argues there's more work to do to really nail down that humans made those cut marks, including looking for artifacts along the riverbank.
LESNEK: You know, finding things like stone flakes or charcoal - really unambiguous indications that humans were present in addition to the cut marks.
DANIEL: Lesnek says this work joins a slowly growing body of evidence that humans populated South America earlier than we once thought. It's not at all a settled matter. But if true, this places people in the Americas during the last glacial maximum, before the vast ice sheets began to retreat.
LESNEK: So it can tell us about the really long history that humans have with climate change.
DANIEL: And, she says, the resilience of people in the face of that change.
For NPR News, I'm Ari Daniel.
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