Fossilized bones from an extinct, shelled mammal offer us the latest clue about when humans arrived in South America.
Scientists have long argued over when people first set foot on the continent because "the evidence is so scarce," says Miguel Delgado, an archaeologist at the National University of La Plata in Argentina. For a while, he says, the consensus hovered around 13,000 years ago — but a slow accumulation of research has been pushing that date back earlier.
Now, in a study published in PLOS One, Delgado and his colleagues have used the new fossil fragments from the armored glyptodont to support other work placing humans in South America at least 21,000 years ago. This was at the end of the Pleistocene, when they would have had to navigate a planet in dramatic climatic flux.
The Last Glacial Maximum occurred about 20,000 years ago, after which glaciers began to retreat worldwide, including from across southern South America. If people were present on the continent then, says Alia Lesnek, a geologist at CUNY Queens College who wasn't involved in the study, "it can tell us about the really long history that humans have with climate change" — and their resilience in the face of it.
"For me," she says, "[it] opens up a lot of new questions about the relationship between how humans traveled and their migration pathways and their settlement patterns — and how that relates to glaciers and climate change."
An unexpected find
The Reconquista River flows through the western outskirts of Buenos Aires, Argentina. Back in 2016, a bulldozer had dug up the riverbank to expand the canal. Shortly after that work was finished, Guillermo Jofré, a paleontologist with the Repositorio Paleontológico Ramón Segura, went for a stroll there.
That's when he stumbled upon a handful of exposed fossil bones that belonged to an ancient armadillo-like mammal called a glyptodont. "This animal was heavily armored," Delgado says. "They had large tail[s] and short limbs."
Finding those bones was a fortuitous accident because when Jofré looked closer, he saw something unexpected — small marks on the bones.
These marks could have been chipped into the bones by rocks or other bones scraping across them, or by rodents or carnivores biting or scratching them, or — perhaps — by prehistoric humans who had done 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, the tail, vertebrae and the pelvis.
Back at the lab, they analyzed the specimens, peering at them under microscopes, analyzing the chemistry of the sediments and also measuring the cut marks in detail, rendering them as 3D models. The results were unmistakable, according to Delgado.
"We realized," he says, "the shape of [those] marks are quite similar to cut marks made experimentally by humans." 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.
"The most important evidence is the location of the marks themselves," he says, "in parts of the bones with denser flesh." These areas of concentrated meat are where people would have wanted to cut and eat the animal. "So this is showing us a logical butchering sequence."
A bold conclusion
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.
It means that if humans were responsible for the cut marks, they must have been present back then, too. "So it's one of the oldest [pieces of] evidence of human presence here in South America," Delgado says.
During this time, at the end of the Pleistocene, numerous large animals populated the harsh and frigid landscape, including giant sloths, mastodons and saber-toothed felines (all of which shared the Earth with humans until some 10,000 years ago). Delgado says that prehistoric people living then may have contributed to the extinction of these species. The dramatic changes in the environment as the glaciers retreated didn't help, however.
"I think this is a really exciting step forward," Lesnek says. "But I think a little bit more work does need to be done to fully support the conclusions that they're making — because they really are bold conclusions — to make sure that, yes, this was humans and not some other process making [the cut marks]."
She says future work should focus on additional dating to "securely establish" the age estimate of the fossils and a search for human artifacts along the riverbank. "Finding things like stone flakes or charcoal," she says. "Really unambiguous indications that humans were present, in addition to the cut marks." (Delgado agrees with that assessment and is already planning further analyses and excavations with his colleagues.)
Overall, Lesnek is excited by these and other findings in the field. "The more research we do," she says, "the farther and farther this keeps getting pushed back."
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