New research revealed that the first humans to arrive in the Americas lived alongside giant ground sloths for thousands of years, challenging previous theories about human migration and megafauna extinction. The study, published in the scientific journal Proceedings of the Royal Society B, indicates that humans may have inhabited South America as early as 27,000 years ago.
At a laboratory at the University of São Paulo, researcher Mírian Pacheco holds in her palm a round sloth fossil the size of a US penny. Noting its surprisingly smooth surface, deliberately polished edges, and a small hole near one edge, she explains that these features suggest human modification. “We believe it was intentionally altered and used by ancient people as jewelry or adornment,” Pacheco stated.
These artifacts from the Santa Elina archaeological site in central Brazil are about 27,000 years old. The bones of giant ground sloths found there show signs of having been manipulated by humans, offering compelling evidence of early human presence in the Americas, more than 10,000 years earlier than previously thought. Pacheco's team analyzed the chemical changes that occur when a bone becomes a fossil to determine when the sloth osteoderms—the bony structures on the sloths' backs—were likely modified. “We discovered that the osteoderms were carved before the process of fossilization, in fresh bones, meaning within days to years after the death of the sloths, but not thousands of years later,” she said.
This discovery suggests that ancient people were carving these bones shortly after the animals died, pointing to direct interaction between humans and giant ground sloths. Such evidence challenges the long-held "Clovis-first" theory, which posited that the earliest humans arrived in the Americas between 11,000 and 13,000 years ago via an ice-free corridor in North America after crossing the Bering land bridge from Asia.
“There’s still a big debate,” Pacheco acknowledged, highlighting the ongoing discussions among scientists about the timing of human arrival in the Americas. The findings contribute to a growing body of evidence that humans may have inhabited the Americas much earlier than previously believed.
Other archaeological sites support this revised timeline. At New Mexico's White Sands National Park, researchers have uncovered human footprints dated to between 21,000 and 23,000 years ago, alongside tracks of giant mammals from the same period. Daniel Odess, an archaeologist at White Sands, interprets one set of tracks as showing "a giant ground sloth going along on four feet" when it encounters the footprints of a small human who has recently dashed by. The huge ground sloth "stops and rears up on hind legs, shuffles around, then heads off in a different direction."
“There was this idea that humans arrived and killed everything off very quickly—what's called 'Pleistocene overkill.' But new discoveries suggest that humans were existing alongside these animals for at least 10,000 years, without making them go extinct,” Odess noted. This suggests that the lives of the first Americans were vastly different from previous assumptions, as they may have spent millennia sharing prehistoric savannas and wetlands with enormous beasts such as mastodons, saber-toothed cats, and dire wolves.
Briana Pobiner, a paleoanthropologist at the Smithsonian's Human Origins Program, reflected on the shifting perspectives: “It was a nice story for a while, when all the timing lined up. But it doesn't really work so well anymore.” The fossil record shows widespread decline of megafauna starting around the same time humans are believed to have arrived, with North America losing 70% of its large mammals and South America more than 80%. However, the new evidence indicates that humans and megafauna coexisted for thousands of years without humans causing their immediate extinction.
“Really compelling evidence from more and more older sites keeps coming to light,” said Richard Fariña, a paleontologist at the University of the Republic in Montevideo, Uruguay. He noted that while anything older than about 15,000 years still draws intense scrutiny, the accumulating data supports an earlier human presence.
Tom Dillehay, a Vanderbilt University archaeologist and longtime researcher at Monte Verde in Chile—where researchers discovered 14,500-year-old stone tools, preserved pieces of animal hides, and various edible and medicinal plants buried beneath a peat bog—said, “Monte Verde was a shock. You’re here at the end of the world, with all this organic stuff preserved.”
The examination of additional archaeological sites and the inclusion of more diverse scholars across the Americas have raised new questions, especially about timing. In the last 30 years, new research methods—including ancient DNA analysis and new laboratory techniques—have upended the old narrative and challenged the "Clovis-first" model.
David Meltzer, an archaeologist at Southern Methodist University, expressed caution regarding some of the new findings. Referring to the White Sands footprints, he said, “They’ve made a strong case, but there are still some things about that site that puzzle me. Why would people leave footprints over a long period of time, but never any artifacts?”
While the exact timing of humans’ arrival in the Americas remains contested and may never be known, it seems clear that if the first people arrived earlier than once thought, they didn’t immediately decimate the giant beasts they encountered.
Los Angeles Times, The Sun, and CBS News reported on the research, among other websites.
This article was written in collaboration with generative AI company Alchemiq