SAO PAULO (AP) — Sloths weren't always slow-moving, furry tree dwellers. Their prehistoric ancestors were huge — weighing up to 4 tons (3.6 metric tons) — and when startled, they would brandish huge claws.
For a long time, scientists believed that the first humans to reach the Americas quickly killed these giant ground sloths through hunting, along with many other megafauna such as mastodons, saber-toothed cats, and dire wolves that once roamed North and South America.
But new research from several sites is beginning to suggest that people came to the Americas earlier—and perhaps much earlier—than previously thought. These findings suggest a markedly different life for these early Americans, one in which they may have spent thousands of years sharing prehistoric savannas and wetlands with huge beasts.
“There was this idea that humans arrived and killed everything very quickly, which is called 'Pleistocene exaggeration,'” said Daniel Odis, an archaeologist at White Sands National Park in New Mexico. But the new discoveries suggest that “humans were around until “These animals have been around for at least 10,000 years, without causing their extinction.”
Some of the most exciting clues come from an archaeological site in central Brazil, called Santa Elena, where the bones of giant ground sloths show signs of being manipulated by humans. Sloths like these lived from Alaska to Argentina, and some species had bony structures on their backs, called osteoderms — somewhat similar to the plates of a modern armadillo — which were probably used to make ornaments.
In a laboratory at the University of São Paulo, researcher Mirian Pacheco holds in her hand a round, penny-sized fossil of a sloth. She notes that its surface is surprisingly smooth, the edges appear to have been deliberately polished, and there is a small hole near one edge.
“We believe they were intentionally modified and used by the ancients as jewelry or adornments,” she said. Three similar “necklace” fossils are clearly different from the unworked osteoderms found on the table – they have a rough surface and without any holes.
These artifacts from Santa Elena are approximately 27,000 years old, more than 10,000 years before scientists believe humans reached the Americas.
Originally the researchers wondered whether the artisans were actually working on ancient fossils. But Pacheco's research strongly suggests that ancient people were carving “new bones” shortly after animals died.
Her findings, along with other recent discoveries, could help rewrite the story of how humans first arrived in the Americas, and their impact on the environment they found.
“There is still a lot of controversy,” Pacheco said.
Scientists know that the first humans appeared in Africa, then moved to Europe, Asia and the Pacific, before finally making their way to the final continental frontier, the Americas. But questions remain about the final chapter in the story of human origins.
In high school, Pacheco received the theory embraced by most archaeologists throughout the twentieth century. “What I learned in school is that Clovis was the first,” she said.
Clovis is a site in New Mexico where archaeologists in the 1920s and 1930s found distinctive projectile points and other artifacts dating between 11,000 and 13,000 years ago.
This date coincides with the end of the last ice age, a time when an ice-free corridor likely emerged in North America, raising an idea about how early humans moved to the continent after crossing the Bering land bridge from Asia.
Because the fossil record shows the widespread decline in American megafauna that began around the same time—North America lost 70% of its large mammals, and South America lost more than 80%—many researchers have speculated that the arrival of humans led to mass extinctions.
“It was a beautiful story for a while, when all the timing lined up,” said Brianna Popiner, a paleoanthropologist with the Smithsonian Institution's Human Origins Program. “But it doesn’t work so well anymore.”
In the past 30 years, new research methods—including ancient DNA analysis and new laboratory techniques—along with the examination of additional archaeological sites and the inclusion of more diverse scientists throughout the Americas, have turned the old narrative on its head and raised new questions, especially about timing. . .
“Anything older than 15,000 years is still under intense scrutiny,” said Richard Farina, a paleontologist at the University of the Republic in Montevideo, Uruguay. “But really compelling evidence from more and more ancient sites keeps emerging.”
In São Paulo and at the Federal University of São Carlos, Pacheco studies the chemical changes that occur when a bone turns into a fossil. This allows her team to analyze when sloth osteoderms were most likely modified.
“We found that the osteoderms were carved before the fossilization process” in “fresh bones” — meaning anywhere from a few days to a few years after the sloth died, but not thousands of years later.
Her team also tested and ruled out several natural processes, such as abrasion and animal gnawing. The research was published last year in the journal Proceedings of the Royal Society B.
One of her collaborators, paleontologist Thais Pansani, who recently worked at the Smithsonian Institution, is analyzing whether similar-aged sloth bones found at Santa Elena were charred by man-made fires, which burn at different temperatures than natural wildfires. .
Their preliminary findings suggest that fresh sloth bones were present in human camps, whether they were intentionally burned during cooking, or simply nearby, is not clear. They also test and rule out other possible causes of black marks, such as natural chemical discoloration.
The first site widely accepted as older than Clovis was at Monte Verde, Chile.
Buried under a peat bog, researchers discovered 14,500-year-old stone tools, pieces of preserved animal skins, and numerous edible and medicinal plants.
“Monte Verde was a shock. You are here at the end of the world, with all this organic material preserved,” said Vanderbilt University archaeologist Tom Dillihay, a longtime Monte Verde researcher.
Other archaeological sites indicate earlier dates of human presence in the Americas.
Among the oldest sites is Arroyo del Vizcaino in Uruguay, where researchers are studying apparently man-made “cut marks” on animal bones dating back about 30,000 years.
In White Sands, New Mexico, researchers discovered human footprints dating back to between 21,000 and 23,000 years ago, as well as similar footprints from giant mammals. But some archaeologists say it's hard to imagine that humans would repeatedly pass a site without leaving behind any stone tools.
“They make a strong case, but there are still some things about this site that puzzle me,” said David Meltzer, an archaeologist at Southern Methodist University. “Why do people leave footprints over a long period of time, but never any artifacts?”
Odess at White Sands said he expects and welcomes such challenges. “We didn't aim to find the oldest thing, we just followed the evidence where it led us,” he said.
While the exact timing of humans' arrival in the Americas remains disputed—and may never be known—it seems clear that if the first humans arrived earlier than previously thought, they did not immediately wipe out the giant monsters they encountered.
The white sand footprints preserve a few moments of their early interactions.
As Odis interprets it, a set of tracks shows “a giant ground sloth walking on four feet” when it comes across the footprints of a small, recently crushed human. “The huge animal stops, stands on its hind legs, moves, and then heads in a different direction.”
The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute's Science and Education Media Group. AP is solely responsible for all content.
