Sloths weren’t always slow-moving, furry tree dwellers. Their prehistoric ancestors were massive, weighing up to 4 tons (3.6 metric tons), and when startled, they brandished immense claws.
For a long time, scientists believed that the first humans to come in the Americas hunted down these huge ground sloths, as well as many other massive species such as mastodons, saber-toothed cats, and dire wolves that previously roamed North and South America.
However, new study from many locations suggests that people arrived in the Americas sooner — possibly much earlier — than previously assumed. These findings suggest that these early Americans lived a very different life, possibly spending millennia sharing prehistoric savannas and marshes with massive creatures.
“There was this idea that humans arrived and killed everything off very quickly—what’s called ‘Pleistocene overkill,'” said Daniel Odess, an archaeologist at New Mexico’s White Sands National Park. However, new discoveries indicate that “humans were living alongside these animals for at least 10,000 years, without causing them to go extinct.”
Some of the most tantalizing findings come from Santa Elina, an archaeological site in central Brazil where huge ground sloth bones show signs of human manipulation. Sloths like these previously roamed from Alaska to Argentina, and some species had bone structures on their backs known as osteoderms, which looked similar to the plates on current armadillos and may have been utilized as decorations.
In a lab at the University of Sao Paulo, researcher Mírian Pacheco holds a circular, penny-sized sloth fossil. She observes that the surface is astonishingly smooth, the edges look to have been purposefully polished, and there is a little hole near one edge.
“We believe it was intentionally altered and used by ancient people as jewelry or adornment,” she informed us. Three identical “pendant” fossils are clearly distinguishable from unworked osteoderms on a table, which are rough-surfaced and lack any perforations.
These artifacts from Santa Elina are around 27,000 years old, more than 10,000 years before experts previously believed that humans arrived in the Americas.
Initially, scholars wondered if the craftsmen were working on old fossils. However, Pacheco’s research strongly shows that ancient people carved “fresh bones” soon after the animals died.
Her findings, together with other recent finds, may help rewrite the story of how humans first arrived in the Americas — and the impact they had on the environment they discovered.
“There’s still a big debate,” Pacheco added.
Scientists know that the first people appeared in Africa, then spread to Europe and Asia-Pacific before making their way to the Americas, the last continental frontier. However, uncertainties persist about the concluding chapter of the human origins myth.
Pacheco was taught in high school the view that most archaeologists believed during the twentieth century. “What I learned in school was that Clovis was first,” she told me.
Clovis is a location in New Mexico where archaeologists discovered characteristic projectile points and other artifacts dating back 11,000 to 13,000 years.
This date coincides with the end of the last Ice Age, when an ice-free corridor is thought to have arisen in North America, raising the possibility that early humans arrived on the continent after crossing the Bering land bridge from Asia.
And, because the fossil record shows that the decline of American megafauna began about the same period — with North America losing 70% of its huge mammals and South America losing more than 80% — many academics concluded that humans’ presence caused mass extinctions.
“It was a nice story for a while, when all the timing lined up,” said paleoanthropologist Briana Pobiner from the Smithsonian Institution’s Human Origins Programme. “But it doesn’t really work so well anymore.”
In the past 30 years, new research methods — including ancient DNA analysis and new laboratory techniques — coupled with the examination of additional archaeological sites and inclusion of more diverse scholars across the Americas, have upended the old narrative and raised new questions, especially about timing.
“Anything older than about 15,000 years still draws intense scrutiny,” said Richard Fariña, a paleontologist at the University of the Republic in Montevideo, Uruguay. “But really compelling evidence from more and more older sites keeps coming to light.”
In Sao Paulo and at the Federal University of Sao Carlos, Pacheco studies the chemical changes that occur when a bone becomes a fossil. This allows her team to analyze when the sloth osteoderms were 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 sloths died, but not thousands of years later.
Her team also tested and ruled out several natural processes, like erosion and animal gnawing. The research was published last year in the journal Proceedings of the Royal Society B.
One of her collaborators, paleontologist Thaís Pansani, recently based at the Smithsonian Institution, is analyzing whether similar-aged sloth bones found at Santa Elina were charred by human-made fires, which burn at different temperatures than natural wildfires.
Her preliminary results suggest that the fresh sloth bones were present at human campsites — whether burned deliberately in cooking, or simply nearby, isn’t clear. She is also testing and ruling out other possible causes for the black markings, such as natural chemical discoloration.
The first site widely accepted as older than Clovis was in Monte Verde, Chile.
Buried beneath a peat bog, researchers discovered 14,500-year-old stone tools, pieces of preserved animal hides, and various edible and medicinal plants.
“Monte Verde was a shock. You’re here at the end of the world, with all this organic stuff preserved,” said Vanderbilt University archaeologist Tom Dillehay, a longtime researcher at Monte Verde.
Other archaeological sites suggest even earlier dates for human presence in the Americas.
Among the oldest sites is Arroyo del Vizcaíno in Uruguay, where researchers are studying apparent human-made “cut marks” on animal bones dated to around 30,000 years ago.
At New Mexico’s White Sands, researchers have uncovered human footprints dated to between 21,000 and 23,000 years ago, as well as similar-aged tracks of giant mammals. But some archaeologists say it’s hard to imagine that humans would repeatedly traverse a site and leave no stone tools.
“They’ve made a strong case, but there are still some things about that site that puzzle me,” said David Meltzer, an archaeologist at Southern Methodist University. “Why would people leave footprints over a long period of time, but never any artifacts?”
Odess at White Sands said that he expects and welcomes such challenges. “We didn’t set out to find the oldest anything — we’ve really just followed the evidence where it leads,” he said.
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.
And the White Sands footprints preserve a few moments of their early interactions.
As Odess interprets them, one set of tracks shows “a giant ground sloth going along on four feet” when it encounters the footprints of a small human who’s recently dashed by. The huge animal “stops and rears up on hind legs, shuffles around, then heads off in a different direction.”