SAO PAULO, Brazil — Unlike the slow and furry creatures we see today, sloths once had enormous ancestors that weighed up to four tons. These prehistoric giants, when threatened, displayed their formidable claws. For a long time, it was believed that the introduction of the first humans to the Americas led to the rapid extinction of these massive ground sloths and other megafauna like mastodons and saber-toothed cats.
Emerging research, however, indicates that humans may have arrived in the Americas much earlier than previously believed, suggesting that early inhabitants coexisted with large animals for thousands of years rather than hastening their extinction. Daniel Odess, an archaeologist at White Sands National Park in New Mexico, articulated that the once-held belief of “Pleistocene overkill”—the idea that humans quickly exterminated all megafauna—may be inaccurate. He noted that evidence suggests humans shared the landscape with these enormous creatures for at least 10,000 years without causing their extinction.
Recent finds at an archaeological site in central Brazil known as Santa Elina have added weight to this new perspective. Here, researchers discovered the bones of giant ground sloths that show signs of human alteration. These sloths were widely distributed across the Americas, and some species possessed bony structures on their backs, referred to as osteoderms. These could have been used for decorative purposes, similar to the plates of modern armadillos.
In a laboratory at the University of Sao Paulo, researcher Mírian Pacheco examined a sloth fossil she described as smooth and polished, suggesting it was purposefully modified. She suspects that this and similar fossils—dated around 27,000 years ago—were intentionally crafted for use as jewelry, predating the conventional timeline for human arrival in the Americas by over 10,000 years.
Initially, some speculated whether these artifacts were made from old bones. However, Pacheco asserts that humans were likely working on freshly deceased sloth remains, carving them shortly after the animals died. This research could prompt a reevaluation of human migration into the Americas and their environmental impact.
The debate regarding the timeline for human settlement in the Americas continues. While it is established that the first humans emerged in Africa and migrated to Europe, Asia, and eventually the Americas, the exact sequence of events remains fluid. Pacheco’s education grounded her in the belief that Clovis culture marked the earliest evidence of human presence in North America, dating back to 11,000 to 13,000 years ago at a site in New Mexico.
This timeline correlates with the end of the last Ice Age, when an ice-free corridor likely opened in North America, allowing humans to migrate from Asia via the Bering land bridge. Coinciding with this migration was the notable decline of large mammals—a trend often linked to the arrival of human hunters.
Paleoanthropologist Briana Pobiner at the Smithsonian Institution remarked that while this narrative once seemed aligned with evidence, it now appears less reliable due to new findings and methods of analysis. Over the past three decades, advancements in techniques such as ancient DNA extraction and more thorough investigation of archaeological sites—combined with contributions from a wider array of researchers—have challenged the established timeline and introduced uncertainties regarding human migration periods.
Richard Fariña, a paleontologist in Uruguay, noted that anything before approximately 15,000 years is still subject to considerable scrutiny but emphasized that compelling evidence is increasingly emerging from older archaeological sites. Pacheco’s research into the chemical changes in fossilized bones aims to pinpoint the timeline of when sloth osteoderms were modified. Her findings indicate that these bones were likely altered shortly after the animals died, ruling out various natural explanations for their marks.
Prominent older sites, such as Monte Verde in Chile, have garnered attention for their age. Tools and organic materials found there date back around 14,500 years, unsettling initial beliefs about human arrival in the Americas. Other sites, like Arroyo del Vizcaíno in Uruguay, offer evidence of human interaction with animals dating back around 30,000 years, raising questions about the longevity of human presence prior to the Clovis era.
While the research continues, discrepancies exist regarding whether early humans left behind artifacts in certain locations. David Meltzer, an archaeologist, observes the perplexities surrounding footprints discovered in New Mexico alongside traces of giant mammals, posing the question of why no tools were found among these signs of passage.
Despite ongoing discussion about the specifics of early human arrival to the Americas, it becomes increasingly clear that if these early travelers arrived sooner than previously thought, they did not immediately instigate the extinction of the colossal fauna. The evidence, including footprints preserved at White Sands, suggests moments of interaction between human beings and the sloths. Odess describes one set of tracks showing a giant ground sloth responding to a small human’s presence by rearing back and changing direction—a glimpse into their complex cohabitation of the landscape.