SAO PAULO — Contrary to the popular image of sloths as sluggish, furry creatures inhabiting trees, their ancient ancestors were far more formidable. These prehistoric animals could weigh as much as 4 tons (3.6 metric tons) and possessed massive claws that could be wielded when threatened.
Scientists long believed that the initial influx of humans into the Americas led to the rapid extinction of giant ground sloths and other colossal creatures such as mastodons, saber-toothed cats, and dire wolves. However, fresh research originating from various excavation sites indicates that humans might have arrived much earlier in the Americas than previously assumed. This new evidence suggests that early inhabitants potentially coexisted for thousands of years with these gigantic animals.
“The prevailing notion was that humans made quick work of the mega fauna they encountered, a phenomenon termed ‘Pleistocene overkill,’” noted Daniel Odess, an archaeologist at White Sands National Park in New Mexico. Recent findings, however, point to the notion that “humans lived alongside these species for at least 10,000 years without causing their extinction.”
Valuable evidence has emerged from an archaeological site in central Brazil named Santa Elina, where researchers discovered bones of giant ground sloths showing evidence of human alteration. These enormous sloths once ranged from Alaska down to Argentina, with some species sporting unique bony growths on their backs called osteoderms, which may have been utilized for decorative purposes.
Mírian Pacheco, a researcher at the University of Sao Paulo, examined a small, round sloth fossil in her lab and revealed that it had an unexpectedly smooth texture. The edges appeared polished, and a tiny hole was present near one side. “We suspect it was modified intentionally and used by ancient peoples as jewelry or ornaments,” she explained. In contrast, different fossils from the same site display rough surfaces and lack any alterations.
Artifacts recovered from Santa Elina are estimated to be around 27,000 years old, predating earlier beliefs about human arrival in the Americas by more than 10,000 years. Initially, researchers questioned whether the artisans working at the site were simply manipulating ancient fossils, but Pacheco’s investigations suggest that the modifications occurred on “fresh bones,” only a short time after the sloths had died.
Pacheco’s discoveries, along with new findings, could significantly alter the understanding of when humans initially set foot in the Americas, as well as the impact they had. “The discussions are ongoing,” she stated.
While it’s established that the first humans originated in Africa before migrating to Europe and Asia-Pacific, the timeline and details surrounding their later arrival in the Americas remain unclear. Pacheco recalled receiving education about the Clovis people in school. “What I understood then was that Clovis was the first group,” she mentioned.
Clovis is known as a key archaeological site in New Mexico, where unique projectile points and artifacts dating between 11,000 and 13,000 years ago were unearthed. This timeframe coincides with the conclusion of the last Ice Age when an ice-free corridor likely opened up in North America, suggesting a route for early humans traversing the continent after crossing the Bering land bridge from Asia.
The fossil record indicates that the decline of large animals in the Americas began around the same period, with North America losing about 70% of its big mammals and South America experiencing nearly 80% loss. This led many researchers to link human migration with these mass extinctions.
“For a while, it made a coherent story when all the timings aligned,” stated paleoanthropologist Briana Pobiner from the Smithsonian Institution’s Human Origins Program. “But that narrative has become less fitting in recent times.”
The last three decades have seen advancements in research techniques, including ancient DNA analysis and enhanced laboratory methods, along with a broader examination of archaeological sites and more inclusive scholarship across the Americas, challenging the previous narrative and sparking new debates on when humans first arrived.
“Evidence older than 15,000 years is often met with skepticism,” remarked Richard Fariña, a paleontologist at the University of the Republic in Montevideo, Uruguay. “Yet increasingly compelling evidence is surfacing from various ancient sites.”
At universities in Sao Paulo and the Federal University of Sao Carlos, Pacheco investigates chemical transformations occurring in bones as they fossilize, helping her team to discern when the sloth osteoderms were modified. “Our research indicates that the osteoderms were reshaped before fossilization took place,” suggesting alterations happened shortly after the sloths perished, not millennia later.
Additionally, her team has ruled out other natural processes like erosion or animal damage to explain markings on the sloth bones. The study was published last year in a scientific journal. A colleague, paleontologist Thaís Pansani, is focusing on determining whether some of the similarly aged sloth bones from Santa Elina exhibit signs of being affected by human-induced fires, which burn at distinct temperatures compared to natural fires.
While earlier dates for human presence have been suggested, such as the site at Monte Verde in Chile where tools and preserved animal remains dating back 14,500 years were found, researchers are probing even older potential sites. At Arroyo del Vizcaíno in Uruguay, evidence indicates possible human modifications on animal bones around 30,000 years ago. In New Mexico’s White Sands, human footprints estimated to be 21,000 to 23,000 years old were discovered, alongside tracks from large mammals, although some archaeologists express skepticism about the lack of stone tools at the site despite repeated human visits.
Odess from White Sands emphasizes the importance of such discussions. “We didn’t aim to uncover the oldest artifacts; we have simply followed the evidence where it leads,” he remarked.
Although the precise timeline of human arrival in the Americas remains subject to debate and might remain uncertain, current evidence suggests that if the first humans entered the continent sooner than believed, they did not cause the immediate extinction of the enormous wildlife they came across.
The footprints at White Sands capture a few fleeting moments of these early interactions. Odess interprets one series of tracks to represent “a giant ground sloth moving along on four feet” before encountering the tracks of a smaller human who had recently dashed through the area. The massive creature appears to have paused, rose on its hind legs, and then changed directions.