Meet the Scientist Decoding Human History in South America Through Giant Ground Sloth Fossils

In a dusty rock shelter in Santa Elina, Brazil, two layers of giant ground sloth bones have been swallowed into the earth. The oldest sloth fossils in the deposit date back 27,000 years. But they are far from untouched — three bones were modified with drilled holes, shaved sides and purposeful laceration marks.

Thaís R. Pansani, a Brazilian paleontologist and recent postdoctoral fellow in the Human Origins Program at the National Museum of Natural History, is currently investigating these altered fossils. The implications could be monumental, suggesting that humans may have arrived in South America thousands of years earlier than estimated. The markings additionally call into question our previous hypothesis that the human arrival to the Americas roughly 13,000 years ago catalyzed the extinction of giant land mammals.

Unlike anthropologists who primarily examine human history through pieces of material culture, like tools, Pansani explores our ancestors’ migration into South America via the traces they engraved on Pleistocene megafauna bones.

Throughout the ice age, the region that became modern-day Brazil was an arid landscape. The drier climate birthed savannas that supported a diverse assemblage of mammals, including giant sloths of varying sizes and their bipedal neighbors: ancient modern humans.

In the 20th century, archeologists proposed the Overkill Hypothesis, the idea that human overconsumption was the primary cause of large mammal extinctions in the Americas. This theory was supported by the discovery of spear points and piles of mammoth bones in Clovis, New Mexico. At its discovery in 1932, Clovis was the earliest known archaeological site in the Americas, dated around 13,000 to 12,000 years old. Scientists believed it provided proof that the emergence of humans led to the demise of the megafauna, especially in North America.

“Now, we know that it's way more complex than that,” Pansani said. “We were here way before that.”

In September, Pansani published research on a modified giant ground sloth tooth discovered over 1,200 miles east of the zooarchaeological layers in Santa Elina. The tooth, shaved into a triangular point with parallel markings on the smoothed surface, was radiocarbon dated to just under 14,000 years ago. This discovery indicated humans were interacting with megafauna for more than ten thousand years before the giant animals disappeared from South America.

What does your research tell us about human interactions with giant ground sloths before their extinction?

We know that the extinction of megafauna, in South America at least, was around 10,000 years ago. This is at least 17,000 years after the arrival of the humans who modified the giant ground sloth bones in Santa Elina. So, there is evidence of humans existing with these mammals in a shared ecosystem and hunting or scavenging them in a sustainable manner.

However, we still don't know what our relationships with the Pleistocene megafauna were like in general. 

For a long time, we thought that the very first humans in the Americas were associated with the Clovis culture. We found weapons and associated bones, some with cut marks made using stone tools. So that supported the Overkill Hypothesis. We thought that humans got to the Americas from Asia, following and hunting the large animals until they were extinct. But this is an oversimplified takeaway.

Do we know why humans were modifying giant ground sloth bones?

There is evidence of symbolic thinking in the ornaments made from giant ground sloth bones. These cultures were interacting with the giant sloths not just for subsistence. In Santa Elina, some of the oldest giant ground sloth bones were not only modified, but they were worn from extensive use. With the modified tooth, we know this part of the animal was handled outside the function of eating. It’s not a rib, it’s not a long bone that shows cut marks because people were hunting them. It’s just a tooth.

It's beautiful to think about, but we can only speculate about the motives behind the alterations. But we're learning more with these studies. 

How does your research highlight some of the shortcomings of the Overkill Hypothesis?

The idea that human arrival immediately drove megafauna to extinction across the Americas does not align with recent discoveries.

I think that we need to be very careful with remembering that even these early dates for human occupation of the Americas are open to change. For a long time, everybody was talking about how megafauna went extinct 11,000 years ago, and humans got to the Americas 13,000 years ago. So the Overkill Hypothesis makes sense from this perspective.

But we keep seeing evidence of people showing up earlier, maybe 27,000 years ago. Similarly, new studies show the possible survival of some megafauna in Brazil until around 5,000 years ago. This demonstrates that humans existed on the continent and could manage the ecosystem and live together with these animals. Our interactions with these creatures were more complex than we thought even 20 years ago. 

Density may be the key. Maybe there was a peak in human density along the late Pleistocene and early Holocene in the Americas, and that is when we began “over hunting.”

I hope to contribute to further discoveries.  We have so many megafauna specimens in museum collections that require further research, especially in South America and Brazil in particular. I'm one of the first people in the country who wants to go to other museums and review all the evidence for human interactions with these bones.

This interview has been edited for length and clarity.  

Meet a SI-entist: The Smithsonian is so much more than its world-renowned exhibits and artifacts. It is a hub of scientific exploration for hundreds of researchers from around the world. Once a month, we’ll introduce you to a Smithsonian Institution scientist (or SI-entist) and the fascinating work they do behind the scenes at the National Museum of Natural History.

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Chihiro Kai | READ MORE

Chihiro Kai is a Science Writing Intern at the Smithsonian National Museum of Natural History. She covers natural history research for the museum's online magazine, Smithsonian Voices. Chihiro is a double major in Journalism and Evolutionary Biology, Ecology and Organismal Biology at the University of Kansas. She is particularly fond of dinosaurs and was a research volunteer for two field seasons at the KU Vertebrate Paleontology Lab. Previous to the Smithsonian, Chihiro was a freelance reporter covering the Tokyo 2020 Olympic and Paralympic Games, specializing in the COVID-19 pandemic and Japanese domestic and international policy. One of her favorite pastimes is watching old "Bill Nye the Science Guy" episodes.