A new discovery in Uruguay has possibly pushed the earliest evidence for human presence in South America back by thousands of years. Scientists studying a 33,000-year-old heel bone from a giant ground sloth have discovered what looks like trauma from a human-made weapon, suggesting that ancient hunters were pursuing megafauna long before the Last Glacial Maximum.
The fossil belonged to extinct giant ground sloth Lestodon armatus, and was found at the Arroyo del Vizcaíno site, located near Sauce, Uruguay. The bone has one clearly defined circular indentation that's approximately 0.8 inches (21 millimeters) in diameter and nearing 1.6 inches (41 millimeters) in depth. This cone-shaped wound has features that suggest it could only have been produced by a penetrating weapon produced by early humans, according to a new study published in the Swiss Journal of Palaeontology.
- How to Hunt a Giant Sloth – Ancient Tactics Revealed in Human Footprints
- Giant Sloth Death Pit Reveals Death By Feces Ingestion
The 33,000-year-old calcaneus (heel bone) of Lestodon armatus showing the distinctive circular indentation that may have been created by a human weapon. The white box indicates the impact zone analyzed by researchers using advanced imaging techniques (R.A. Fariña et al., Swiss Journal of Palaeontology)
Revolutionary Evidence from a Prehistoric Bone Bed
The Arroyo del Vizcaíno site has yielded over 2,000 skeletal elements from at least 15 megafaunal species, with the majority belonging to Lestodon armatus, a massive ground sloth that could reach 13 feet (4 meters) in length. The site represents an extraordinarily dense accumulation of prehistoric fauna, with radiocarbon dating consistently placing the remains at around 33,000 calibrated years before present.
Dr. Richard Fariña, a paleontologist at the Universidad de la República in Uruguay and lead author of the study, employed cutting-edge analytical techniques to examine the mysterious indentation. The research team used CT scanning, high-resolution silicone casting, and microscopic analysis to create detailed 3D models of the wound structure, as reported by Archaeology Magazine.
The microscopic examination revealed fascinating details about the nature of the injury. Fine parallel striations within the cavity indicated that the penetrating object had rotated and shifted during insertion, creating a spiral pattern characteristic of a thrusting weapon. More intriguingly, organic fiber residues were discovered lodged within the indentation, suggesting the presence of plant materials that may have been part of a wooden spear or shaft.
Microscopic examination revealed organic fibers and conchoidal fractures within the wound channel, indicating the penetrating object rotated during insertion. (R.A. Fariña et al., Swiss Journal of Palaeontology (2025))
Challenging the Timeline of Human Migration
This discovery carries profound implications for our understanding of when humans first arrived in the Americas. The widely accepted timeline places the earliest human presence at around 23,000 years ago, based on footprints found at White Sands National Park in New Mexico. However, the Uruguay evidence pushes this back by nearly 10,000 years, suggesting humans were hunting megafauna in South America well before the Last Glacial Maximum (26,500-19,000 years ago).
The angle of penetration of approximately 60 degrees relative to the ground suggests a close-range hunting scenario. This would be consistent with early humans using thrusting spears or similar weapons to target large prey animals. The researchers propose that the weapon likely had a rounded tip made of hardened wood, bone, or ivory, attached to a wooden shaft.
Alternative explanations for the indentation, including carnivore bites, accidental impacts, or natural erosion, were systematically ruled out by the research team. Carnivore teeth would have created irregular, crushing wounds rather than the smooth, circular perforation observed. Similarly, accidental impacts typically produce irregular fractures, not the clean, cone-shaped cavity found in the sloth bone.
- Evidence Mounts in Favor of Early Inhabitants of the Americas Over 20,000 Years Ago
- Unprecedented Wave of Large-mammal Extinctions Linked to Prehistoric Humans
Mounted skeleton of Lestodon armatus, showing the massive size of these extinct ground sloths that early humans may have hunted. (Chensiyuan / CC BY-SA 4.0)
Implications for Megafaunal Extinction
The evidence from Arroyo del Vizcaíno adds a crucial piece to the puzzle of megafaunal extinction in South America. The site contains over 40 bones with cut marks and other signs of human modification, suggesting sustained interaction between early humans and giant ground sloths over an extended period, as documented in previous research by Fariña and colleagues.
Lestodon armatus was one of the largest ground sloths, weighing up to 4 tons and standing over 6.5 feet (2 meters) tall. These massive herbivores were endemic to South America and represented formidable prey for early human hunters. The evidence suggests that rather than simply scavenging already-dead animals, humans were actively pursuing and killing these giants.
The discovery also provides insights into early hunting technologies. The organic residues found within the wound channel indicate that early Americans were crafting sophisticated composite weapons, possibly using fire-hardened wooden points or bone and ivory tips secured to wooden shafts. These tools would have been essential for successfully hunting such large and dangerous prey.
The Arroyo del Vizcaíno site location in Uruguay, where over 2,000 megafaunal remains have been discovered in a dense bone bed. (ResearchGate)
Ongoing Research and Future Discoveries
While the evidence is compelling, the research team acknowledges that more investigation is needed to definitively prove human agency in the sloth's death. The site continues to yield new discoveries, including additional cut-marked bones and possible lithic fragments that may further support the human presence hypothesis.
The Arroyo del Vizcaíno site represents just one of several controversial pre-Last Glacial Maximum sites in the Americas. Similar evidence from locations in Mexico, Brazil, and Chile suggests that the story of human migration to the Americas is far more complex than previously understood. These discoveries challenge the traditional "Clovis First" model and support the growing evidence for much earlier human presence in the New World.
As archaeological techniques continue to advance and more sites are investigated, our understanding of when and how humans first arrived in the Americas will undoubtedly continue to evolve. The 33,000-year-old sloth bone from Uruguay stands as a testament to the ingenuity and adaptability of our earliest ancestors, who successfully established themselves in South America during one of the most challenging periods in Earth's recent history.
Top image: Left, A giant sloth reproduction. Right; The 33,000-year-old calcaneus (heel bone) of Lestodon armatus showing the distinctive circular indentation that may have been created by a human weapon. The white box indicates the impact zone analyzed by researchers using advanced imaging techniques. Source: Left; CC BY 2.0 Right; R.A. Fariña et al., Swiss Journal of Palaeontology (2025)
By Gary Manners
References
Fariña, R.A., Hayes, E., Lemoine, L.A. et al. (2025). An indentation in a 33,000-year-old right calcaneus of the ground sloth Lestodon (Xenarthra, Folivora) from Uruguay and its possible human agency. Swiss Journal of Palaeontology 144, 31. Available at: https://sjpp.springeropen.com/articles/10.1186/s13358-025-00379-0
