The Rise and Fall of the Giant Sloths
What 35 million years of evolution reveal about resilience, extinction, and the role of humans.
When we think of sloths today, we picture small, slow-moving creatures hanging upside down in rainforest canopies. But if we could travel back in time, we’d find a very different sloth. Some were as large as modern-day elephants, roaming open landscapes and shaping ecosystems as part of South America’s megafauna.
Sloths repeatedly evolved large and small body sizes according to their grade of terrestriality/arboreality. At the bottom, Bradypus tridactylus; at the left, Hapalops elongatus; in the background, Megatherium americanum. Illustration by Diego Barletta.
From Tree Huggers to Ground Giants.
A groundbreaking new study, led by scientists from CONICET (Argentina’s National Scientific and Technical Research Council), the University of Buenos Aires, and partner institutions in Brazil and Europe, maps out the full evolutionary journey of sloths from their terrestrial beginnings 35 million years ago to their near-disappearance 12,000 years ago.
Megatherium americanum skeleton from Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Buenos Aires, Argentina. Photograph by Alberto Boscaini.
The study, published in Science, combined fossil records, ecological data, and cutting-edge evolutionary models to show how sloths repeatedly reinvented themselves: shrinking, growing, and adapting to new habitats, before suffering a sudden collapse triggered by humans.
“Their extinction timeline mirrors human expansion,” explains paleontologist Alberto Boscaini. “No prior climatic crisis erased them; this points to anthropogenic pressures as the final blow.”
From Earthbound Heavyweights to Canopy Featherweights
Early sloths were likely large, terrestrial animals weighing 70 to 350 kg. Over millions of years, their descendants branched into all kinds of sizes and lifestyles. Some evolved into giants like Megatherium, weighing over 4 tons and dominating Pleistocene landscapes. Others went in the opposite direction by becoming lighter and arboreal, like the sloths we know today.
The key driver behind these dramatic size shifts? Where they lived.
Life on the ground favored large bodies. Life in the trees, however, imposed weight limits. As sloths took to the branches, their bodies rapidly downsized. The result was a lineage that danced between extremes: massive and slow-evolving on land, nimble and fast-evolving in the trees.
“This group turned volatility into opportunity,” says co-author Daniel Casali from São Paulo University. Sloths adapted their size and behavior again and again, adjusting to new diets, climates, and ecosystems.
A South American Success Story
Throughout their history, sloths thrived across the Americas, from the Andes to the Caribbean. As climate and tectonics reshaped the continent, sloths shifted with it: digging burrows, climbing trees, even exploring marine environments. They survived glacial cycles and ecological upheaval.
By the Pleistocene, the sloth family tree was bursting with diversity: giant grazers, nimble climbers, and everything in between. Their success was built on adaptability. But then, something changed.
The Human Factor
About 15,000 years ago, as Homo sapiens expanded into the Americas, large ground sloths began to disappear. By 12,000 years ago, they were gone from the mainland. In a second wave, Caribbean sloths, smaller and more isolated, vanished too.
Despite surviving for tens of millions of years of natural change, they could not withstand the rapid, disruptive forces brought about by humans: hunting, habitat alteration, and ecosystem destabilization.
“Resilience has limits,” warns evolutionary biologist Ignacio Soto. “Sloths thrived when environmental shifts were gradual. But today’s crises are fast and human-driven. Their story reminds us: survival isn’t just about adapting, it’s about time.”
What We Can Learn Today
This research doesn’t just rewrite the sloth family history; it gives us a powerful lens to look at biodiversity today. It shows that even the most adaptable species can be pushed past their limits when change is too rapid, too widespread, and too human.
Ground sloths once played vital roles in ecosystems. Their loss reshaped landscapes, leaving behind just two surviving branches of a once-thriving tree.
The lesson is urgent: resilience is not infinite. Evolution takes time. Conservation must act faster.A huge thanks to Alberto Boscaini for sharing his research with SloCO.
