Sloths weren’t always the slow, tree-hugging creatures we know today. For millions of years, they came in a wild variety of shapes and sizes, from tiny climbers to massive ground beasts as big as elephants.
Their evolutionary journey, shaped by shifting habitats and climates, was abruptly halted when humans entered the scene, leading to the extinction of their most colossal members.
Sloth Diversity: From Tree-Dwellers to Towering Giants
Long before today’s slow-moving, tree-dwelling sloths, these fascinating animals came in all shapes and sizes—from tiny climbers to towering ground-dwellers the size of elephants. New research reveals that this incredible range in sloth body size was shaped mostly by where they lived.
And their dramatic disappearance? Likely the result of rising human pressures, especially hunting, which wiped out the largest species. The few sloths that survive today may owe their survival to the fact that they lived high in the treetops, out of reach from early human hunters.
A Lost Empire of Sloths
Today, just two small tree-dwelling sloth species live in the rainforests of Central and South America. But not long ago, sloths were everywhere. During the late Cenozoic era, over 100 different kinds of sloths roamed the Americas, thriving in forests, grasslands, and more. Some were massive, standing up to six meters (20 feet) tall and weighing several tons. Yet by the end of the last Ice Age, nearly all of them had vanished.
Uncovering 35 Million Years of Sloth Evolution
To understand what drove this dramatic rise and fall, researcher Alberto Boscaini and colleagues looked back across 35 million years of sloth history. Using a combination of fossil data, DNA and protein sequences, and advanced evolutionary modeling, they traced the evolutionary path of 67 different sloth genera. They tested how factors like habitat, diet, climate, and predators influenced changes in body size over time.
Big or Small? It All Came Down to Habitat
The results were striking. Whether sloths lived on the ground or in trees turned out to be the biggest factor in how large they became. Early sloths were mostly large, land-based grazers. But as forests spread and open spaces gave way to tree cover, sloths evolved smaller bodies and took to the canopy, often independently in separate lineages. Some still grew large again, likely as a response to colder climates and shifting ecosystems.
A Sudden Fall: Sloths and the Human Arrival
For millions of years, sloths flourished. Their body size diversity peaked during the Pleistocene. But then, around 15,000 years ago, their numbers crashed. This collapse doesn’t line up with major climate shifts, but it does match the timeline of humans arriving in the Americas. The evidence strongly points to human hunting as the key factor behind the extinction of the giant, ground-dwelling sloths. Their smaller, treetop cousins survived—barely—while the giants disappeared forever.
Explore Further: They Were 8,000-Pound Sloths With Claws and Armor – Then Humans Showed Up
Reference: “The emergence and demise of giant sloths” by Alberto Boscaini, Daniel M. Casali, Néstor Toledo, Juan L. Cantalapiedra, M. Susana Bargo, Gerardo De Iuliis, Timothy J. Gaudin, Max C. Langer, Rachel Narducci, François Pujos, Eduardo M. Soto, Sergio F. Vizcaíno and Ignacio M. Soto, 22 May 2025, Science.
DOI: 10.1126/science.adu0704
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