35 Million Years of Sloths & How Giants Rose and Fell

A new study sheds light on the rise and fall of these prehistoric titans and why some survived in the trees while others didn’t

One of my favorite memories from my time behind the scenes at the Smithsonian wasn’t about a skull or a skeleton. It was about poop. 

Yes, poop.

Or rather, a giant ball of it. We kept a fossilized sloth coprolite in the collections: rough, fibrous, and about the size of a grapefruit. Visitors would hold it in their hands, totally puzzled. When I told them it was ancient poop from a giant ground sloth, their faces always dropped in a mix of horror and fascination. 

Then they’d laugh: “I guess it’s just compost by now!”

That coprolite always reminded me how far sloths have come… and fallen. We tend to think of them today as sleepy, slow-moving creatures tucked away in rainforest canopies. But their extinct relatives once shaped ecosystems, roamed deserts and mountains, and in some cases, dug caves with their claws. 

A new study published in Science unravels just how these sloths got so big, what habitats pushed them in that direction, and why their reign came to such an abrupt end.

The research, led by Dr. Alberto Boscaini and a global team of paleontologists, builds a detailed picture of sloth evolution over the past 35 million years. By combining DNA, protein sequences, and detailed bone measurements from over 400 fossils across 17 museums, they reconstructed a total-evidence evolutionary tree. 

That allowed them to map shifts in body size against climate change, habitat preferences, and extinction patterns.

Their findings challenge some long-held assumptions and confirm others with new clarity. The biggest driver of sloth size? Where they lived. 

As co-author Dr. Rachel Narducci of the Florida Museum puts it, “They looked like grizzly bears but five times larger.” And these mega-sloths weren’t dangling from branches. They were firmly planted on the ground.

The researchers found that terrestrial sloths evolved to be big, sometimes elephant-big, through slow, steady evolutionary changes. In fact, gigantism emerged at least three separate times, especially in lineages that settled into open, often harsh environments. 

In contrast, arboreal sloths, the kind we know today, stayed small and evolved much faster. It turns out that life in the trees imposes strict weight limits. Climb too high with too much bulk, and gravity becomes unforgiving.

The team’s modeling revealed three evolutionary “sweet spots” for body size: arboreal sloths averaged just under 15 pounds, semi-arboreal ones hovered around 170, and ground-dwellers shot up to an average of over 750. The largest ever, Megatherium, weighed in at around 8,000 pounds.

But size wasn’t just about the trees. Climate played a major role, too. During the Middle Miocene Climatic Optimum, a global warming event about 15 million years ago, sloths shrank. Later, as the world cooled, ground sloths bulked up again. Bigger bodies help conserve heat and water, two essential traits when roaming deserts, scaling the Andes, or surviving in the subarctic.

Some even ventured into the ocean. Thalassocnus, a marine-adapted sloth, had dense ribs like manatees to help with buoyancy. Others, like the Shasta ground sloth, preferred desert caves, where they left behind mountains of poop so thick they puzzled early paleontologists.

So what happened to these ecological giants?

The study finds no clear correlation between their extinction and climate shifts during the last 130,000 years. Instead, the data points squarely at human arrival. About 15,000 years ago, as Homo sapiens expanded across the Americas, ground sloths disappeared. Fossil sites show signs of butchering. Their large size, once a shield against most predators, became a liability.

However, the losses didn’t stop on the mainland. Caribbean sloths, some of the last holdouts, vanished around 4,500 years ago, shortly after humans reached the islands.

Meanwhile, the only survivors were the small, canopy-dwelling sloths we know today. Likely too small to be worth the hunt, and tucked away in dense forests, they were spared.

The sloth story isn’t just about extinction. It’s about how life experiments with form and function, and how sometimes, those experiments end not because they failed, but because they couldn’t withstand the ripple effects of a new force: us.

Yes, we’re again the Villains of the story. 

That fossilized poop I used to pass around was funny, sure. But it was also a clue to a deeper story, a window into a world where massive mammals shaped ecosystems, only to vanish in the blink of evolutionary time. Now, all that remains are their bones, their burrows, and their droppings. And if we’re lucky, the lessons they left behind.

Published in Fossils et al. Follow to learn more about Paleontology and Evolution.

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I’m thrilled you’re here. Stay curious, and thank you for sharing this journey with me!

Best,

Sílvia P-M, PhD Climate Ages