Science & Technology

Giant Sloths Once Ruled the Americas

When you think of a sloth, do you think of a small, slow-moving creature known to stay still so long that it grows moss on its back? Or do you think of a towering giant with enormous claws that was big enough to feed an entire prehistoric family? If all you know is the modern sloth, then it's time you met the ground sloth — your ancestors certainly did.

House(-Sized Sloth) Hunters

In April, 2018, paleontologists published definitive proof of something they had suspected for years: humans hunted giant sloths, and these are the footprints that prove it. It's almost like watching a shadow puppet show from a distance of 11,000 years — through these human and sloth footprints, a drama unfolds that's a triumph for the smaller creatures and a tragedy for the large one. Ten of the human footprints were found embedded in the sloth's, oriented in the same way and placed so far apart that the person would have to be moving pretty awkwardly. It's a clear suggestion that this prehistoric New Mexican was deliberately attempting to hide their tracks.

A similar scene probably played out time and time again, all over the Americas. Scientists have long believed that humans played a role in driving the ground sloth to extinction, and prior to finding these damning footprints, sloth bones with marks possibly indicating human tools had been dated to 30,000 years ago. Today, there are only six surviving species of sloth, but the ground sloths of the past were a lot more diverse and a lot more widespread. Covering two continents from Tierra del Fuego at the very tip of Argentina to the northernmost reaches of Alaska, they came in all shapes and sizes. Some were subterranean, some adapted to an aquatic lifestyle, and some, like Megatherium, were literally the size of an elephant.

Megatherium might have been more than just a victim of human hunters. It might have been a predator itself. According to an analysis from 1996, the arms and claws of that towering behemoth might have been better suited for stabbing and slashing than grasping heavy branches. We do know that these sloths ate plants (they're the whole reason avocados evolved, after all), but cows, deer, and other herbivores have been observed scavenging bodies, so a little meat eating wouldn't be unheard of. They might also have been used to defend their territory against other sloths, or to flip over trespassing glyptodons. One thing's for sure: no matter how aggressive the sloths were, we wouldn't want to be on the wrong side of those massive slashing claws.

Super-Speedy Evolution

Today's sloths are a long ways off from the prehistoric sloths of the Americas. The only way they could be more different is if it turned out the ground sloths were super fast travelers as well. They weren't — not notably so, anyway. But they probably weren't especially slow either. And anyway, there's one thing that they do very quickly: evolve. Sloth generations grew to those massive sizes rapidly. Some lineages were found to have gained an average of 100 kilograms (220 pounds) every million years. That's some of the fastest rates of change in the entire animal kingdom.

It's enough to make you wonder what happened to those giants, if they were so good at rapidly adapting to their environment. The six surviving sloth species can be divided into the two- and three-toed varieties. What's really interesting is that those two made their slow escape to the trees separately, but settled on basically the same body shape. That also happened quite rapidly — and likely kept the small, slow creatures from being hunted by the humans that terrorized their more massive relatives. Maybe sloths move so slowly because they're already so far ahead of the rest of us.

Giant sloths weren't the only prehistoric mammals to roam the Americas. Walk alongside them in "Atlas of a Lost World" (free with a trial membership to Audible), which explores the continents as they were between 20 and 40 thousand years ago. We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.

Giant Sloths and Early Humans

Written by Reuben Westmaas May 17, 2018

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