Eons

Over 10,000 years ago, around the end of the Late Pleistocene epoch, on a salt flat in what's now New Mexico, a band of humans stalked a giant sloth.

The footprints they left behind in the soft ground look like at least one person was following in hot pursuit, planting their own footprints right inside the sloths.

Another set of prints, coming from a different direction, was made by someone walking on their tiptoes like a cartoon guy sneaking up on someone.

The giant sloth itself seems to have been understandably upset about the whole situation.

Its tracks abruptly zig-zag, pointing to a possible attempt at evasive maneuvers.

And it even seems to have reared up on its hind legs and pivoted a few times, probably to swing its front claws around defensively when the pesky humans got too close.

Now, we dont know for sure what exactly was happening here.

Was it a hunt?

A deadly game of cat and mouse?

Or just a bunch of bored teenagers harassing a giant sloth for fun on an otherwise slow Ice Age afternoon?

Its hard for us to even imagine what these now-extinct animals would have been like, yet for thousands of years, we lived alongside them.

See, one the most ironic and frustrating things about the Pleistocene is that, unlike every prior epoch of natural history, many of its mysteries used to be common knowledge.

Which means that some researchers are essentially trying to rediscover details about ancient Pleistocene species and environments that we humans just forgot.

And giant sloths are a perfect example of this.

Even though we once knew them pretty well, all of that firsthand knowledge is now lost to the mists of time including some of the most basic aspects of their biology and behavior.

So what did ancient people once know about these bizarre megafauna, that weve since forgotten?

And what did the sloths know about ancient people that weve forgotten about ourselves?

Your average 10-year-old kid from the Pleistocene Americas would probably have been able to tell you in detail how various giant sloth species looked, acted, sounded, and smelled.

Nowadays though, giant sloth experts have to go to great lengths to rediscover just a small fraction of that lost knowledge.

Because unlike a lot of other Pleistocene megafauna who have fairly similar surviving relatives like mammoths and modern elephants, for example the closest living relatives of giant sloths arent much like them at all.

Todays six species of small and sleepy tree-dwellers are radically different from their giant extinct relatives.

Plus, we know that after first diverging around 35 million years ago, sloths radiated into over a hundred genera that ranged widely in body size, lifestyle, and habitat.

Some clambered through tropical rainforests, some roamed deserts and mountains, some lived on isolated islands, and some even took to the seas, becoming at least partially aquatic.

And perhaps the most impressive of all were the giants.

There were at least 30 species of them that thrived well into the Pleistocene, with the biggest rivaling Asian elephants in size.

But almost all of that sloth diversity is now gone.

And since theres nothing like giant sloths around today, and their fossils can only tell us so much, theres a whole lot of pretty fundamental stuff about them that we just no longer get. Take their diets, for example.

You might assume one of the few things we can be confident about is that all sloths are obligate herbivores they eat plants and only plants.

And, well, yeah, that was the presumption about all sloths, both living and extinct, for a long time.

Until 2021, that is, when new evidence forced us to rethink even this basic aspect of giant ground sloth biology.

Turns out, some of them may have also had a taste for flesh.

See, researchers had been trying to unravel a much broader long-standing mystery about Pleistocene South America: its ecosystems just dont seem to make sense.

They had too many big mammal herbivores and too few big mammal carnivores.

This left them with an ecological imbalance.

How could those ancient environments have produced enough plant biomass to support such a weirdly big-herbivore-heavy food chain?

Well, maybe they didnt.

Maybe, instead, hidden in the fossil record, were omnivores and carnivores disguised as herbivores.

But which ones?

Since we cant just ask the people who lived alongside them back then, in 2021, researchers turned to chemistry to find the imposters.

To figure out if giant sloths mightve been among South Americas missing meat-eaters, the researchers analyzed the preserved fur of two different species.

The first was Darwins giant ground sloth, Mylodon darwinii, and the second was the Shasta ground sloth, Nothrotheriops shastensis.

Specifically, they studied the nitrogen isotopes of specific amino acids found in preserved hair samples.

See, because you are what you eat, the ratio of those isotopes reflect where an animal sits in the food chain.

And by comparing those nitrogen isotope ratios to modern species whose diets and ecological roles we understand, the researchers could get a general idea of those sloths place in their food chains.

Now, Nothrotheriops came back as a bona-fide herbivore though with a particularly strong signal of desert plant consumption.

But Mylodons nitrogen values didnt look like an herbivore at all.

This animal was an omnivore, with values matching the modern American pine marten, which eats a mixed diet of plants and meat.

Now, this doesnt mean that Mylodon was a bloodthirsty predator.

Its much more likely that they were opportunistic scavengers just not passing up the chance of some free meat and eggs if they came upon them.

But still, if Mylodon and possibly other giant sloth species werent totally reliant on plants and could incorporate meat into their diets too, it potentially helps explain how ecosystems could support them.

Plus, the feeding habits of megafauna have big ecological impacts on things like vegetation structure, soil conditions, and carbon cycling.

So rediscovering that at least some sloths were actually opportunistic omnivores rather than obligate herbivores has implications for how we reconstruct ancient environments that we once lived in ourselves, but no longer remember.

And if the image of a giant sloth with a taste for flesh isnt horrifying enough for you, instead imagine one thats hairless, leathery, and almost twice as big.

See, while its true that modern sloths are more-or-less fluffy, it has been argued that the biggest sloths of all might have actually been completely naked.

Because unlike our friends Mylodon and Nothrotheriops who happened to leave behind samples of fur, most giant ground sloth species did not.

These naked guys mightve included the elephant-sized Megatherium and Eremotherium.

In 2002, some scientists hypothesized that these species probably lost their fur entirely as they evolved to such enormous sizes.

They based this hypothesis on mathematical modeling.

See, as you get bigger, you get worse at radiating heat, so with a thick coat of fur, the calculations suggested that theyd be too warm in most of their environments.

Instead, the scientists argued the very largest sloths were mostly hairless, like modern elephants, hippos, and rhinos.

Not everyone buys this idea though, and until we can analyze some actual preserved skin furry or otherwise we may never know for sure.

And, in fact, in 2025, some other scientists challenged the hairless hypothesis with their own modelling, using new, significantly lower estimates of giant ground sloth metabolism and body temperatures.

When they included this new data, their simulations found the exact opposite: both species would have needed a coat of fur to stay warm enough.

Though, interestingly, they also found that Eremotherium specifically which had an especially huge range might have varied in coat thickness and coverage from region to region, and maybe even season to season.

Whatever the answer turns out to be, this would have once been the most obvious thing about them, but its yet another thing weve forgotten as a species.

And ancient people in the Americas would have probably found it hilarious that, more than 10,000 years later, were struggling to figure out something that was once clear from a mile away.

But heres the thing, those same people would probably also be surprised at how mysterious we find them, too.

Because one of the biggest mysteries about the Americas during the Pleistocene is when, exactly, people showed up.

And its a question that, surprisingly, studying giant sloths is helping us to unravel.

See, for a long time, the mainstream consensus was that people only got to the Americas during the dying days of the Pleistocene, around 13,000 years ago.

And the idea was that they had made the journey by crossing a land-bridge from Asia that became ice-free around that time.

But in recent years, it has become increasingly clear that humans actually arrived in the Americas much earlier than this.

For example, stone tools found in whats now Idaho seem to have been crafted at least 16,000 years ago.

And in 2021, some of the human footprints from that ancient salt flat in New Mexico that we mentioned earlier, known as Whitesands, were dated to between 21,000 and 23,000 years ago.

While the earliest human arrival in the Americas is a complex mystery that deserves a whole episode to itself, ironically, its something that giant sloths would have known the answer to.

Because we humans tend to have less than subtle effects on ecosystems when we show up, especially on big megafauna.

And they would have definitely noticed our arrival.

For example, in 2023, researchers reported a strange pattern in giant sloth remains that may represent some of the oldest evidence of human activity anywhere in the Americas They were studying osteoderms small bony structures that had once been embedded in the skin of a giant sloth species called Glossotherium phoenesis.

These came from a site in central Brazil and dated back 25,000 to 27,000 years.

Weirdly, the researchers found that three of the osteoderms seem to have been polished.

They also had small holes drilled into them before they fossilized, something that probably couldnt have happened naturally.

Instead, the researchers proposed that this was the work of previously unknown ancient people who had deliberately modified these osteoderms, possibly stringing them together to be worn as pendants.

If its true, this would be the oldest evidence of people in South America, and it would mean that humans were living with and messing with giant sloths for even longer than we thought.

We still have a lot to rediscover about the Pleistocene people of the Americas, and the giant sloths they lived alongside.

But it's clear that, in a sense, we live in a state of ecological amnesia, about both them, and ourselves.

And it shows us that knowledge doesnt last forever, its fragile.

When species fade away into extinction, many of the things we once knew about them often do as well.