Home >science >health >Following 11,000-year-old footprints in sand

My favourite scene in the delightful animated film Zootopia involves sloths. Maybe you remember it too. In urgent need of some information, the two main characters walk into a government office that is manned by sloths (“slothed by sloths"?). Then the sloths’ sloth-like slowness at everything is the catalyst for much hilarity over the next few minutes.

Hilarious, because sloths are synonymous with being slow. I have personal experience of this. Years ago, I spent an afternoon watching a sloth wander about high in a tree in Limon, Costa Rica. Its speed, or lack thereof, was very apparent, and that’s a natural and good-natured metaphor for government offices we’ve all known and loved.

I’ve never heard sloths characterized as anything but slow. But just recently, I read about a sloth—maybe sloths—that once left footprints in the sand. White sand. Not metaphorically, but really.

Somewhere in the White Sands National Monument in New Mexico, USA, scientists found several sloth footprints. You and I might have looked at these in some mild curiosity—after all, it’s not every day that we run across the pug marks of a sloth—and moved on. But the scientists were slightly more curious, because these prints were made a while ago. Not last year or the year before, either, but several thousand years ago.

Digest that for a few seconds if you will. Finding such marks in the clay, recognizing and then dating them, are remarkable feats in themselves. And then consider the edifice the scientists built upon these marks, the story they believe they tell us.

The White Sands Monument has footprints of all kinds of now-extinct animals: ancient cattle-like, dog-like and cat-like creatures, mammoths and giant sloths, and humans too, for that matter. Thousands and thousands of such impressions, actually.

David Bustos, a naturalist with the US National Park Service, and several palaeontologist colleagues zeroed in on a “trackway" of distinctive kidney-bean-shaped sloth footprints.

In a recent paper (“Footprints preserve terminal Pleistocene hunt?", Science Advances, 25 April 2018, bit.ly/2r9c4Tj), they explain that because of the way the clay in the area has moved about and settled over the years, they found it hard to use conventional radiocarbon methods to date the prints. Instead, they used a combination of other relatively accurately dated sediments in the area, other similar archaeological sites and the time when this particular breed of prehistoric sloth went extinct — and this process suggests that sloths walked in that clay on a Wednesday evening about 11,000 years ago.

So what, you probably are wondering. So a few large animals once made their way slowly over this bed of clay that adjoins a lake, and apart from this happening very long ago, what makes this incident interesting? What makes it worth studying and writing about? (All right, I made up the Wednesday evening bit.)

Here’s what: in one of those large sloth prints, Bustos and his team found the impression of a smaller foot. A smaller sloth? Maybe a baby, following its mother about? No, this foot is unmistakably… human.

Something about that touched a quiet but deep chord in me, and maybe in you too? We’ve all stepped in others’ footprints, trying to match the length of their stride, or just being playful, or to follow along to see where they went. To think that some human ancestor did the same thing thousands of years ago is, oddly enough, to immediately feel a connect across those long centuries.

And over in White Sands, there are actually more than ten of these superimposed pairs — human foot placed deliberately inside a sloth pug mark and aligned with it. This, says the Bustos paper, “indicat[es] that the human trackmaker was walking intentionally within the sloth track." What’s more, with some clever analysis of the impressions, the scientists were able to show that the human(s) came along either at the same time or very soon after the sloth(s).

Were humans stalking a sloth here? Really, just what happened in this remote corner of what would become the White Sands Monument, on that long-ago Wednesday evening?

That’s just what Bustos and his colleagues set out to deduce, and their effort is a tour de force of inference built on tiny clues.

The sloth tracks show that they generally walked in straight lines, or wide curves. But that’s only where the clay shows no human prints nearby (or indeed, superimposed). Where there are human prints, the sloth tracks show it suddenly changed direction, suggesting that the humans were being a nuisance and the animal was trying to evade them. Just like you might change direction, if you were out on a walk and a few dogs rushed at you, yapping annoyingly.

But then at several places, the sloth prints form circular patterns in the clay. In each such pattern, there are one or a few footprints at the centre, but not the full foot like elsewhere. Instead, they seem to have been made by just the heel, or the foot as it pivots. Around these are what look like several wrist and claw impressions. Together, the patterns suggest the giant sloth had come to a stop and was “flailing" about. Annoyed by the pesky humans, unable to evade them any longer, it finally turned to face them. Dragging its claws along the ground, it reared up onto its hindlegs in a defensive posture, ready and willing to strike out and fight.

And indeed, approaching one of these circular patterns is a series of human footprints, but with toe impressions clearly visible. Does this tell a tale of an intrepid hunter on tiptoe, perhaps trying to match the height of the upright and angry sloth, perhaps trying to launch a spear as the animal swung its long and powerful arms around? Possibly, though this interpretation stretches the evidence enough that one of the paper’s authors, Matthew Bennett of Bournemouth University, told The Washington Post that it amounts to “paleo-poetry".

Was this a real hunt? And if it was, did the humans eventually kill the sloth? Unlikely, because as the authors of the paper point out, “the vast majority of hunts by modern hunter-gatherers are unsuccessful", and there’s little reason to believe that was not the case 11,000 years ago. In any case, running up on tiptoe to throw a spear or three is one thing, but actually finishing off the sloth would have been hard work.

Ancient sloths were massive and “densely-muscled", and “their strong arms and sharp claws gave them a lethal reach and clear advantage in close-quarter encounters." That is, humans who got too close to the animal risked being torn apart by those flailing claws. In addition, there is no sign of a sloth carcass, or bones, nearby. Then again, that could also be because the environment in the White Sands area “rapidly degrade[s] bones; thus preservation of bones [from this era] remains improbable."

Think about all this for a minute. All this inference, from a number of impressions in the mud of the White Sands Monument; impressions that you and I, untrained as we are, may not even have looked at twice. But to people who know what to look for, they offer clues to a rich and vivid story from 11,000 years ago. You can almost hear the humans yelp in excitement, the sloth roar in fear and anger.

So is it a true story, then? Wouldn’t we all love to know! Yet that question was best answered by the late palaeontologist Lewis Binford, whom I quoted in an earlier column here (bit.ly/2HRZAXq): “The question ‘Is this true?’ doesn’t lead anywhere. The question to ask is ‘Does this open up new learning opportunities?’"

And if you ask that question of the White Sands tracks, there are possible answers.

What the tracks demonstrate is a near-certain interaction between sloths and humans. They tell us the two species most probably lived at the same time, in some proximity to each other.

A troop of our human ancestors may have stalked and annoyed one or more sloths, at least hoping to kill and eat the animal (even if this particular hunt did not succeed). If this was their usual behaviour, and if these interactions happened often, it’s likely humans helped drive the giant sloth to extinction.

Plenty to learn, in other words. You see, it’s easy to laugh at sloths today, and the genial fellows in the Zootopia office are there for just that reason. But 11,000 years ago sloths may not have been quite as genial, quite as slow-moving, quite as hilarious.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners.

His latest book is Jukebox Mathemagic: Always One More Number. His Twitter handle is @DeathEndsFun.

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