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There’s something about footprints. In fiction, the most famous is probably the one Robinson Crusoe found: a single footprint in the sand on an island where he had been the only resident for a very long time. Naturally, it terrified him. If you read Daniel Defoe’s classic, you’ll learn the rest of the story.

But there are real-life footprints too, less terrifying but they tell stories nevertheless. Some years ago, I wrote here about footprints of sloths in the White Sands National Monument in New Mexico, US (bit.ly/3F54Vbz). You and I and anyone else, I think, might have been mildly intrigued—it’s not every day that I run across either sloths or their footprints—but no more. But these prints attracted the interest of a team of scientists. Why? Because they were 11,000 years old. But it wasn’t just that. Inside one of the large sloth footprints was the impression of a smaller foot—and this one was human. And there were several more like that, human print inside sloth print.

In fact, the researchers concluded that all those years ago, some small human—a child, most likely—“was walking intentionally within the sloth track". I hope that vision of a playful kid grabbed at you like it grabbed at me. There was much more they deduced from the sloth prints. Specifically, about that long-ago encounter between sloths and humans; about the possibility that humans helped drive that species of sloth into extinction.

Besides, these particular human footprints supported the generally accepted theory of when humans appeared in the Americas. Various tools found in the town of Clovis, also in New Mexico, have been dated to about 13,000 years ago, and those are the oldest such tools found on the continent. This made sense as far as the theory went, because the last Ice Age ended about that long ago. Glaciers began retreating then, withdrawing to far northern latitudes. This allowed humans who had crossed from Siberia into Alaska to start moving further south—into North and eventually South America.

Like I said, it’s a generally accepted theory, and most evidence found so far has supported it. Until ... some of the same scientists have just reported that some of those human footprints in White Sands are not 11,000, but 23,000 years old (bit.ly/3maQydk). That is, there were already humans south of those glaciers long before they retreated. How did that happen? But before answering that question, let’s explore two related angles to this story.

First, the Clovis theory has been challenged in more recent years. Up and down the Americas, archaeologists have found other sites that seem to suggest a human presence there from earlier than 13,000 years ago. There’s one site in the state of Washington where the rib of an ancient mastodon seems to have the tip of a spear embedded in it. That bone was found to be 14,000 years old. There’s what looks like a campsite in far-southern Chile. Carbon-dating of the wood showed it was 14,600 years old. Both these finds were written about in the late 1970s. Putting even them in considerable shade, though, is a site in southern California. The archaeologists who studied it reported in 2017 that “hammerstones and stone anvils occur in spatio-temporal association with fragmentary remains of a single mastodon". And a “radiometric analysis of multiple bone specimens" from the site threw up a truly incredible age: 130,000 years.

Still, there’s also been plenty of doubt cast on these finds. The tools scattered about, for one example—were they really just oddly-shaped rocks or bits of bone? The marks on the bones, for another—were they caused naturally, or perhaps even by the modern archaeologists as they worked? Can we be sure the mastodons were actually slaughtered by humans?

Second, while footprints are remarkable, almost tangible evidence of a human presence, it’s also famously difficult to determine their age. In effect, they are just part of the layers of rock in White Sands, and as this paper’s lead author, Matthew Bennett, told Nature magazine, “Dating [rock layers] is a nightmare." Nevertheless, two years ago, his co-author David Bustos found that one set of these footprints seemed to lead “right into layers of rock-hard sediment"—and this particular spot offered up another way to estimate its age.

Scattered about in the rock, besides the footprints, were preserved seeds of a weed called spiral ditchgrass. This is an aquatic plant, and great clumps of it must have once flourished on the shores of an ancient lake here that’s long since dried up. But a large number of seeds were left behind and preserved in the sediment over the millennia. Bustos and his colleagues gathered up plenty of them, because organic matter is relatively easy to carbon-date.

When they figured out the age of the seeds, the scientists were startled. For they dated back to before the last Ice Age. Knowing that this result would attract scrutiny and criticism, just like the Chile, California and Washington sites have done, they worked to nail down the age more accurately and surely. They dug a trench at the site, exposing layers of rocks going several feet down. Six of those layers had footprints. The deepest of those—thus the oldest—lay below a layer of seeds that they dated to 22,800 years ago.

Thus the footprints are at least 22,800 years old. Much the same analysis showed that the youngest of the footprints are about 21,000 years old. All in all, much older than the Clovis implements; much before the retreat of the continent-wide expanse of glaciers at the end of the Ice Age.

So if this finding holds up, the immediate question is, how did these ancient humans get to New Mexico? Their ancestors had most likely crossed into Alaska on a “land bridge" that was later submerged when the ice sheets melted. If you look at a map, you can see the detritus of this bridge: a series of tiny islands that stretch between Siberia and Alaska, today separated by long stretches of the Bering Sea. But after having walked across all those millennia ago, the ancient humans were confined to Alaska by the glaciers that covered much of the rest of North America, and were not to melt for several more millennia. It’s hardly likely that they walked across thousands of miles of ice to New Mexico. So how did they get there?

One theory is that they travelled south in small boats, along the Pacific coast of North America. At some point, perhaps in what is today California, the ice cover ended. There, more than likely sick of sailing, these ancestors of ours came ashore, abandoned their boats, and walked inland. Steadily inland. Probably a thousand miles east is White Sands. When they got there, they found a lake with ditchgrass and other plants luxuriant on the shore, sloths and other possible prey wandering about. Pleasant spot, no doubt. So perhaps they decided to stay put for a while. Their kids gambolled about as kids do, chasing sloths or at any rate playing in their footsteps.

And they left tracks in the sand. 23,000 years later, a few curious archaeologists find the tracks and learn about these early humans.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun

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