Evidence published this week not only changes the timing of human existence in southwestern North America but suggests that people remained in that location for at least 2,000 years. It’s a stunning revelation that indicates humans lived in areas below the ice sheets covering much of North America and the world at that time, evidence that could change our understanding of when people arrived on this continent, where they migrated, how they impacted the ecosystem, and how they responded to climate change.
White Sands National Park in New Mexico is known for its beautiful white gypsum dunes and its wealth of fossil footprints. Many exciting discoveries have been made about specific trackways over the past several years, but this is the first time researchers have dug a trench into the ground to study what lies beneath. This paper, published in Science, suggests people lived there approximately 23,000 years ago — 10,000 years before accepted dates of North American human occupation.
Within the layers of sediment, the researchers discovered a number of prints that they largely attribute, based on size, to teenagers and children. Only a few footprints seem to have the dimensions of an adult foot. None of the prints indicate they were moving exceptionally fast or particularly slowly. The authors propose that, if they were anything like some societies today, the teens may have been doing chores with the younger children in tow, playing around them. In a few layers, there are proboscidean prints and a dire wolf print. More than one researcher mentioned that, in comparison to other tracks studied at the site, these are remarkably unremarkable, in that they depict seemingly ordinary life thousands of years ago
Dating human presence is fraught with controversy. From the dating methods themselves to the artifacts associated with a site, there are many reasons scientists might challenge new research. But the authors involved in this paper believe their conclusions are solid, particularly because eight layers of unquestionably human footprints in the sediment are hard to contest. Multiple lines of evidence support their dates, the most significant of which come from ancient seeds.
The trench revealed layers of seeds — tiny, delicate remnants from the aquatic grass, Ruppia cirrhosa — still attached to their stems, and even one footprint in which the crushed grass seeds are embedded within it, offering further evidence that the plants and humans were contemporaneous.
Seeds have specific ways of moving through ground. They can move up through the soil or down, depending upon a number of environmental factors. So a few seeds here and there may not be a reliable way to determine age. Clumps of seeds, however, are a different story. And in the case of this specific plant, where separating the delicate stem from the tiny seed wouldn’t take much, the fact that they were found largely attached means that they didn’t move. It therefore meant the team could radiocarbon date these seeds.
Knowing they didn’t move through layers of the sediment was only one important step. Making sure the dates were correct was another, because aquatic plants are notorious for producing ages significantly older than they may actually be. This is due to carbon within the water the plants ingest. The depths of large lakes, for instance, tend to have older carbon, because there isn’t a lot of exchange with the surrounding atmosphere. It’s a well-known phenomenon referred to as the hard-water or reservoir effect, and one familiar to Kathleen Springer and Jeff Pigati, co-authors and geologists with the US Geological Society (USGS). Because these footprints indicate that ancient people were walking on the edge of a lake, where water would have constant interaction with the atmosphere, and because there were no great jumps in the ages throughout the sediment, it suggests there wasn’t a significant hard-water effect in these dates. The stratigraphy, they said, reads like a book: oldest to youngest, with no variation in between, even though the samples were only separated by a few centimeters of sediment. The team used other methods to determine the age of these prints, but the seeds were instrumental.
“You get these dates back,” said Springer in a video interview, “and you just kind of go, wow, there’s something happening here! You can see the change in the sedimentation where it’s getting drier in the layers that contained the trackways. People clearly were walking about — and they weren’t walking in a lake! They were walking where the lake edge had receded.”
Springer and Pigati initially recommended digging a trench, as it was “the only way,” Springer explained, “we could prove these human trackways are in the subsurface that would also allow us to find in situ datable material above and below.”
The two geologists are experts on deciphering ecosystem responses to climatic events in the southwestern U.S., and what they saw in the trench pointed to sudden climate warming, enough that it impacted the local lake.
“[W]hen that warming occurred,” Pigati said in a video interview, “the lake level dropped and exposed this big flat area for people to walk across. That’s what allowed the tracks to be there in the first place. This entire story is driven by climate change.”
Springer agreed. “What really got us excited was the realisation that there was a very strong climatic signature in this sequence, as well as the prospect of investigating climate signals in future work all over the basin.”
Finding the optimal spot to dig the trench was the responsibility of co-author and research scientist at Cornell University Tommy Urban, who conducted a search using ground-penetrating radar.
“We had surveyed dozens of areas,” he described in an email. “This one appeared to be clearly stratified with potentially multiple layers of prints. This improves the odds of getting a sequence of dates.”
He was surprised not just by the age of human occupation but the 2,000 years they were there.
“It means,” he wrote, “that people were using this area for a very long time, and thousands of years before humans were thought to have been present on the continent. We had always considered the possibility, though.”
Human evolution is of particular interest to Sally Reynolds, co-author, principal academic in hominin palaeoecology at Bournemouth University, and head of the Institute for Studies of Landscape and Human Evolution. In a video interview, she expressed her fascination with how humans evolved from “such humble beginnings” to “such formidable predators.”
“Which is really an amazing achievement,” she mused, “if you think that we don’t have strong teeth, we can’t run fast, we’re not camouflaged.”
She was part of the team that studied giant ground sloth and human tracks at White Sands, tracks that indicated humans may have been stalking the sloth. She was also part of the team that studied yet another trackway in which a human carried a toddler across White Sands. Those tracks were intersected by enormous proboscideans (either mammoths or mastodons). In one case, footprints of a giant ground sloth indicate it was walking toward the area where the human was, seemed to notice that a human was nearby, and walked away in a completely different direction.
“We are making the sloths think twice about getting anywhere near a human being, and that is exactly what a prey animal does to avoid a predator,” Reynolds said. “So it tells us a lot about human’s place in the ecosystem at the point when we arrived in the Americas. The prints at White Sands are so unique in terms of getting us behaviour and not just morphology, it means that we can actually sense these sorts of attitudes of one species to the other.”
But in terms of the most recent research, she wonders of ancient people: “How did they end up that far south that early? [This was] much further than we expected, much earlier than we expected. Which means that we’re underestimating the ability of Homo sapiens to expand, [to] migrate. Clearly, we’re a very adaptable species. And proof of this is that we’re considering migrating to another planet!”
This recent research is particularly important to David Bustos, co-author and resource program manager at White Sands, who has worked at the park for over 15 years and is very familiar with the trackways there. He has always wanted to know just how old the footprints are. He describes White Sands as being “so subtle.”
“At first,” he said in a phone call, “you see large concentrations of tracks here and there, but as you look further and invest more time, you start to understand why they overlap or where the animals are going. Things start to come together, and it gets more and more exciting as more of the story reveals itself.”
But, he continued, “it’s really sad in a lot of ways, because the reason these stories are rapidly revealing themselves is because of soil erosion. We see all these incredible stories, but then we know they’re going to be gone soon after.” Sometimes, he adds, it’s only within a year or two. “It’s a race to record.”
“The evidence of people in the Americas during the Last Glacial Maximum is limited and hotly contested,” wrote Kathryn Krasinski, assistant professor of anthropology at Adelphi University who was not involved in the new research. “Typically, the issues surround whether the evidence is indicative of past human activity or how the age of that activity was determined. Now the scientific community will have the opportunity to evaluate the depositional context of the footprints in relation to the seeds that were radiocarbon dated.”
These footprints hold not only scientific value for some but a deep spiritual connection for others. Learning more about them and actually seeing them has been an incredible experience for Kim Charlie, member of the Pueblo of Acoma and the first woman to sit on the board of the Tribal Historic Preservation Office for the Pueblo of Acoma. New laws mean that the National Park Service needs to consult with and inform members from all 23 Native American communities in New Mexico and any other tribes outside of the state that have a connection to White Sands when the park wishes to make any changes. It is through these consultations that Charlie has become involved with White Sands.
She says of the human footprints, “These are thousands and thousands of years old, but we still have that connection, I would say, to these people who lived there once a long time ago.”
“Sometimes,” she explained, referencing the ancient people of White Sands as ancestors, “when you go back, open-hearted, you can feel it. We Native Americans have that. You can feel it. And it’s just such a wonderful feeling. It’s like they say, ‘I’m here. I’m here if you need my help.’”
She mentioned the so-called ghost-tracks of White Sands — tracks that only appear under certain environmental conditions — and said, “We Native Americans always know that they’re there. And they will show you. They will give you some kind of guidance. Saying, ‘Here we are. SEE.’”