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Footprints left in layers of clay and silt in White Sands National Park in New Mexico may be between 23,000 and 21,000 years old. It is based on radiocarbon dating of the remains of grass seeds buried in the layers of sediment above and below the tracks. If the dates are correct, the tracks are evidence that people were walking along the now dry Lake Otero at the height of the last Ice Age, when miles of ice covered the northern half of the continent. And that would mean people have to have arrived in North America – and headed for an area well south of the ice – before the ice caps expand enough to close the road.
Arrive in front of the ice caps
Bournemouth University archaeologist Matthew Bennett and his colleagues have found a total of 61 human footprints east of an area called Alkali Flat, which was once the bed and shore of an ancient lake. Over time, as the lakeshore expanded and contracted with climate change, it left behind distinct layers of clay, silt, and sand. Seven of those layers, in the area recently excavated by Bennett and his colleagues, contained traces of humans as well as those of a long-lost megafauna.
Some layers of sediment contained remnants of ancient grass seeds mixed with the sediment. Bennett and his colleagues dated the radiocarbon seeds from the layer just below the oldest indentations and from the layer just above the newer ones. According to the results, the oldest impressions were made some time after 23,000 years; the most recent were made some time before 21,000 years ago. At that time, the northern half of the continent lay several kilometers under massive patches of ice.
Ice caps had completely covered most of Canada and the northernmost United States about 26,000 years ago, and they would not begin to thaw and recede until about 20,000 years ago.
“These data provide definitive evidence of human occupation of North America south of the Laurentian Ice Sheet during the Last Ice Maximum,” Bennett and colleagues wrote in their recent article. And everyone who lived in what is now New Mexico during that time, known as the Last Ice Maximum, must have arrived before the ice caps closed the route from Asia to the Americas.
If so, we may need to rethink our species’ role in extinction of megafauna like mammoths and giant land sloths. “It also raises the possibility of a human role in poorly understood megafauna extinctions previously thought to their arrival,” wrote Bennett and his colleagues.
The search for the first Americans
North and South America were the last continents reached by humans; as far as we know, none of our other hominids have ever been there. At present, the earliest widely accepted evidence of the existence of people in the Americas comes from a scattering of sites along the western coasts of both continents, and it dates from 13,000 to 16,000 years ago.
Our understanding of how and when people first entered the Americas has changed dramatically in recent years. Until a decade ago, it seemed that early Americans were part of the Clovis culture, named for the distinctive projectile tips they left behind near what is now Clovis, New Mexico. . All available evidence indicates that the Clovis people headed south through a corridor that opened up in the middle of the ice caps about 13,000 years ago.
But then, as is usually the case (at least on a good day), archaeologists found new evidence, such as a 14,000-year-old set of footprints in Argentina, a single 14,600-year-old footprint in Chile, a 14,500-year-old footprint – an ancient site in Florida and 16,000-year-old stone tools in western Idaho. This evidence pushed the date of arrival back a few thousand years, suggesting that the Clovis were in fact not the first to arrive. It also gave the impression that the early Americans had in fact skirted the edge of the ice caps along the Pacific coast.
Currently, most of the evidence suggests that people arrived in North America around 16,000 years ago and followed the coastline to the south of the ice caps. However, if Bennett and his colleagues are right, the newly discovered leads at White Sands could drastically change what we think we know yet again. 23,000-year-old footprints can only mean that people were already living in what is now New Mexico before the ice caps separated the southern half of the continent from the rest of the world for the next few thousand years. . It’s possible, if not likely, that another wave of newcomers arrived as the ice caps receded again, but there might already be someone here to meet them.
Proof for extraordinary claims
Extraordinary claims require extraordinary evidence, as the late astronomer Carl Sagan would say. And Bennett and his colleagues certainly make an extraordinary claim. If there is to be a scientific debate over the White Sands traces, it is likely to focus on dating the sediment layers involved.
The seeds mixed in the layers above and below the White Sands tracks provided a convenient way to date the tracks. But aquatic plants, like the grass species that Bennett and his colleagues dated, can sometimes look older than they are. If the water is full of dissolved calcium carbonate from much older diatoms or other aquatic life, it can give the impression that the C-14 ratio in plants is too low. This is called a hard water effect (or a reservoir effect).
To verify their results, Bennett and his colleagues compared radiocarbon dating of terrestrial and aquatic plants from the region around Alkali Flat. The aquatic dates matched the terrestrial ones, meaning that the aquatic plants that have grown in the area for several thousand years probably did not suffer from a hard water effect.
The new claim is also less far-fetched and supported by much stronger evidence than some others. For example, a group of California archaeologists insist they found a 130,000-year-old mammoth slaughter site, which would put humans in California long before we have any evidence that our species is ‘was even far from Africa as from Europe. . And the case of the improbably old Californian site depends entirely on the use of certain round stones as hammers.
Meanwhile, the 23,000-year-old traces of White Sands seem to match a paper published last year that described stone tools unearthed in a 30,000-year-old layer of sediment in a cave in Mexico.
Ice Age Races
If people were walking around New Mexico during the Last Ice Maximum, who were they and what were they doing? From what the measurements of their feet tell us about their stature, most of the people who have taken off the tracks at White Sands appear to have been teenagers and children. If so, they may have gone to fetch water or collect food or other resources.
“One hypothesis is the division of labor, in which adults are involved in skilled tasks while ‘fetch and carry’ is delegated to adolescents. Children accompany adolescents, ”wrote Bennett and his colleagues.
Most children’s feet appear to be flatter than those of most people living today, suggesting that they often went barefoot. The toes of the slopes feel a bit stretched out, which usually happens when someone slips while walking, such as on a muddy shore of a lake.
The environment of White Sands has been, for thousands of years, perfect for preserving footprints. Last year, the same team of researchers found 10,000 to 15,000 year old traces of a small teenage girl or woman crossing mammoths and a giant land sloth while carrying a small child. In 2019, researchers used radar to spot hidden tracks. And in 2018, they followed in the footsteps of hunters chasing giant sloths.
Science, DOI 2021: https: /10.1126/science.abg7586; (About DOIs).
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