Preserved footprints found in New Mexico’s Lake Otero Basin were dated to the last ice age between 23 and 21 thousand years ago, according to a study published in September 2021 in the journal Nature.
The footprints were found in an ancient lakebed alongside evidence of giant sloths and mammoths, which the study concluded was the “earliest unequivocal evidence of human occupation anywhere in the Americas” during the height of the last glacial cycle, known as the Last Glacial Maximum.
New research by scientists from DRI, Kansas State University, the University of Nevada, Reno, and Oregon State University, now dispute the initial study, suggesting that the footprints could have been left thousands of years later than originally claimed.
Charles Oviatt, emeritus professor of geology at Kansas State University and one of the new study’s authors said: “I read the original Science article on the human footprints at White Sands and was initially struck, not only by how tremendous the footprints were on their own, but how important accurate dating would be. I saw potential problems with the scientific tests of the dates reported in the Science paper.”
Archaeologists and historians use a number of methods to determine the timing of historic events. Based on these methods, scientists tend to agree that the earliest known dates of humanity’s colonisation of North America lie between 14 and 16 thousand years ago. If the original claims are correct, current chronological models in fields as varied as paleogenetics and regional geochronology would need to be re-evaluated.
By studying ancient DNA from human fossils and using rates of genetic change (a sort of molecular clock using DNA), paleogeneticists surmise that the American Southwest was first occupied no earlier than 20 thousand years ago. If the footprints are older, it throws into question the use and integrity of these genetic models. It’s possible that the ages from one study at a single site in a New Mexico lake basin are valid, and that age estimates from a variety of other fields are invalid, the authors write, but more robust evidence is needed to confirm the claims.
At the centre of the debate are tiny seeds used to date the footprints using radiocarbon dating methods, in which researchers examine Carbon-14 that originates in the atmosphere and is absorbed by plants through photosynthesis.
These carbon isotopes decay at a constant rate over time, and comparing the amount of Carbon-14 in the atmosphere to the amount present in fossilised plant material, scientists can determine their approximate age.
However, the plant seeds come from Ruppia cirrhosa, an aquatic plant that grows underwater, and therefore obtains much of its carbon for photosynthesis not directly from the atmosphere as terrestrial plants do, but from dissolved carbon atoms in the water.
“While the researchers recognise the problem, they underestimate the basic biology of the plant,” says Rhode. “For the most part, it’s using the carbon it finds in the lake waters. And in most cases, that means it’s taking in carbon from sources other than the contemporary atmosphere – sources which are usually pretty old.”
This method is likely to throw off the radiocarbon-based age estimates, as the results will be much older than the plants themselves. Ancient carbon enters the groundwater of the Lake Otero basin from eroded bedrock of the Tularosa Valley and the surrounding mountains and occurs in extensive calcium carbonate deposits throughout the basin.
The authors demonstrated this effect by examining Ruppia plant material with a known age from the same region. Botanists collected living Ruppia plants from a nearby spring-fed pond in 1947 and archived them at the University of New Mexico herbarium. Using the same radiocarbon dating method, the plants that were alive in 1947 returned a radiocarbon date suggesting they were about 7400 years old, an offset resulting from the use of ancient groundwater by the plant.
The study suggests that if the Ruppia seeds dated from the human footprints were also offset by roughly 7400 years, their real age would be between 15 and 13 thousand years old – a date which aligns with ages of several other known early North American archaeological sites.
The dating of the footprints can be resolved through other methods, including radiocarbon dating of terrestrial plants (which use atmospheric carbon and not carbon from groundwater) and optically stimulated luminescence dating of quartz found in the sediment, the authors write.
“These trackways really are a great resource for understanding the past, there’s no doubt about that,” says Rhode. “I’d love to see them myself. I’m just cautious about the ages that the researchers put to them.”
Desert Research Institute
Header Image Credit : Jeff Pigati & Kathleen Springer, USGS