|These footprints in New Mexico have the potential to rewrite the history of the Americas. BENNET AND OTHERS/SCIENCE|
The preserved footprints of a life that roamed millennia ago can be found on the vast expanse of an ancient lakebed in New Mexico. Giant sloths and mammoths left their imprints, as did signs of our ancestors. According to a study published in September 2021, these footprints are "definitive evidence of human occupation of North America" during the last ice age, which lasted between 23,000 and 21,000 years ago. A new study now calls into question the evidence of such a young age.
In Quaternary Research, scientists from DRI, Kansas State University, the University of Nevada, Reno, and Oregon State University warn that the dating evidence is insufficient for claims that would fundamentally alter our understanding of when and how humans first arrived in North America. Using the same dating method and materials as the previous study, the new research suggests that the footprints could have been left thousands of years later than previously thought.
"When I first read the original Science article on the human footprints at White Sands, I was struck not only by how incredible the footprints were on their own, but also by how important accurate dating would be," says Charles Oviatt, emeritus professor of geology at Kansas State University and one of the study's authors. "I detected potential flaws in the scientific tests of the dates reported in the Science paper."
"It really calls into question a lot of what we thought we knew," says David Rhode, Ph.D., a paleoecologist at DRI and co-author of the new study. "That's why it's critical to really nail down this age, and why we're arguing for more evidence."
To determine the timing of historic events, archaeologists and historians employ a variety of techniques. Scientists generally agree that the earliest known dates of human colonization of North America are between 14,000 and 16,000 years ago, after the last ice age. Current chronological models in fields as diverse as paleogenetics and regional geochronology would need to be reevaluated if the original claims are correct.
"You need to pay attention to how people got into North America between 23,000 and 21,000 years ago," Rhode says. "At the time, there was a huge, mile-high ice range covering Canada to the north, and the pathway down the Pacific Coast wasn't very accommodating either—so people may have had to come here much earlier than that."
Paleogeneticists hypothesize that the American Southwest was first occupied no earlier than 20,000 years ago by studying ancient DNA from human fossils and using rates of genetic change (a type of molecular clock using DNA). If the footprints are older, the use and integrity of these genetic models are called into question.
The authors write that it's possible that the ages from a single study at a single site in a New Mexico lake basin are correct and that age estimates from a variety of other fields are incorrect, but that more robust evidence is needed to confirm the claims.
The tiny seeds of an aquatic plant used to age the footprints are at the center of the debate. Radiocarbon dating methods were used to determine the timeframe for the seeds, in which researchers examined a type of carbon known as Carbon-14. Carbon-14 enters the atmosphere and is absorbed by plants via photosynthesis.
These carbon isotopes decay at a constant rate over time, and scientists can estimate their age by comparing the amount of Carbon-14 in the atmosphere to the amount present in fossilized plant material. However, the plant species used, Ruppia cirrhosa, grows underwater and thus obtains much of its carbon for photosynthesis from dissolved carbon atoms in the water rather than directly from the atmosphere, as terrestrial plants do.
"While the researchers recognize the issue, they underestimate the plant's basic biology," Rhode says. "For the most part, it's utilizing the carbon found in lake waters." And in most cases, that means it's absorbing carbon from sources other than the current atmosphere—sources that are typically quite old."
This method is likely to produce radiocarbon-based plant age estimates that are much older than the plants themselves. Ancient carbon enters the Lake Otero basin's groundwater from eroded bedrock in the Tularosa Valley and surrounding mountains, and it is found in extensive calcium carbonate deposits throughout the basin.
The authors demonstrated this effect by examining Ruppia plant material from the same region with a known age. Living Ruppia plants were collected from a nearby spring-fed pond in 1947 and archived at the University of New Mexico herbarium. Using the same radiocarbon dating method, the plants alive in 1947 yielded a radiocarbon date indicating they were around 7,400 years old, an offset resulting from the plant's use of ancient groundwater.
The authors point out that if the ages of the Ruppia seeds dated from human footprints were also shifted by about 7,400 years, their true age would be between 15,000 and 13,000 years old—a date that corresponds to the ages of several other known early North American archaeological sites.
Other methods, such as radiocarbon dating of terrestrial plants (which uses atmospheric carbon rather than carbon from groundwater) and optically stimulated luminescence dating of quartz found in the sediment, can be used to resolve the dating of the footprints, the authors write.
"There's no doubt that these trackways are a fantastic resource for understanding the past," Rhode says. "I'd like to see them for myself." I'm just concerned about the ages assigned to them by the researchers."
More information: Charles G. Oviatt et al., Quaternary Research, A critical assessment of claims that human footprints in the Lake Otero basin, New Mexico, date to the Last Glacial Maximum (2022). qua.2022.38 (DOI: 10.1017/qua.2022.38).