New Evidence Calls Into Question Timing of Human Arrival in North America

Prehistoric cavemen

Footprints believed to be evidence of Ice Age humans in North America need more precise dating, according to new research.

New research questions the age of preserved human footprints found in New Mexico’s Lake Otero Basin, which could revolutionize our understanding of the arrival of humans in North America if accurately dated.

An ancient lake in New Mexico contains well-preserved traces of various animals that lived there thousands of years ago, including giant sloths and mammoths, as well as humans. A study previously published in September 2021 suggested that these human footprints provide “conclusive evidence” of human occupation of North America during the “last ice age,” dating from 23,000 to 21,000 years ago. However, a new study casts doubt on this claim.

Recently, a group of scientists published a study in the journal Quaternary research warning that the dating evidence for ancient human footprints found in a New Mexico lake is not strong enough to support claims that would significantly change our understanding of when and how humans first arrived in North America. The researchers used the same dating methods and materials as the previous study, but their findings suggest the footprints may have been left thousands of years later than first thought.

“I read the original Science article on human footprints at White Sands and was initially struck not only by how enormous the footprints were, but also how important accurate dating would be,” said Charles Oviatt, professor emeritus of geology at Kansas State University and one of the new. the authors of the study. “I saw potential problems with the scientific tests of the dates mentioned in the scientific article.”

“It really calls into question what we think we know,” said David Rhode, Ph.D., a DRI paleoecologist and co-author of the new study. “That’s why it’s important to really pinpoint this age, and why we suggest we need better evidence.”

Archaeologists and historians use a variety of methods to determine the timing of historical events. Based on these methods, scientists tend to agree that the earliest dates for human colonization of North America are between 14,000 and 16,000 years ago, after the last ice age. If the initial claims are correct, current chronological models in fields such as paleogenetics and regional geography should be reevaluated.

“23,000 to 21,000 years ago is when you have to really look at how people got to North America,” says Rhode. “At that time, there was a huge, kilometer-high ice ridge that covered Canada to the north, and the path down the Pacific coast was also not very convenient, so people probably had to come here much earlier. than that.”

Studying ancient[{” attribute=””>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 center of the debate are the tiny seeds of an aquatic plant used to age the footprints. The timeframe for the seeds was identified using radiocarbon dating methods, in which researchers examine a type of carbon known as Carbon-14. Carbon-14 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 fossilized plant material allows scientists to determine their approximate age. But the plant species used, Ruppia cirrhosa, 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 recognize 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 give radiocarbon-based age estimates of the plant that are much older than the plants themselves. Ancient carbon enters the groundwater of the Lake Otero basin from the 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 authors note that if the ages of 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 the 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.”

Reference: “A critical assessment of claims that human footprints in the Lake Otero basin, New Mexico date to the Last Glacial Maximum” by Charles G. Oviatt, David B. Madsen, David Rhode and Loren G. Davis, 2 September 2022, Quaternary Research.
DOI: 10.1017/qua.2022.38



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