The footprints were discovered in an ancient lakebed alongside evidence of giant sloths and mammoths. The study concluded that they were the “earliest unequivocal evidence of human occupation anywhere in the Americas” during the Last Glacial Maximum.
New research from DRI, Kansas State University, the University of Nevada, Reno, and Oregon State University calls the initial study into question. They suggested that the footprints could have been left thousands of years later than previously thought.
“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,” said Charles Oviatt, emeritus professor of geology at Kansas State University and one of the new study’s authors. “I detected potential issues with the scientific tests of the dates reported in the Science paper.”
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 colonisation of North America are between 14 and 16 thousand years ago. Current chronological models in paleogenetics and regional geochronology would need to be re-evaluated if the original claims are correct.
Paleogeneticists hypothesise was first occupied no earlier than 20 thousand years ago by studying ancient DNA from human fossils and using rates of genetic change. 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.
The tiny seeds used to date the footprints using radiocarbon dating methods, are at the centre of the debate. Here researchers examined Carbon-14 that originates in the atmosphere and is absorbed by plants through photosynthesis.
These carbon isotopes decay at a constant rate over time. Scientists can estimate their age by comparing the amount of Carbon-14 in the atmosphere to the amount present in fossilised plant material.
However, the plant seeds come from Ruppia cirrhosa. It is an aquatic plant that grows underwater. This obtains much of its carbon for photosynthesis from dissolved carbon atoms in the water. And not directly from the atmosphere, as terrestrial plants do.
“While the researchers recognise the issue, they underestimate the plant’s basic biology,” Rhode says. “For the most part, it’s utilising the carbon found in lake waters.” And in most cases, that means it’s absorbing carbon from sources other than the current atmosphere. These are sources that are typically quite old.”
This method is likely to throw off radiocarbon-based age estimates. Because the results will be 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. There it accumulates in extensive calcium carbonate deposits.
The authors demonstrated this effect by examining Ruppia plant material from the same region with a known age. In 1947, botanists collected and archived living Ruppia plants from a nearby pond at the University of New Mexico herbarium. Using the same radiocarbon dating method, the plants alive in 1947 yielded a radiocarbon date. This indicates they were around 7400 years old, an offset resulting from the plant’s use of ancient groundwater.
If the Ruppia seeds dated from human footprints were also 7400 years off, their true age would be between 15 and 13 thousand years old. It is a date that corresponds to the ages of several other known early North American archaeological sites.
Other methods, can be used to resolve the dating of the footprints. This includes radiocarbon dating of terrestrial plants and optically stimulated luminescence dating of quartz found in the sediment,
“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.”