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Hometech-scienceHumans lived in America earlier than thought, ancient footprint study finds

Humans lived in America earlier than thought, ancient footprint study finds

Researchers say they have confirmed the age of a set of ancient human footprints in New Mexico, finding them to be between around 23,000 and 21,000 years old.

The latest results, published in the journal Science, indicate that humans were present in the Americas earlier than previously accepted, challenging consensus views on a topic that has long been hotly debated among experts.

“If these footprints were made by humans between 23,000 and 21,000 years ago, it would be one of the most significant finds in the history of American archaeology,” Todd Surovell, a researcher in the Department of Anthropology at the University of Wyoming, who was not involved in the latest Science study, told Newsweek.

The generally accepted range for the date of human arrival in the Americas over the past couple of decades has been roughly 13,000 to 16,000 years ago depending on the specific archaeological sites individual scholars either take into account or reject.

Some recent studies have provided evidence for human occupation of the Americas even earlier than this range—during, or even prior to, the Last Glacial Maximum (LGM), which spanned the years between around 26,000 and 19,000 years ago. The LGM was a period time in Earth’s history when ice sheets and glaciers reached their greatest extent during the last ice age.

However, evidence indicating human settlement of the Americas earlier than 16,000 years ago has often been met with skepticism and such findings are the subject of significant debate.

The ancient human footprints in New Mexico—located in White Sands National Park at the site of an ancient lakebed—fall into this category. In 2021, a team of scientists, including authors of the latest study, dated 61 human footprints in the park, finding that they had been left behind between around 23,000 and 21,000 years ago.

This indicated that humans were present in the region for roughly 2,000 years during the height of the LGM. However, these results proved to be controversial, with many researchers casting doubt on the accuracy of the ages.

“The immediate reaction in some circles of the archeological community was that the accuracy of our dating was insufficient to make the extraordinary claim that humans were present in North America during the Last Glacial Maximum,” Jeff Pigati, a research geologist with the U.S. Geological Survey (USGS) and co-lead author of the new Science study, said in a press release.

In the 2021 study, researchers used a radiocarbon dating technique on ancient seeds from the common aquatic plant Ruppia cirrhosa that were found in the fossilized footprints. This is what sparked the controversy over the age of the impressions because aquatic plants can acquire carbon from dissolved carbon atoms in the water rather than ambient air, potentially making them appear older than they really are.

The study authors were confident in the ages they had come up with but began trying to independently evaluate their results using multiple lines of evidence in order to address some of the criticisms.

In the latest study, Pigati, co-lead author Kathleen Springer—another USGS research geologist—and colleagues used radiocarbon dating of conifer pollen taken from the exact same geological layers as the original seed samples, enabling a direct comparison.

They decided to use conifer pollen because these plants are terrestrial and, thus, are not affected by the same issues that arise when using aquatic plants like Ruppia cirrhosa. This is down to the fact that their carbon comes directly from the atmosphere via photosynthesis.

“The drawback of dating pollen is that pollen is tiny, but the methods that we used in this study allowed us to isolate 75,000 grains of pure pollen per sample, which was enough to obtain robust radiocarbon ages,” the researchers said.

The researchers found that the pollen samples were statistically identical to the corresponding seed ages, lending support to the original dates put forward in the 2021 study. In addition, the scientists used another dating technique known as optically stimulated luminescence, which dates the last time quartz grains were exposed to sunlight.

The luminescence dating revealed that the quartz samples collected within the footprint-bearing layers had a minimum age of around 21,500 years, further supporting the radiocarbon results.

Given that there are now three separate lines of evidence pointing to the same approximate age for the footprints, it is highly unlikely that all are incorrect, indicating that the original age range of 23,000 to 21,000 years old is accurate, according to the researchers.

“Our results from radiocarbon dating of pollen and luminescence dating of the sediments show that the chronologic framework originally established for the White Sands footprints is robust and reaffirms that humans were present in North America during the Last Glacial Maximum,” the researcher said. “We think our study from White Sands closes the case and resolves the controversy.”

“People were in the southern part of North America during the Last Glacial Maximum 23-21,000 years ago. That means that they were there prior to the massive ice sheets closing. Our findings extend the known range of human occupation in North America by thousands of years, which has implications for migratory routes, ancient DNA studies and linguistics. Given time, we hope to build community confidence in the ages that we’ve developed from White Sands.”

Researchers that Newsweek contacted have praised the latest findings and the quality of the evidence put forward in the study.

“The authors have done an excellent job responding to critiques of the dating of the site,” Surovell said. “At this point, we can be very confident that these deposits are more than 20,000 years old. I hesitate to say that we can be ‘certain’ because there is always the possibility that there are unidentified problems with the dating, but when three independent methods produce congruent results, chances are very high that they have identified the correct age of the site.”

The findings “would suggest that there is a lot we do not understand about human colonization of the Americas,” Surovell said. “In my mind, the biggest questions are about human demographics. If people are in the deserts of New Mexico at the height of the last ice age, why do they remain largely invisible everywhere else in the Americas for 8,000 to 10,000 years?”

The researcher said it is also important to note that other archaeologists have raised questions about excavation methods and whether these are truly human footprints.

“What this site will ultimately require for widespread acceptance is independent investigation of the locality and independent replication of its findings,” he said.

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