New research strengthens case for age of ancient New Mexico footprints
Researchers used a technique called radiocarbon dating to determine that organic matter in the remains of wetland muds and shallow lake sediments near the fossilized foot impressions is between 20,700 and 22,400 years old. That closely correlates to previous findings, based on the age of pollen and seeds at the site, that the tracks are between 21,000 and 23,000 years old.
The footprints, whose discovery was announced in 2021, indicate that humans trod the landscape of North America thousands of years earlier than previously thought, during the most inhospitable conditions of the last Ice Age, a time called the last glacial maximum.
The age of the footprints has been a contentious issue.
Asked how the new findings align with the previous ones, University of Arizona archaeologist and geologist Vance Holliday, the study leader, replied: "Spectacularly well."
Homo sapiens arose in Africa roughly 300,000 years ago and later spread worldwide. Scientists believe our species entered North America from Asia by trekking across a land bridge that once connected Siberia to Alaska. Previous archaeological evidence had suggested that human occupation of North America started roughly 16,000 years ago.
The hunter-gatherers who left the tracks were traversing the floodplain of a river that flowed into an ancient body of water called Lake Otero. The mud through which they walked included bits of semi-aquatic plants that had grown in these wetlands.
Radiocarbon dating is used to determine the age of organic material based on the decay of an isotope called carbon-14, a variant of the element carbon. Living organisms absorb carbon-14 into their tissue. After an organism dies, this isotope changes into other atoms over time, providing a metric for determining age.
"Three separate carbon sources - pollen, seeds and organic muds and sediments - have now been dated by different radiocarbon labs over the course of the trackway research, and they all indicate a last glacial maximum age for the footprints," said Jason Windingstad, a University of Arizona doctoral candidate in environmental science and co-author of the study published this week in the journal Science Advances.
The original 2021 study dated the footprints using radiocarbon dating on seeds of an aquatic plant called spiral ditchgrass found alongside the tracks. A study published in 2023 used radiocarbon dating on conifer pollen grains from the same sediment layers as the ditchgrass seeds.
But some scientists had viewed the seeds and pollen as unreliable markers for dating the tracks. The new study provides further corroboration of the dating while also giving a better understanding of the local landscape at the time.
"When the original paper appeared, at the time we didn't know enough about the ancient landscape because it was either buried under the White Sands dune field or was destroyed when ancient Lake Otero, which had a lot of gypsum, dried out after the last Ice Age and was eroded by the wind to create the dunes," Holliday said.
Today, the landscape situated just west of the city of Alamogordo consists of rolling beige-colored dunes of the mineral gypsum.
"The area of and around the tracks included water that came off the mountains to the east, the edge of the old lake and wetlands along the margins of the lake. Our dating shows that this environment persisted before, during and after the time that people left their tracks," Holliday said.
The area could have provided important resources for hunter-gatherers.
"We know from the abundant tracks in the area that at least mammoths, giant ground sloths, camels and dire wolves were around, and likely other large animals. Given the setting, there must have been a large variety of other animals and also plants," Holliday added.
The climate was markedly different than today, with cooler summers and the area receiving significantly more precipitation.
"It is important to note that this is a trackway site, not a habitation site," Windingstad said. "It provides us a narrow view of people traveling across the landscape. Where they were going and where they came from is obviously an open question and one that requires the discovery and excavation of sites that are of similar age in the region. So far, these have not been found."
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