Dozens of stunning ancient footprints left on the shores of an Ice Age lake have reignited a long-running debate about when the first humans arrived in America.
Two years ago, a team of scientists concluded that human footprints sunk in the mud of White Sands National Park in New Mexico were more than 21,000 years old. The provocative find threatened dominant thinking about when and how people migrated to the Americas. Shortly after, there arose technical debate on the method used to estimate the age of footprintswhich was based on an analysis of plant seeds embedded in those footprints.
Now, a study published in the journal Science confirms the initial finding with two new lines of evidence: thousands of pollen grains and an analysis of quartz crystals in the sediments.
“It’s pretty much a master class on how to do this,” said Edward Jolie, an anthropological archaeologist at the University of Arizona who studied the White Sands footprints in the field but was not involved in the new study. “As Carl Sagan said, ‘extraordinary claims require extraordinary evidence’. “They have some extraordinary evidence.”
Lorena Becerra-Valdivia, a member of the Oxford Radiocarbon Accelerator Unit, said the results support her modeling work, which suggested that People first crossed into present-day North America before 29,000 years ago, possibly traveling across the ocean.
“If anything, early finds like the White Sands footprints should inspire more scientific research in what is a dynamic and changing field,” Becerra-Valdivia said.
Some critics who raised concerns about the initial study said they were encouraged by the follow-up analyzes but were not convinced.
“I don’t agree that this has resolved the issue of timing, but they have made progress,” said Loren Davis, an anthropologist at Oregon State University. “Knowing the age of this is important, because if these researchers are right and people really were in New Mexico 23,000 years ago, or even 21,000 years ago, it means we have to change our fundamental understanding of some things.”
Fossil footprints were first seen in the Tularosa basin in New Mexico in the early 1930s and were initially thought to be evidence of a bigfootsaid David F. Bustos, resource programs manager at White Sands National Park. It turned out to be from a giant ground sloth, a 2,000-pound mammal that became extinct about 10,000 years ago. Researchers also found footprints of mammoths, a giant (also called “dire”) wolf, and other ice age creatures.
Bustos said he first saw fossilized footprints in the basin that looked human in 2009, and a growing team of scientists began studying them. Those footprints gave life to a vivid snapshot of the Pleistocene, the epoch that began 2.6 million years ago and ended 11,700 years ago.
The thousands of footprints found at White Sands are an extraordinary but evanescent record of life around Lake Otero, the body of water that rested within the basin during the Pleistocene. Ancient traces are remains of complex interactions. Children played. Humans stalked giant sloths. One person walked a mile, carrying a child and placing him on the ground from time to time. But the fossilized footprints are slowly being destroyed by erosion: they are so soft they can be cut with a butter knife.
“It was hard to believe that humans could be walking alongside mammoth footprints nearby, and that the footprints could be the same age,” Bustos said.
To place these interactions in time, Kathleen Springer and Jeffrey Pigati, U.S. Geological Survey scientists who typically spend their time studying paleoclimate, joined the team of scientists working on the prints.
They began the work in January 2020, taking seed samples from an aquatic plant called ditch that was interspersed with the footprints. Using careful geological studies and radiocarbon dating, they obtained a surprising result: The footprints were between 21,000 and 23,000 years old.
“It was something very important, momentous and shook the world of archaeologists,” Springer said.
During decades, Experts believed that the first people of America migrated from Siberia across the Bering Strait across a land bridge exposed during the Last Glacial Maximum, sometime between 26,500 and 19,000 years ago. The land bridge was submerged when the ice sheets melted about 13,000 years ago. These people were thought to have developed the 13,000-year-old Clovis culture in New Mexico, which was characterized by the use of distinctive stone points.
In recent decades, archaeologists have found evidence that alters this “Clovis first” hypothesis. But the thinking that has continued to dominate the field is that people’s journeys to the Americas would have been blocked by ice sheets, making it unlikely that they would arrive much sooner by land.
The White Sands footprints, which appear to have been left during the Last Glacial Maximum, would require a radical rethinking of long-held assumptions about how people got here, something that has been welcomed by some scholars and rejected by others.
For many indigenous peoples, the study is simply a confirmation of things they already knew thanks to knowledge passed down from generation to generation, said Kim Pasqual-Charlie, a member of the Pueblo of Acoma who has visited the site many times.
“These are our ancestral footprints”said Pasqual-Charlie.
Paulette Steeves, an indigenous archaeologist and professor at Algoma University, has compiled a database of evidence of earlier human presence in the Americas. She said that The White Sands find is just one strand of a growing body of evidence that people were in the Americas much earlier than archaeologists long believed.
“Think about the rest of the world [y] how much our understanding of human evolution has grown and been informed by increased archaeological work in the advancement of the sciences. However, in the Americas, it has remained static,” Steeves said. “When it comes to adding indigenous voices and expanding the time frame for indigenous peoples in the Americas, there is still a lot of racism and prejudice in American archaeology.”
But there was a known problem when dating a site that used ditch seeds. Because ditch is an aquatic plant, it absorbs carbon dissolved in the water during photosynthesis. That could include older carbon sources that can make seeds appear artificially old. Oregon State’s Davis worked with a team that took trench samples from 1947 and analyzed them using radiocarbon dating. The results suggested that the plants were 7,400 years older than themselves.
So he and other scientists asked for additional lines of evidence.
“We’re talking about a possible paradigm shift in relation to the peopling of North America,” said David Rachal, a geoarchaeology consultant who also criticized the original study and is skeptical of the new one. “We have good models to say when people showed up and entered the scene. “If this goes backwards, it will alter everything we think we know.”
Springer and Pigati were well aware of the shortcomings of the ditch as a dating method and had always planned to see if other streams of evidence supported their initial study.
For monitoring, they collected ancient pollen from coniferous trees that was embedded around the footprints. This type of material would not have the same problem as aquatic plants, because trees absorb carbon from the atmosphere. They also used a technique called “optically stimulated luminescence” to measure the energy accumulated in quartz crystals within the White Sands sediments. This method allowed them to calculate the last time the mud containing the footprints was exposed to sunlight or heat.
The pollen study was an arduous task that required scientists from four laboratories spread across the United States to work together to prepare and analyze the age of 75,000 pollen grains. Using radiocarbon dating, they found conifer pollen dating back to 22,600 to 23,400 years ago, matching their early results.
His study of the quartz crystals showed that they were 21,500 years old, give or take 2,000 years.
“I think this study is so far the most compelling evidence for early human presence in the Americas,” said Bente Philippsen, leader of the National Laboratory for Age Determination at the Norwegian University of Science and Technology.
Others raised technical questions about methods and sampling. Rachal said it was possible that the pollen had been “reworked”, meaning that older pollen could have entered the samples. Davis said he was happy to see the analysis of the quartz crystal, but he wanted to see more samples taken from different layers.
Springer and Pigati argued their evidence is strong and they will continue working at the site.
“At first it didn’t bother me, but you could say I’m really starting to feel a nudge,” said Pasqual-Charlie, of the Town of Acoma. “How much more evidence is needed to confirm this? We existed back then. “We’ve been in the southwest region for a long time.”
Jolie, of the University of Arizona, said the debate over dating methods can continue for years, and that’s part of how science advances. But he added that for him, as a scientist and a person of mixed Oglala Lakota and Hodulgee Muscogee ancestry, the site brings to life a crucial period in human history that had been off-limits, erased by the passage of time and the changing landscape. .
“It’s a fun way to think about a common past shared by many Native people,” Jolie said. “You can visualize little children splashing in the mud. “There’s nothing like seeing a little child’s footprint in the sand.”
(c) The Washington Post