Expanded excavations at White Sands National Park in New Mexico have provided researchers an opportunity to refine the age of the oldest human footprints in North America and establish a deeper understanding of the landscape and environmental conditions at the time humans were present there.
The new study , published in Science Advances by a team of researchers including the University of Arizona, the United States Geological Survey and other partners, reveals a dynamic environment indicating the humans who left their tracks along an ancient lake shore experienced abrupt changes in temperature and water availability within their lifetimes.
The research team previously discovered and confirmed the White Sands human footprints to be the oldest evidence of human presence in North America, placing people on the continent thousands of years earlier than once thought.
The research details innovative extraction procedures used to isolate up to 75,000 individual pollen grains per sample and refined dating methods used at the USGS Radiocarbon Laboratory. This resulted in a more precise age of the site and reaffirmed the White Sands footprints date to between 23.5 and 21.7 thousand years ago, or during the Last Glacial Maximum. The new chronology was key to unlocking the rest of the story, which includes a deeper understanding of the abrupt environmental changes that early humans in North America faced.
The researchers used the new chronology to investigate the climate and environmental changes at a timescale equivalent to a human generation, or approximately 50 years, which is rare for terrestrial paleoclimate archives of this time period from the southwestern U.S.
"The White Sands sediments are unique in that they allow us to reconstruct environmental conditions during the last ice age at a very high temporal resolution, so that we can see fluctuations that were relevant for the humans present at that time," said co-author Jessica Tierney, a professor at the U of A Department of Geosciences .
Tierney applied her expertise in the analysis of geochemical signatures captured in sediments, which she combined with fossil pollen from the plants growing in the surrounding landscape, to reconstruct temperature, precipitation, aridity and plant communities. The results show the climate at White Sands was dynamic. Humans witnessed a complete turnover of the landscape from conifer forest to sagebrush and oak steppe and the recession of Lake Otero, a massive lake that would have covered most of what is White Sands National Park during the last ice age. By measuring isotopes – elemental atoms that differ by their number of neutrons, in this case, hydrogen – in leaf waxes of preserved plant material, Tierney's group at the U of A discovered that there were also swings in water availability within a century that included severe aridity during an abrupt warming event also recorded in ice core samples from Greenland.
"This tells us that local changes in aridity at White Sands were quite large and followed global climate shifts," Tierney said.
During the last glacial maximum, when the humans left their footprints in what likely was a muddy plain, temperatures in the Southwest were generally cooler, around 50 degrees Fahrenheit, as opposed to today's annual mean temperatures around 60 degrees in that area, Tierney explained. During the sudden shift documented by pollen in the sedimentary layers, vegetation changed from pine forest to pinyon and juniper, all within about 100 years.
Tierney said that varying amounts of pollen left behind by Ruppia, an aquatic plant that likes to grow in desert streams and springs, indicate Lake Otero's water level went up and down over the course of about 1,500 years.
"Eventually the lake shrunk more and more until it gave way to a flat playa," she said. "And throughout that whole sequence, people were walking there, leaving their tracks alongside those of mammoths, giant sloths and dire wolves."
Having an exquisitely preserved record of the environmental conditions and changes, such as rainfall, temperatures and vegetation in the same place with tracks left by humans and wildlife at the same time is extremely unique for the Southwest, if not globally, Tierney added.
"Even though these footprints have been buried in the gypsum mud for more than 20,000 years, they look like someone walked out on the playa yesterday," Tierney said. "This is an incredibly rare opportunity to be able to do paleoclimate reconstruction and combine it with archeology in the same exact place."
Next, the researchers want to delve deeper into what exactly happened at the time – what mechanisms may have been at work that would have caused a warming event in Greenland to cause drying in the American Southwest.
"This study fulfilled a long-term research goal at White Sands – to expand the originally published footprint site and sample for environmental indicators to help further our understanding of the complete context of conditions at the height of the Last Glacial Maximum," said Kathleen Springer, USGS research geologist and co-lead author on the paper. "It's remarkable that early humans in the region were real-time witnesses to a rapidly changing environment discernible within a human lifetime – that is really food for thought."
Ed Jolie, Clara Lee Tanner Associate Professor of Anthropology at the U of A and a study co-author, said the findings "humanize the deep Indigenous past of the Americas." Jolie, who is Oglala Lakota and Hodulgee Muscogee, is also associate curator of ethnology at Arizona State Museum.
"This research continues to remind scientists and the wider public that these resilient ancient human populations are the ancestors of contemporary Indigenous peoples, providing important scientific evidence in support of sovereign Native nations' oral traditions and reinforcing the fact that we have been here since time immemorial," Jolie said.
