People really were walking around in the southwestern US during the middle of the last Ice Age, according to a recent study that double-checked the dates on a set of surprisingly ancient human footprints at White Sands National Park.

Many thousands of years ago, someone walked along the muddy shore of an ancient lake at what’s now White Sands. They crushed ditchgrass seeds and grains of conifer pollen beneath their feet with every squishing, slippery step. Bournemouth University archaeologist Matthew Bennett and his colleagues (including the authors of the current study) unearthed eight layers of tracks at the site in early 2020; they radiocarbon-dated the ditchgrass seeds from the oldest layer of footprints to 23,000 years old and the youngest layer to around 21,000 years old.

Their 2021 paper sparked immediate debate.

People weren’t supposed to be there yet

USGS research geologist Jeff Pigati and his colleagues (including Bennett and other co-authors of the 2021 paper) recently radiocarbon-dated conifer pollen—mostly from fir, spruce, and pine—from the same ancient ground surface as the tracks and the ditchgrass seeds. They also used another type of dating, called optically stimulated luminescence (a type of dating that measures when a grain of quartz was last exposed to sunlight) on sediment samples from between the oldest two layers of tracks. The results lined up very well with Bennett and his colleagues’ original radiocarbon dates; the tracks couldn’t be any younger than about 21,500 years old.

A growing pile of evidence suggests that the first people to set foot in the Americas spread south along the Pacific coast sometime between 20,000 and 16,000 years ago, following the slowly receding western edge of the ice sheet. If Pigati and his colleagues are right, people walked at White Sands between 23,000 and 21,000 years ago. That’s a big enough claim on its own, but it carries even bigger implications.

“This obviously shows that people were in what is now Southern New Mexico 23,000 years ago, well south of the coalesced Laurentide and Cordilleran ice sheets,” USGS research geologist Kathleen Springer, a coauthor of both studies, told Ars Technica. “So that means they had to have been there before those big walls of ice closed, or they got there some other way.”

The ice sheets sealed off the northern third of the continent around 26,000 years ago.