In 2006, a cluster of mysterious dark spots on a lakebed of White Sands National Park in New Mexico caught the attention of archaeologists. The shapes stroked their curiosity until they eventually excavated the site three years later. Waiting for them was one of the rarest and soon-to-be controversial discoveries in history—a set of fossilized human footprints. 

The preserved markings were found on the shore of a lake that existed during the most recent ice age, and could be one of the earliest signs of biped migration to North America. Some experts claim they are the steps of the Clovis people, the continent’s first human inhabitants and the ancestors for most Native Americans. The Clovis are thought to have made the journey to North America 13,000 to 13,500 years ago using a land bridge that connected Asia to Alaska. From there, they continued to move as far down south as Central and South America. 

“This was groundbreaking to the archaeologic community, and it was also a tough pill to swallow,” says Kathleen Springer, a research geologist for the United States Geological Survey (USGS) who helped analyze the fossilized steps. “Having 23- to 21,000-year-old footprints is much earlier than the prevailing paradigm of Clovis or pre-Clovis that are known in this part of North America.”

The finding initially received some pushback. When the results were first revealed in 2021, concerned archaeologists wrote comments and papers challenging the results, citing the need for better evidence. More specifically, they criticized the study method and the decision to use radiocarbon dating on the seeds of an aquatic plant that was excavated from the same site. 

Part of the debate came down to an isotope that’s often used in archaeological work. Carbon-14 forms in the air and is introduced to photosynthetic plants and the animals that eat them. When flora and fauna are alive, they have the same amount of carbon-14 as the Earth’s atmosphere; when they die, it decays in their remains. Scientists can then measure how much of the isotope is left and use that metric to calculate an organism’s approximate age. But as some experts have pointed out, aquatic plants like the ones sampled at White Sands can get carbon from the water they live in, which can skew the measurements and make a specimen seem older than it really is.

“It’s called the hard water effect, and it’s a really well-known problem with radiocarbon dating,” explains Jeffrey Pigati, a USGS research geologist who co-authored both studies with Springer. He says the general argument with the first paper is that there were large hard-water effects that made them overestimate the age of the footsteps when they should have been around 15,000 or 17,000 years old.

The COVID pandemic delayed many of the follow-up experiments Pigati and Springer wanted to complete when investigating the site in 2020. Three years later, they finally did with two new methods that corroborate their original estimate of the footprints’ age: radiocarbon dating of pollen and luminescence dating.

To avoid heavy-water effects, the team extracted pollen grains from the same sediment as the White Sands footprints. According to Pigati, this is a time-consuming and laborious process because it involves breaking down rock into one cubic centimeter of material and separating pollen from other organic material before measuring carbon-14 levels. Additionally, pollen is extremely light—experts need to sample thousands of grains to meet the minimum mass requirement for a single radiocarbon measurement. In total, they successfully isolated 75,000 pollen grains. When the they compared the measurements to ones from the seeds of the aquatic plant, the ages matched.

The second technique was optically stimulated luminescence (OSL) dating. Unlike radiocarbon dating, OSL dating is based on the buildup of luminescence properties in quartz crystals over time; in some rare cases, it can date sediments as far back as 400,000 years ago. The USGS team dated three different mineral samples from the same area where the footprint was discovered and calculated ages that were similar to the ones measured in the seeds.

Still, this does not mean we have a complete picture of our species’ migration to North America. Paulette Steeves, an archaeologist and author of The Indigenous Paleolithic of the Western Hemisphere, who was not involved in the White Sands research, says there are archaeological sites in both North and South America that date to as early as 11,000 to 200,000 years ago. While she argues it’s not the oldest sign of human habitation in the Americas and may not be proof of the first Indigenous group, “the White Sands footprints site is a great addition to the record of early people in the Western Hemisphere.”

The footprints are just one piece of the puzzle. Archaeologists still don’t know exactly how people lived in the middle of an ice age and weathered harsh climate. Future projects at White Sands could include tracking the footprints to a campsite or further scouring the area for stone tools that could give some insight into their survival. “Every day we’re working out there is amazing because you never know what is going to be discovered,” Pigati says. “This is all a part of science in action.”