So, when USGS researchers and an international team of scientists claimed an age of 23,000 to 21,000 years for seeds found in the footprints — thousands of these footprints are preserved in an alkali flat in White Sands National Park — the pushback from archeologists was extreme.

One point of contention was that the seeds came from a common aquatic plant, spiral ditchgrass (Ruppia cirrhosa). Dating aquatic plants can be problematic, Wahl said, because if the plants grow totally submerged, they take up dissolved carbon rather than carbon from the air. Dissolved carbon can be older because it comes from surrounding bedrock, potentially causing the age measured by radiocarbon dating to be older than the material analyzed actually is. Hence, the reanalysis.

“The immediate reaction in some circles of the archeological community was that the accuracy of our dating was insufficient to make the extraordinary claim that humans were present in North America during the Last Glacial Maximum. But our targeted methodology in this current research really paid off,” said Jeff Pigati, USGS research geologist and co-lead author of the new study.

For the newly published follow-up study, the researchers focused on radiocarbon dating of conifer pollen — more than 75,000 pollen grains per sample from fir, spruce and pine, which are terrestrial. Wahl, Marie Champagne and Jeff Honke of USGS worked with Susan Zimmerman at Lawrence Livermore National Laboratory to painstakingly isolate the pollen grains through several steps including physical and chemical separation, followed by purifying via flow cytometry at the Flow Cytometry Core Facility at Indiana University (FCCF). The carbon isotope composition of each sample was then determined using mass spectrometry at the Center for Mass Spectometry (CAMS). Importantly, the pollen samples were collected from the exact same layers as the original Ruppia seeds, so a direct comparison could be made. In each case, the pollen age was statistically identical to the corresponding seed age.

“Pollen samples also helped us understand the broader environmental context at the time the footprints were made,” Wahl said. “The pollen in the samples came from plants typically found in cold and wet glacial conditions, in stark contrast with pollen from the modern playa, which reflects the desert vegetation found there today.”

In addition, Harrison Gray on the USGS team used a different type of dating called optically stimulated luminescence, which dates the last time quartz grains were exposed to sunlight. Using this method, the team found that quartz samples collected within the footprint-bearing layers had a minimum age of about 21,500 years, providing further support to the radiocarbon results.

“We were confident in our original ages, as well as the strong geologic, hydrologic and stratigraphic evidence, but we knew that independent chronologic control was critical,” noted Kathleen Springer, USGS research geologist and co-lead author of the paper.

With three separate lines of evidence pointing to the same approximate age, it is highly unlikely that all are incorrect or biased and, taken together, they provide strong support for the 23,000-to-21,000-year age range for the footprints.

"Critics asked us to provide more than one piece of evidence, and we've given them a total of three," Wahl said. "So, we're feeling really good."