Some of the sediment layers contained the remains of ancient grass seeds mixed with the sediment. Bennett and his colleagues radiocarbon-dated seeds from the layer just below the oldest footprints and the layer just above the most recent ones. According to those 2021 results, the oldest footprints were made sometime after 23,000 years ago; the most recent ones were made sometime before 21,000 years ago.
At that time, the northern half of the continent was several kilometers below massive sheets of ice. The existence of 23,000-year-old footprints could only mean that people were already living in what’s now New Mexico before the ice sheets sealed off the southern half of the continent from the rest of the world for the next few thousand years.
Ancient human footprints found in situ at White Sands National Park in New Mexico.
Credit:
Jeffrey S. Pigati et al., 2023
Other researchers were skeptical of those results, pointing out that the aquatic plants (Ruppia cirrhosa) analyzed were prone to absorbing the ancient carbon in groundwater, which could have skewed the findings and made the footprints seem older than they actually were. And the pollen samples weren’t taken from the same sediment layers as the footprints.
So the same team followed up by radiocarbon-dating pollen sampled from the same layers as some of the footprints—those that weren’t too thin for sampling. This pollen came from pine, spruce, and fir trees, i.e., terrestrial plants, thereby addressing the issue of groundwater carbon seeping into samples. They also analyzed quartz grains taken from clay just above the lowest layer of footprints using a different method, optically stimulated luminescence dating. They published those findings in 2023, which agreed with their earlier estimate.
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