How Old Is a Footprint?
The oldest widely reported human footprints in the Americas lie in a dried lake bed at White Sands, New Mexico. Nobody disputes that people made them; the argument is only how long ago. One team dates the tracks to the last Ice Age, thousands of years before humans were thought to have reached the continent [1]. Critics reply that the dated plant may have absorbed ancient carbon, inflating the age [2]. For an outsider the stake is the entire timeline of the first Americans.
The field in brief
Quaternary geochronology is the science of dating the last 2.6 million years, the span that holds the ice ages and the arrival of modern humans. Its workhorse is radiocarbon dating, which measures the decay of carbon-14 in once-living material to estimate age. The method assumes the sample drew its carbon from the atmosphere. When an organism instead takes up carbon dissolved from old rock or groundwater, it inherits a built-in age called the radiocarbon reservoir effect, or hard-water effect. A second method, optically stimulated luminescence, or OSL, dates when mineral grains such as quartz were last exposed to sunlight, and it needs no carbon at all. The tracks fall within the Last Glacial Maximum, the coldest peak of the last Ice Age near 20,000 years ago. That timing matters because the long-standing Clovis-first model held that humans reached the Americas only about 13,000 years ago.
The fight
In 2021 a team led by Matthew Bennett radiocarbon-dated seeds of the aquatic plant Ruppia cirrhosa found above and below the tracks, and reported an age of roughly 23,000 to 21,000 years [1]. The next year Charles Oviatt, David Madsen, David Rhode and Loren Davis published a formal challenge [2]. They warned that the seeds grew underwater and could carry a reservoir offset [2]. As a test they radiocarbon-dated Ruppia collected alive in 1947, which returned an apparent age of about 7,400 years [2]. Rhode put the problem plainly, saying the plant takes in carbon “from sources other than the contemporary atmosphere – sources which are usually pretty old” [6].
The original team answered in 2023 with materials the objection cannot reach [3]. Radiocarbon on terrestrial pollen and OSL on quartz grains converged on the same Ice Age window [3]. In 2025 Vance Holliday’s group added radiocarbon on organic lake mud, independent of the seeds, at 20,700 to 22,400 years [4]. Holliday said the accumulated evidence is “really hard to explain all this away” [5]. Michael Waters still declined to sign, saying he remains “concerned about the radiocarbon ages” [5].
What the fight reveals
The weight of evidence now favors the Ice Age age. Three independent lines point to the same window the seeds first indicated: terrestrial pollen radiocarbon, luminescence, and lake-mud radiocarbon [3][4]. That convergence is hard to reconcile with a large hidden offset [4].
One point remains genuinely unresolved. The skeptics never claimed the seed dates were certainly wrong, only that the carbon’s origin was unverified, and that narrow doubt still stands for Waters as the surrounding evidence grows [2][5]. Settling it will require a date both camps accept as immune to the reservoir question [2].
- Bennett, M.R., et al. (2021). Evidence of humans in North America during the Last Glacial Maximum. Science 373(6562): 1528–1531. The original claim: radiocarbon on Ruppia cirrhosa seeds dates the tracks to ~23,000–21,000 years, placing humans in the Americas during the Last Glacial Maximum.
- Oviatt, C.G., Madsen, D.B., Rhode, D., and Davis, L.G. (2022). A critical assessment of claims that human footprints in the Lake Otero basin, New Mexico date to the Last Glacial Maximum. Quaternary Research. The reservoir-effect challenge: living Ruppia collected in 1947 returned a false radiocarbon age of ~7,400 years, implying the footprint dates may be inflated.
- Pigati, J.S., Springer, K.B., et al. (2023). Independent age estimates resolve the controversy of ancient human footprints at White Sands. Science 382(6666): 73–75. Answer to the critics: radiocarbon on terrestrial pollen and OSL on quartz independently confirm the ~23,000–21,000-year age.
- Holliday, V.T., et al. (2025). Paleolake geochronology supports Last Glacial Maximum (LGM) age for human tracks at White Sands, New Mexico. Science Advances 11(25): eadv4951. A third independent line: radiocarbon on organic lake mud yields 20,700–22,400 years, matching the seed dates.
- Live Science (2025). Hotly debated human footprints in New Mexico are more than 21,000 years old, closely matching prior estimates. Source of the 2025 principal quotes: Holliday (“really hard to explain all this away”) and Waters (still “concerned about the radiocarbon ages”).
- Popular Science (2024). Scientists still are figuring out how to age the ancient footprints in White Sands National Park. Source of David Rhode’s plain-language statement of the reservoir problem in aquatic plants.