Around 23,000 years ago, in present-day New Mexico, a group of hunters and gatherers roamed the shores of a lake surrounded by conifers and sagebrush, leaving their footprints in the mud alongside tracks of mammoths, wolves and giant sloths. These bare footprints, described in 2021 in the journal Science, are now preserved in the rock in the white gypsum desert of White Sands National Park. In a new study published on Friday, October 6, in the same journal, Matthew Bennett (University of Bournemouth, United Kingdom) and his colleagues confirm their age, using two new dating techniques.

Since this discovery was announced, its age has been the subject of intense debate. These footprints challenge one of the dogmas of New World archaeology, which posits that humans did not inhabit the Americas before 13,500 years ago, the time when the "Clovis" culture is documented, characterized by distinctive lanceolate stone points. According to this theory, Paleo-Indians arrived in America from Asia via the Bering Strait, a route considered impassable during the last glacial maximum (approximately 26,500 to 19,000 years ago).

The footprints at White Sands contradict this prevailing view. The initial dating, based on the analysis of radiocarbon in seeds of an aquatic plant called spiral rush, found mixed with the footprints, faced criticism. Radiocarbon dating can be influenced by the absorption of inorganic carbon dissolved in the water by these plants. This carbon may have an older origin, thus affecting the measurement. Recognizing this potential source of error, Bennett and his team also collected fossilized pollen samples, which are not susceptible to this "reservoir effect." They extracted approximately 75,000 pollen grains from each of the three samples, yielding radiocarbon dates ranging from 23,400 to 22,600 years, with a margin of error of plus or minus 2,500 years.

For added reliability, they relied on a third dating method called "optically stimulated luminescence." This technique is based on the principle that minerals like quartz, when shielded from sunlight, accumulate charge based on the radiation they receive from their surroundings. This charge is reset when these crystals are exposed to light or heat. The time elapsed since their burial can be estimated by measuring the luminescence signal they emit when stimulated with monochromatic light. The quartz at White Sands provided a date of 21,500 years (plus or minus 2,000 years) using this method.

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