The date when the first Homo sapiens arrived in the Americas has been a subject of investigation and speculation for decades. First with a reliance on dated archeological evidence, and later supported by genetic studies, an emerging scientific consensus had developed that supported no earlier an occupation of the Americas than 17,500 years ago (see the review article by Waters, 2019, Science 365(6449), DOI: 10.1126/science.aat5447).

This view has been challenged by new dating of Ruppia seeds and pollen in preserved footprints in the White Sands National Park in New Mexico, USA, which would imply that humans were present in the Americas as early as 21,000 to 23,000 years ago (see Pigati et al., 2023, Science 382(6666), DOI: 10.1126/science.adh5007). Although the dates themselves constitute objective data, their meaning often requires substantial interpretation of the stratigraphy, depositional environment, and uncertainties in the dating methods themselves.

In a recent article published in Quaternary Research, https://doi.org/10.1017/qua.2023.74, David Rachal, Robert Dello-Russo and Matthew Cuba explore the possibility that the dated Ruppia plant materials were deposited in an unstable, dynamic shoreline environment, after having grown in, and been transported from, more distant locations (i.e., Salt Creek) where they had been previously subjected to the confounding effects of hard water, which is known to result in anomalously old radiocarbon dates. This hypothesized depositional scenario complicates the interpretation of the raw dates, because the original source of the dated material and its age relationship to the footprints may not be straightforward. Rachal and colleagues suggest that resolving the stratigraphic and geomorphic context issues remains a necessary, but as-yet incomplete, task. These uncertainties will need to be resolved before the ages of the plant materials, and the sediments that contain them, can be fully embraced as representing the age of the footprints with which they are associated.

Figure description – (A) A photo of a Ruppia meadow in the south reach of Salt Creek in the Tularosa Basin. (B) A photo of Ruppia plant material collected from Salt Creek. (C) A photograph of a transported Ruppia ball embedded into a Ruppia seed layer, the source of the previously dated seeds. 

Article link: https://doi.org/10.1017/qua.2023.74

Literature

1. Pigati, J.S., et al., 2023.  Independent age estimates resolve the controversy of ancient human footprints at White Sands. Science 382. https://doi.org/10.1126/science.adh5007.

• Waters, M., 2019. Late Pleistocene exploration and settlement of the Americas by modern humans. Science 365. https//doi.org./10.1126/science.aat5447.