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The peopling of the Americas is one of my favorite subjects in anthropology. Lately, we’ve seen a whole slew of studies that focus on this topic but through a genetic lens, the most impactful of which indicates that people began migrating to the Americas roughly 16,000 years ago.

But there are some inconsistencies with this date, especially the 325 foot-like prints found in Valsequillo Basin’s Toloquilla rock quarry in Mexico. These footprints were within a soft, damp volcanic ash along a lakeshore shortly after a volcanic eruption. This type of volcanic tuff is known as the Xalnene. Samples of the Xalnene tuff were sent off to the Berkeley Geochronology Center for argon-argon and paleomagnetic dating. The results, published in this 2005 Nature study, “Age of Mexican ash with alleged ‘footprints’,” indicated the ash layer to be 1.3 million years old, making many wonder as to whether the indentations were even made by humans.

The team that initially found the prints have revisited the dating of the Xalnene tuff at Toloquilla. They say that the ‘dating of the ash is complicated by the fact that an eruption occurred underwater.’ The team reanalyzed the tuff by optically stimulated luminescence (OSL), a method that requires sampling deep within the tuff and in complete darkness. The samples are then irradiated with an atomic reactor and when ultraviolet light is shone onto the irradiated samples, the resulting fluorescence reveals how long it has been since the rock was last exposed to sunlight—or volcanic heat.

The results of the OSL redating were shared at last week’s meeting of the American Geophysical Union in Florida. Right below the ash, the sediments date to 70,000 and 100,000 years old. The sediments above, date to 9,000 to 40,000 years old. With this time range, it is possible to confer the OSL dates with carbon-14. The team did just that, radio-carbon dating the ages of shells in the sediments above and below the ash layer. The dates of all three layers therefore suggest the footprints were made about 40,000 years ago.

This date indicates humans were in the Americas 25,000 years before the coalescence dates from the most recent genetic studies, and 27,000 years before the Clovis culture. So it begs one to ask if the foot prints are even made by humans? The team scanned the foot prints with a laser scanner and constructed images to compare to footprints made by volunteers trotting on a beach in the United Kingdom.

Ultimately, this study challenges argon-argon and paleomagnetic dating. Paul Renne, of the University of California, Berkeley’s Geochronology Center and Rafael Suárez of the Museo Nacional de Historia Natural y Antropología in Montevideo, Uruguay both are critical… they wonder why there isn’t an archaeological record for 27,000 years.

    Renne, P.R., Feinberg, J.M., Waters, M.R., Arroyo-Cabrales, J., Ochoa-Castillo, P., Perez-Campa, M., Knight, K.B. (2005). Geochronology: Age of Mexican ash with alleged ‘footprints’. Nature, 438(7068), E7-E8. DOI: 10.1038/nature04425
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