Footprints may rewrite the history of the Americas

12 July 2005

Scientists from Oxford’s Research Laboratory for Archaeology and the History of Art (RLAHA) have been centrally involved in the dating of a group of human footprints which have shattered theories of how humans first colonised the Americas.

Dating by the Oxford team has shown that the footprints, unearthed in the Valsequillo Basin of Central Mexico by scientists from Liverpool John Moores University, are around 40,000 years old. Traditionally it has been thought that the Americas’ first settlers crossed the Bering Straits, from Russia to Alaska, at the end of the last ice age – around 11,000 years ago. The latest findings, presented earlier this month at the Royal Society’s Summer Exhibition, challenge this theory, providing new evidence that humans settled in the Americas as early as 40,000 years ago.

Scientists from the RLAHA used state-of-the-art Accelerator Mass Spectrometry (AMS) radiocarbon dating and Optically Stimulated Luminescence (OSL) dating techniques to date sediments above and below the footprints. The dating work took over a year to complete.

AMS enables samples of milligram size to be dated. Tiny samples of freshwater shellfish found in sediments at the Valsequillo site were dated in Oxford.

Drs Tom Higham and Christopher Bronk Ramsey, directors of the Radiocarbon Accelerator Unit at the RLAHA, were responsible for the AMS radiocarbon dating. Dr Higham said: ‘The dating of the human evidence from this site is absolutely critical to the interpretation of human prehistory in the Americas. At present, the earliest definitive human evidence from the American continent is Monte Verde, in Chile, which dates to 12,500 years ago. The dates from the Valsequillo footprint site are substantially earlier. They invite us to reconsider the issue of earlier human presence in the Americas, at a time which is similar to the arrival of anatomically modern humans in Europe, southeast Asia and Australia.’

Dr Jean-Luc Schwenninger of the Luminescence Dating Laboratory used luminescence dating techniques, originally developed at Oxford, to date single grains of mineral inclusions found within the volcanic ash upon which the human footprints were found. The team found close agreement between the ages obtained through luminescence dating and those derived using radiocarbon dating. The luminescence work was particularly challenging because of the rarity of suitable minerals in the sediments at Valsequillo.

Further dating research is planned thanks to a recent grant from the Natural Environment Research Council which will enable new fieldwork and collection of more samples from the footprint site.

For more information on the footprints research visit the Mexican Footprints website. [www.mexicanfootprints.co.uk]