Walking with ancestors: discovery rewrites American prehistory

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Humans arrived in America 25,000 years earlier than previously thought - at least 40,000 years ago - footprints found near Mexico City have proved.

British and Mexican archaeologists said the discovery of the prints, made in volcanic ash near the town of Puebla, 80 miles south-east of Mexico City, will force a total rewrite of humanity's early migrations and is one of the most important archaeological finds of recent decades.

The layer of volcanic ash in which the 269 footprints are preserved has been dated by two different techniques - radiocarbon and optically stimulated luminescence dating - to between 38,000 and 39,000 years ago. Until now the earliest definite dates for a human presence in the Americas were 15,000 years ago. Given the location of the find, deep in the Americas, it makes it almost certain that humans must have first entered the Americas at least 40,000 years ago.

When combined with existing knowledge on prehistoric climate, the discovery suggests humans may have entered the Americas during a slightly less cold phase in the last Ice Age about 50,000 years ago. They would have walked over the ice-bound Bering Strait or island-hopped to Alaska via the Kuril and Aleutian chains of islands.

This means the migration into the Americas occurred at about the same time as the normally accepted date of the early Aboriginal colonisation of Australia and some archaeologists now believe that the first Americans were Australoid peoples closely related to the early Aborigines.

Given that the new evidence, it is also now conceivable that humans entered the Americas even earlier, perhaps during a much warmer spell about 70,000 years ago. It means archaeologists will now have to take more seriously two claims that a site in Brazil and another in Chile date from 50,000 and 33,000 years ago respectively.

The Mexican footprints were made by four to six individuals - probably two adults and between two and four children - in at least three episodes, several weeks or even months apart. Each time they were walking barefoot along the shore of a large lake, now Lake Valsequillo. The imprints were sealed and preserved by ash from successive eruptions of a nearby volcano, Mount Tolukuilla.

As well as human footprints, the archaeologists also found deer, camel, wolf (or dog) and puma prints.

The discovery, announced in London yesterday, was made by Sylvia Gonzalez and Professor Dave Huddart of Liverpool John Moores University and Professor Matthew Bennett of Bourne-mouth University. Dr Gonzalez said: "It shows our ancestors adapted to new environments much quicker and more easily than we had imagined."