Group of British scientists claimed yesterday to have identified human footprints in central Mexico that are 40,000 years old, almost three times older than the most generally accepted evidence for human settlement in the Americas.

　　The team from universities in Liverpool, Bournemouth, and Oxford are convinced that the footprints are human and represent several adults and children who walked in freshly fallen volcanic ash in the Valsequillo Basin, about 80 miles south-east of Mexico City.

　　Working with international colleagues, they have applied dating techniques on the sediment itself and on finds including a land snail, a water snail and a mammoth tooth, all of which came back with an age of around 40,000 years.

　　The footprints had to be disentangled from animal tracks, more than 250 marks in all. Casts, which Matthew Bennett of Bournemouth University described as "unmistakably human" were produced from laser modelling at the site, employing a technique used to make industrial prototypes.

　　Although the findings were announced after two years of work at the British universities and elsewhere, they are controversial, defying the conviction of many scientists that humans arrived in the Americas not more than 15,000 years ago.

　　The tracks were made in gritty ash from the volcano, protected by later sediment layers, and underwater for long periods. The ash layer has since become as hard as concrete, and was locally quarried as a building material, which is how the footprints resurfaced.

　　Scientists partially excavated the quarry in the 1960s, and found ancient animal bones and hints of a very early habitation site, but with the technology of the day could not date them accurately.

　　The British team revisited the site two years ago. Silvia Gonzalez, from Liverpool John Moores University, first spotted the trail of marks, and described her instant conviction that they were human and very ancient as "like a thunderbolt in my mind".