Human settlers made it to the Americas 30,000 years earlier than previously thought, according to new evidence.
A team of scientists came to this controversial conclusion by dating human footprints preserved by volcanic ash in an abandoned quarry in Mexico.
They say the first Americans may have arrived by sea, rather than by foot.
The traditional view is that the continent's early settlers arrived around 11,000 years ago, by crossing a land bridge between Siberia and Alaska.
Details of the latest findings were unveiled at the UK Royal Society's Summer Science Exhibition.
Dr Silvia Gonzalez of Liverpool's John Moores University and her colleagues found the footprints in the quarry, some 130km (80 miles) south-east of Mexico City, in 2003. But they have only finished dating them this year.
Dr Gonzalez was under no illusions that the finding would be controversial: "It's going to be an archaeological bomb and we're up for a fight," she said.
The team used several methods to date a variety of material from the site near Puebla, Mexico, in order to be sure they were right about the age.
The researchers used radiocarbon dating on shells and animal bones in the sequences and dated mammoth teeth by a technique called electron spin resonance. The sediments themselves were dated by optically stimulated luminescence.
"Some lake sediments were incorporated into the ash and were baked. They look like small fragments of brick and these were the ones we dated in the footprint layer. They gave us a result of 38,000 years," Dr Gonzalez.
mammoth: 一种猛犸，属体形大、有毛且已绝种了的象，尤指体毛发达的猛犸 （猛犸象）曾广泛分布于北半球
electron spin resonance: 电子自旋共振法
optically stimulated luminescence: 光释光方法