By Paul Rincon
BBC News science reporter
Human settlers made it to the Americas 30,000 years earlier than previously thought, according to new evidence.
People left traces of their presence in the sediments of a shoreline
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.
The footprints were preserved as trace fossils in volcanic ash along what was the shoreline of an ancient volcanic lake. They were soon covered in more ash and lake sediments and, when water levels rose, became as solid as concrete.
Dr Gonzalez was under no illusions that the finding would be controversial: "It's going to be an archaeological bomb," she told the BBC News website, "and we're up for a fight."
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.
"We have materials that have been dated below the footprint layer, the footprint layer itself and on top of the footprint layer. Everything is making sense," said Dr Gonzalez.
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.
Under the traditional view, humans trekked from Siberia to Alaska across a land bridge that linked these land masses at the end of the last ice age (between about 10,000 and 12,500 years ago).
Central to the theory, called the Clovis First model, are Clovis points - the tools these settlers used to hunt large beasts, or megafauna, such as mammoths and mastodons.
"The existence of 40,000-year-old human footprints in Mexico means that the Clovis First model of human occupation can no longer be accepted as the first evidence of human presence in the Americas," said co-investigator David Huddart, of Liverpool John Moores.
Dr Michael Faught, an expert in early American archaeology, was reserving judgment until evidence was published: "It would be significant if it were demonstrated, but usually those (early) sites don't hold up well," he told the BBC News website.
But, he added: "There's more and more evidence that Alaska was not the only place people came into the continent."
Dr Gonzalez is a proponent of the Coastal Migration Theory. This proposes that people arrived on the west coast in boats, hugging the coastline from North to South.
But where these settlers came from is still a mystery, she says. Some have proposed that the earliest humans to reach the continent could have come from south-east Asia or even Australia.
Genetic studies of present-day Native American populations support a recent arrival from north-east Asia, which agrees well with an entry through the Beringian land bridge at the end of the last Ice Age.
Dr Gonzalez suggests that the earliest settlers may have become extinct, leaving no genetic legacy today. She thinks these hunters may have been highly mobile, living in small groups, perhaps explaining why they left scant trace of their presence.
Dr Gonzalez and ancient DNA expert Alan Cooper, of the University of Adelaide in Australia, have managed to extract genetic material from three molars belonging to Peņon Woman III, a 13,000-year-old partial skeleton from Mexico. The analysis is still underway.