SCIENTISTS have unearthed a set of ancient human footprints that could shatter theories on how the Americas were first inhabited.

An international team led by Liverpool John Moores University (JMU) claim footprints found in central Mexico are 40,000 years old.

The discovery poses a serious challenge to considered wisdom on the settlement of the continent, which says humans arrived on foot around 11,000 years ago.

The traditional view was that settlers crossed the Bering Straits, from Russia to Alaska, at the end of the last Ice Age - around 11,500 to 11,000 years ago.

But work by the Liverpool scientists could prove the first colonists in fact arrived by water 30,000 years earlier, according to Dr Silvia Gonzalez, who led the JMU team.

It also re-confirms the importance of Central Mexico as one of the most important areas for the study of early human occupation, although it does not challenge accepted theories that Africa was the birthplace of mankind.

Dr Gonzalez's team analysed 269 footprints, both animal and human, found close to the Cirri Toluquilla volcano in the Valsequillo Basin, near the city of Puebla, 130km southeast of Mexico City.

The footprints were preserved as trace fossils in volcanic Xalnene ash along what was the shoreline of an ancient volcanic lake.

The trails became submerged when the water levels rose and the footprints were preserved until quarry workers excavated the lake sediment to use the ash, which is now as hard as concrete, as a building material.

Dr Gonzalez said: "We think there were several migration waves into the Americas at different times by different human groups.

"Most early human occupation sites in the Americas date to the latest Pleistocene or Ice Age, between 10,000 and 12,500 years ago.

"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."

She added: "Our findings support the theory that these first colonists may perhaps have arrived by water rather than on foot using the Pacific coast migration route."

The findings could hold the key to one of the most contentious and long running debates in geoarchaeology - who first colonised the Americas and how?

The previous theory, known as the Clovis First Model, comes from Clovis Points, huge weapons used to hunt mammoths and mastodons which were found all over the American continent.

Ancient human and animal prints are a rarity due to the special conditions required for their preservation, so the discovery is an important addition to the global archive of human prints.

Dr Gonzalez found the footprints on a visit with Professor David Huddart (LJMU) and Professor Matthew Bennett (Bournemouth University) in 2003.

The geoarchaeologist, who is funded by the Natural Environment Research Council's "Changing Climates, Evolving Humans" programme,, called for further urgent research.

Her team will now carry out more extensive investigations to corroborate their initial findings, thanks to an additional £210,000 from the NERC.

They will also calculate the height, pace and stride of the human population present 40,000 years ago.

It could also give a better understanding of the relationship that these early Americans had with megafauna, such as mammoths, camels and other large animals.

The Mexican Footprints research will be presented at the Royal Society's Summer Science 2005 Exhibition from today until Friday, at 6 to 9 Carlton House Terrace, London.