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Scientists have unearthed human footprints in central Mexico which they claim are around 40,000 years old, shattering previous theories on how humans first colonised the Americas.

The researchers hope that their preliminary findings will eventually help shed light on one of the most contentious debates in American history: who was there first and how did they get there?

An international team of geoarchaeologists, led by Dr Silvia Gonzalez from Liverpool John Moores University (LJMU), finally completed dating the footprints, found in 2003, earlier this year.

The footprints were found in an abandoned quarry by Dr Gonzalez, Professor David Huddart (LJMU) and Professor Matthew Bennett (Bournemouth University) in September 2003 and have been subsequently studied by a multinational team of scientists.

The first stage of their research focused on analysing 269 footprints, both animal and human, found close to the Cerro Toluquilla volcano in the Valsequillo Basin, near the city of Puebla, 130 km southeast of Mexico City. Further sites in the area with footprints have been identified in the Valsequillo Basin. Now thanks to the award of £212,000 from the Natural Environment Research Council, Dr Gonzalez and her research team will be able to carry out more extensive investigations to corroborate their initial findings and also calculate the height, pace and stride of the human population present 40,000 years ago. Such research would also give a better understanding of the relationship that these early Americans had with megafauna, such as mammoths, camels and other large animals.

Dr Gonzalez explained: "The footprints were preserved as trace fossils in volcanic ash along what was the shoreline of an ancient volcanic lake. Climate variations and the eruption of the Cerro Toluquilla volcano caused lake levels to rise and fall, exposing the Xalnene volcanic ash layer."

The geoarchaeologist suggests that the early Americans walked across this new shoreline, leaving behind footprints that soon became covered in more ash and lake sediments. The trails became submerged when the water levels rose again, so preserving the footprints.

Now as hard as concrete, the Xalnene ash is used locally as a building material. Dr Gonzalez and her research team were able to see the footprints without carrying out any excavation as quarry workers had already removed between 2-3metres of lake sediments that had been deposited on top of the volcanic ash.

The team's findings pose serious challenges to considered wisdom on the settlement of the continent.

Dr Gonzalez, working as part of the Natural Environment Research Council's 'Environmental Factors in the Chronology of Human Evolution and Dispersal (EFCHED) research programme, said: "We think there were several migration waves into the Americas at different times by different human groups."

This debate has been running for more than a century. The traditional view - known as the Clovis First Model - is 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. Evidence for this theory comes from Clovis Points - tools used to hunt mammoths and mastodonts - found from many locations in the American continent.

The discovery of human footprints in the Valsequillo Basin of Central Mexico challenges this model, providing new evidence that humans settled in the Americas as early as 40,000 years ago.

Professor David Huddart (LJMU) explained: "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."

Dr.Gonzalez continued: "New routes of migration that explain the existence of these much earlier sites now need urgent consideration. 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 Mexican Footprint research is vitally important for the study of the settlement of the Americas as it provides extensively validated data that directly challenges current theories on the peopling of the Americas. It also re-confirms the importance of Central Mexico as one of the most important areas for the study of early human occupation.

The footprints were mapped and scanned using laser technology and have been reproduced at the University of Bournemouth using rapid proto-typing technology, allowing excellent visualisation. This technology has been used to produce physical models of the footprints with sub-millimetre precision.

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

The prints have been preserved through the application of technology normally associated with the design of new consumer products. Professor Bennett says 'that footprint preservation has been possible through the fusion of modern design technology with archaeology, allowing 40,000 year old footprints to be brought to life in London.'

The discovery is being launched at the Royal Society Summer Exhibition which open on 4 July 2005 (4-7 July, 6-9 Carlton House Terrace, London SW1Y 5AG), where the footprints will be available for the press to examine both at the Exhibition and the press launch.

Source: Liverpool John Moores University