23 July 2010, by Adele Rackley
Identifying human footprints correctly is vital for understanding when man first occupied different parts of the world.
Now scientists have developed a new model that can statistically show whether a print was actually made by a human.
The work has already called into question some early dates proposed for the human colonisation of the Americas.
This previous dating evidence came from a series of impressions found in a layer of volcanic ash in an active quarry in Valsequillo, Central Mexico. The shape of the marks, and their roughly linear 'track', led researchers to identify them as human footprints in 2006.
The ash layer has been sampled by several teams, who have dated it to between 38,000 and 1.3 million years old – a big range, but in any case significantly earlier than evidence from other sites, which clusters around 15,000-20,000 years old.
A human footprint will reflect the anatomy of the human foot, but visual identification can be more subjective than scientific. Anything from the type of ground surface to the quality of light can affect their appearance. The evidence from Valsequillo looked convincing, but it couldn't be tested scientifically.
But now a team of researchers from the UK and US has developed a statistical model that can do just that.
'[Footprints] should only be accepted in the geological record when the evidence is unequivocal.'
Sarita Amy Morse, Rutgers, State University of New Jersey
Existing models for studying footprints are designed to look at things like gait and biomechanics, but they start by assuming that the print is human – they don't actually identify it.
The researchers took the novel approach of analysing the 'geometry' of the print, a process which makes no assumptions about origin and is not based on anatomy. Instead it looks at the pattern of indentations and determines whether they were likely to have been made by the pressure and motion of a human foot.
The model was developed specifically to test the veracity of the Mexican prints, and their results are published in Quaternary Science Review. 'I was fortunate to be able to work with the original research team and use their data,' says lead author Sarita Morse, from Rutgers, The State University of New Jersey.
The technique uses laser scanning to produce a 3-D image of the print, then researchers plot five 'landmarks' that together define the print's maximum length, mid-length width and depth.
First the team analysed six 'control' examples from around the world. These included modern humans and examples from Laetoli, Tanzania, around 3.75 million years old.
'We started with populations we know are human and hominid,' explains Morse, 'but even then we were interested to see what a tight range of results they produced.'
When compared to the controls, the Valsequillo prints had distinct differences. They were much broader across the mid-foot region than a human print would be.
And the deepest point tended to be in the area that would have been the arch of the foot – a profile that's not at all consistent with the motion of a human.
'We thought the depth data for the Valsequillo prints might be different, says Morse, 'but we were surprised to see how much wider they came out – even wider than the oldest control example.'
But if they're not human, what did make the prints?
It looks likely that the marks were made by picks used in recent mining activity. 'The picks produce a spalling effect which removes material more widely around the point of impact,' says Morse. 'Actual pick marks are visible in the quarry but this spalling seems to have caused the print-like depressions with their 'mid-foot' deep points.'
'Footprints provide important information on the presence of humans, their anatomy and behaviour,' Morse adds, 'so they should only be accepted in the geological record when the evidence is unequivocal.'