Our ancestors walked upright two million years earlier than thought
20 July 2011, by Tamera Jones
Our ancestors 'almost certainly' walked upright, just like we do, nearly four million years ago – two million years earlier than previously thought, say researchers.
An international team of scientists analysed a track of ancient footprints at Laetoli in Tanzania and concluded that the feet of whoever made them already had most of the features considered essential for an upright gait.
The footprints' maker could push off the ground using their forefoot, instead of using the middle part of the foot, like today's great apes do.
'They show a type of walking that was fully upright and that was driven by the front of the foot, particularly the big toe,' says Professor Robin Crompton from the University of Liverpool, who led the study.
This means the way the likely printmaker walked was closer to our own gait than that of chimpanzees, orang-utans or gorillas.
Researchers think that being able to push off the big toe and being able to walk upright would ultimately have helped our ancestors expand out of Africa and colonise the world.
Until now, most scientists thought these characteristics only emerged in early Homo species around 1.9 million years ago.
'There's no evidence for another hominid existing four million years ago, so it's probably Australopithecus afarensis.'
Professor Robin Crompton, University of Liverpool
The site at Laetoli contains 11 individual footprints, which are 3.66 million years old. Despite their age, the prints are in good condition, and are the earliest known trail made by human ancestors. But trying to figure out the gait of the human ancestor who made them has eluded scientists since they were found in 1974. Previous studies have tended to focus on single footprints , which, says Crompton, are liable to misinterpretation.
'Earlier research on these footprints has looked at single prints, but this approach is prone to error, because of random damage by plant roots, animals and erosion, on top of the variation in foot placement and ground texture from one step to the next,' he says.
To settle the longstanding argument over the gait of their likely maker, Crompton and his colleagues used a new statistical technique, based on methods used in MRI brain imaging, to get a 3D average of 11 of them.
'It was clear to us that we had to look at the central tendency for all 11 prints,' explains Crompton.
They then compared this with data from experimental studies of footprint formation and underfoot pressures generated by modern humans and other great apes while walking. Computer simulation helped them predict the footprints that would have been formed by different types of gait.
Now Crompton and his team say the most likely printmaker was a species called Australopithecus afarensis, which, until now, most scientists thought walked with a crouched posture, and had a foot which worked more like an ape's than a human's.
'There's no evidence for another hominid existing four million years ago, so it's probably Australopithecus afarensis,' Crompton says. 'This human ancestor most probably still used the trees as well as the ground.'
While the study shows that the maker's gait was fully upright, the upper body of Australopithecus afarensis wasn't like ours at all. Instead of having long legs and a short body like modern humans the printmaker would've had the opposite physical build: short legs and a long body.
'This makes it likely that it could only walk or run effectively over short distances,' says Crompton. 'Now we need to figure out when our ancestors could first walk or run the long distances we can today.'
'This is the strongest evidence yet to support the idea that the footprints point to a surprisingly modern foot function,' he adds.
The research is described in Interface, a journal of the Royal Society.
The following was added on 21 July
It turns out that this research has led to a worldwide patent. The image-analysis software the team developed and used in their research could be used in any foot-pressure analysis.
'This means there are applications for airport security, police forensics, and clinical problems of the feet of all kinds,' explains Crompton.
Robin H. Crompton, Todd C. Pataky, Russell Savage, Kristiaan D'Août, Matthew R. Bennett, Michael H. Day, Karl Bates, Sarita Morse and William I. Sellers, Human-like external function of the foot, and fully upright gait, confirmed in the 3.66 Mya-old Laetoli hominin footprints by topographic statistics, experimental footprint-formation and computer simulation, Interface a journal of the Royal Society, published online 20 July 2011, doi:10.1098/rsif.2011.0258
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