By Liz Seward
Science reporter, York
Achilles tendons play a critical role in human running ability, a major conference in York has been told.
A new computer model confirms that skeletons need to store energy in their tendons to be able to run efficiently.
Speaking at the BA Festival of Science, Dr Bill Sellers said further fossil data was needed to shed light on when our ancestors started running.
Running was an important development in human evolution, giving man the ability to chase prey, Dr Sellers explained.
Running is a far more difficult activity than walking.
"It changes your options. If you're an efficient long-distance runner then you might be able to run down your prey," the University of Manchester primatologist explained.
"You're not going to walk down your prey."
Dr Sellers created models of a human skeleton with and without tendons, and looked at their running speeds and energy requirements.
"What we've found is the key thing for simulating running is to have a springy model," he said.
"And what this means is that muscles are attached to bones by tendons at the end and these tendons are big springs that store energy.
"If we make a model without tendons, it turns out that it's rubbish.
"It can't go very fast and it uses an awful lot of food to get from A to B. So you really can't run if you don't have tendons. If you do have tendons, you can run a lot faster and you do it for less fuel."
Scientists do not know when humans developed the ability to run.
The Achilles heel
Studies and models of the bone structure of "Lucy", a 3.2-million-year-old skeleton of a human ancestor from Ethiopia, have shown that she walked upright, but there is no evidence she was a sprinter.
"You look at the Lucy skeleton... and it doesn't look at all like a human. It really is very different; it's got much shorter legs and it's got longer arms and I think there is every chance that Lucy couldn't run," said Dr Sellers.
But he said he suspected Homo erectus, who lived two million years-ago, could sprint. So the Achilles tendon must have evolved at some evolutionary stage between these two species.
To pinpoint when it evolved, the team now needs to know which fossils show evidence of an Achilles tendon.
But this, said Dr Sellers, was difficult: "The problem with hominid fossils is that the vast majority have no feet. And what you really want, to find evidence of an Achilles tendon, is a nice intact ankle bone."
The University of Manchester researcher said he hoped that future findings would discover evidence of Achilles tendons, allowing scientists to solve the mystery of when humans learnt to run.