Two legs good but Achilles' tendon better

Evolution:While our early ancestors stood upright on two legs millions of years ago, humans really only got a spring in their step when the Achilles' tendon evolved.

Dr Bill Sellers of the University of Manchester told the Science Festival that most large primates such as the gorilla and chimp do not have an Achilles' tendon as their lifestyles only require them to be flexible when moving through trees.

However, the situation is entirely different in humans, with the tendon acting as a spring that allows us to run at much greater speeds.

"You need the springs. No springs and you don't run," Dr Sellers said. He used advanced computer technology to model the gait of animals, including humans, and to reconstruct the walking style of ancient fossils based on bone shape, size and where the ends of muscles or tendons were inserted.

"This model is a virtual robot where we can activate muscles and get it to move its legs in a physically realistic fashion. The tricky bit is getting it to actually walk or run without falling over."

The system can learn from its mistakes until the bones and muscles work in unison, allowing the researchers to "watch" a fossil human walk across a computer screen. It showed humans without tendons had only half the running speed of those with the Achilles' tendon.

"Walking like a [ modern] human is much more efficient - it uses a lot less energy when getting from point to point."

He used the system to anticipate the walking style of the famous Lucy fossil skeleton, a 3.2 million-year-old ancestor recovered in Ethiopia.