A critical stage in human evolution that turned our early ancestors into fast and fearsome hunters has been identified by British scientists.

The dramatic shift came with the birth of the spring-like Achilles tendon at the back of the heel, which allowed early humans to break into a powerful run for the first time in history.

The shift saw humans move away from a slow-paced life of scavenging, as their newly-found speed and agility gave them the ability to chase and capture prey.

Evidence from the fossil record suggests humans first developed Achilles tendons more than 2m years ago, before the rise of Homo erectus, the first human ancestor to become a true hunter-gatherer.

Scientists led by Bill Sellers, a biomechanics specialist at Manchester University, made the discovery after modelling the gaits of early humans on computers. The researchers took information on bone and muscle structure from fossil records and used it to reconstruct the likely walking style of Lucy, the famous 3.2m-year-old human ancestor called Australopithecus afarensis, whose remains were discovered in Ethiopia in the 1970s.

The models showed that while Lucy was able to walk upright, she was slower than modern humans because her limbs were shorter.

But the researchers also found that Lucy was unable to run until they adjusted the model to include an Achilles tendon.

"If Lucy had no Achille's tendon, she'd be far too ineffcient to be any sort of pursuit runner, though she could still scavenge already dead prey," Dr Sellers told the British Association festival of science in York yesterday.

"Running is very odd without the Achilles tendon. You don't have the power and you need a lot of power to get off the ground. The tendon lets you store a lot of power so you can spring from step to step," he said.

The researchers found that an Achilles tendon would have allowed early humans to move nearly twice as fast as before. Energy stored in the tendon acts like a spring, and improved the efficiency of their gait by more than 100%.

While most fast land animals have evolved strong heel tendons, the Achilles tendon has a more complex history along the human evolutionary path.

Gibbons, which split with human ancestors 20m years ago have Achilles tendons, but modern chimps and gorillas lack them, probably because they are of little help for animals that walk on all fours or spend time in the trees.

Dr Sellers said fossil experts should now be able to pinpoint the moment humans gained the ability to run by re-examining the remains of our ancient ancestors.

"How we evolved from our common ancestor with chimpanzees six million years ago is a fundamental question. Walking upright seems to be the very first thing that distinguishes our ancestors from other apes, so finding out about this should help us map the evolutionary pathway to modern humans.