Early humans might have been walking upright 3.5 million years ago but even the most athletic individuals would have struggled to keep up with a modern couch potato in a running race, new research has suggested.

Ancient human relatives such as the famous fossil Lucy, a member of the species Australopithecus afarensis, would have found running almost impossible because they probably lacked Achilles tendons, scientists said yesterday.

With the exception of gibbons, all humanity’s closest animal relatives – the chimpanzees, gorillas and orang-utans – have no Achilles tendons and it is highly likely that early hominins such as Lucy did not have them either, Bill Sellers of the University of Manchester said.

A new computer gait simulation of both Homo sapiens and A. afarensis has revealed that Achilles tendons are critical to fast running speeds. If Lucy lacked such tendons, she could still have walked almost as fast as modern humans, but her top running speed would have been half as quick. She would also have expended twice as much energy, causing her to tire quickly.

The findings, which were presented yesterday to the British Association Festival of Science in York, suggest that the evolution of Achilles tendons was critical to our species’s development, allowing us to become hunters.

“It opens all sorts of options,” Dr Sellers said. “If you’re a runner, all of a sudden the idea of hunting becomes possible. Without Achilles tendons, you are rubbish at running. You can’t go very fast, and you use an awful lot of food to get from A to B. Humans and, strangely, gibbons, have great big fat Achilles tendons.

“Pursuit hunting pretty much relies on running. If Lucy had no Achilles tendon, she’d be far too inefficient to have been any kind of pursuer. It wouldn’t have stopped her from scavenging, but she would not have been much of a hunter.”

Anatomical evidence for the evolution of Achilles tendons is sparse because very few hominin fossils have intact feet. The shape of other bones, however, suggest that the features did not evolve until perhaps 2m years ago, probably in Homo erectus, a species thought to be a direct ancestor of modern humans.

“I would be very surprised if Homo erectus couldn’t run,” Dr Sellers said. “If you look at the skeleton, it looks virtually like a modern human from the neck down, so I would be very surprised if there was no Achilles tendon. Lucy, however, lived 3.5m years ago, and doesn’t look at all like humans. I think there’s every chance that Lucy couldn’t run.

“The problem with hominin fossils is that the vast majority have no feet. What we really need is a nice intact ankle bone, but there is no great evidence at the moment.”

The benefit of an Achilles tendon is that it acts like a giant spring, storing energy and producing an efficient running gait. “Efficient running would have been essential to allow our ancestors to move from a largely herbivorous diet to the much more familiar hunting activities associated with later humans,” Dr Sellers said. “What we need to discover now is when in our evolution did we develop an Achilles tendon as knowing this will help unravel the mystery of our origins.

“We have only just started to look at running and so there are still plenty of questions to answer,” said Dr Sellers. “But whilst these very early fossils could walk well, our initial findings suggest that efficient running came about quite a bit later in the fossil record.

While most large mammals have Achilles tendons, the same is not true of great apes, and it is likely that it disappeared as the common ancestors of humans, chimps and orang-utans took to the trees. The large tendon restricts movement at the ankle, making it harder to grasp with the foot, and would have been a handicap.

“If you have an arboreal lifestyle, not having an Achilles tendon could be a good thing,” Dr Sellers said.

Dr Sellers’s computer gait models have previously been used to show that Lucy almost certainly walked upright, and at a speed only a little slower than that of modern humans. He recently published similar research revealing that a small dinosaur named Compsognathus may have been the fastest two-legged creature ever to have lived, with a top speed of almost 40 mph.

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