Long-distance running may have been a driving force behind evolution of the modern human body, scientists say.
People run to keep fit today but our ancestors ran for different reasons
American researchers said humans began endurance running about 2 million years ago to help hunt for prey, influencing the development of the human body.
Previous studies have suggested running was purely a by-product of walking.
But the study, published in Nature, said humans evolved big buttocks, a balanced head and longer legs to help gather food.
Professor Dennis Bramble, of the University of Utah, and Professor Daniel Lieberman, of Harvard University, reported that early human beings may have needed to run long distances to help hunt prey or scavenge animal carcasses on the African savannah.
Without the development from running, humans would be much more like apes with shorter legs, smaller heads and a hunched posture, the scientists said.
While humans are poor sprinters in comparison with many animals, they perform well when it comes to long-distance running.
After examining 26 human body features essential for endurance running, the pair concluded humans may have evolved as they did from their ape-like ancestors because they could run long-distances.
Important attributes for endurance running include skull structure to prevent over-heating, ligaments to give spring, long legs to increase stride length and independent head and shoulder movement to aid balance.
The scientists said because of natural selection, our ape-like ancestors known as Australophithecus, who were good at running, survived, while shorter-legged ancestors died out.
Professor Bramble said: "Today endurance running is primarily a form of exercise and recreation but its roots may be as ancient as the origin of the human genus and its demands a major contributing factor to the human body form.
"Running may have helped hunters get close enough to throw projectiles or perhaps even to run some mammals to exhaustion in the heat."
Professor Chris Stringer, head of human origins at the Natural History Museum in London, said the findings were "plausible" and provided a "valuable fresh look at our anatomy and some of its special features".
"Although it will require much more complete evidence for the evolution of the skeleton of early humans below the neck to test their ideas properly."