Humans were born to run and evolved from ape-like creatures into the way they look today probably because of the need to cover long distances and compete for food, scientists said on Wednesday.
From tendons and ligaments in the legs and feet that act like springs and skull features that help prevent overheating, to well-defined buttocks that stabilize the body, the human anatomy is shaped for running.
"We do it because we are good at it. We enjoy it and we have all kinds of specializations that permit us to run well," said Daniel Lieberman, a professor of anthropology at Harvard University in Massachusetts.
"There are all kinds of features that we see in the human body that are critical for running," he said.
Lieberman and Dennis Bramble, a biology professor at the University of Utah, studied more than two dozen traits that increase humans' ability to run. Their research is reported in the science journal Nature.
They suspect modern humans evolved from their ape-like ancestors about 2 million years ago so they could hunt and scavenge for food over large distances.
But the development of physical features that enabled humans to run entailed a trade off -- the loss of traits that were useful for being a tree-climber.
"We are very confident that strong selection for running -- which came at the expense of the historical ability to live in trees -- was instrumental in the origin of the modern human body form," Bramble said in a statement.
Against the grain
The conventional theory is that running was a by-product of bipedalism, or the ability to walk upright on two legs, that evolved in ape-like human ancestors called Australopithecus at least 4.5 million years ago.
But Lieberman and Bramble argue that it took a few million more years for the running physique to evolve, so the ability to walk cannot explain the transition.
"There were 2.5 million to 3 million years of bipedal walking without ever looking like a human, so is walking going to be what suddenly transforms the hominid body?" said Bramble.
"We're saying 'no, walking won't do that, but running will.'"
If natural selection did not favour running, the scientists believe humans would still look a lot like apes.
"Running has substantially shaped human evolution. Running made us human -- at least in the anatomical sense," Bramble added.
Among the features that set humans apart from apes to make them good runners are longer legs to take longer strides, shorter forearms to enable the upper body to counterbalance the lower half during running and larger disks which allow for better shock absorption.
Big buttocks are also important.
"Have you ever looked at an ape? They have no buns," said Bramble.
Humans lean forward when they run and the buttocks "keep you from pitching over on your nose each time a foot hits the ground," he added.