Running, not walking, helped shape the modern human body ? and the buttocks played a key role in this evolutionary process. A study by US scientists suggests that seemingly unconnected anatomical features made humans exceptionally strong endurance runners and led to the unique shape of the human body.

The traits that transformed humans into efficient runners include a small ridge at the base of the skull, shoulders decoupled from skulls, long elastic tendons in the legs and feet and well-defined buttocks, the researchers said last week in a report published in Nature.

?These features make humans surprisingly good runners,? said Daniel Lieberman, an anthropologist at the Harvard University. The researchers said that running evolved to allow ancestors of humans to compete with other carnivores for access to protein which was required to build large brains. ?Running is one of the most transforming events in human history,? said David Bramble, a biologist at the University of Utah and coauthor of the study. ?We?re arguing that the emergence of humans is tied to the evolution of running.?

The scientists were trying to figure out which anatomical features support fast running in animals such as dogs, horses and cheetahs. They noted that poor runners, such as pigs, lack the ridge at the base of the skull which keeps their head steady while running.

The researchers also picked up other anatomical features that support running rather than merely bipedal walking: shoulders decoupled from the skulls, the long tendons on legs and gluteus maximus (the muscle of the buttocks). ?The gluteus maximus stablises your trunk as you lean forward in a run,? said Lieberman. ?Shoulders decoupled from skulls also makes running easier and more efficient.?