The shoulder may have been one of human's greatest evolutionary successes, allowing us to kill prey by hurling spears or rocks, a study shows.
Focusing on a relatively underappreciated part of the anatomy, scientists said the shoulder was the key part of a throwing mechanism that evolved about 2 million years ago, enabling our puny forebears to capture prey with a projectile weapon.
Researchers at George Washington University in Washington DC used a 3D, high-speed camera to record student baseball players as they threw a ball.
Their model showed that the shoulder acts rather like a slingshot, storing and then releasing energy for the throw.
"When humans throw, we first rotate our arms backwards from the target," said Neil Roach at the university's Center for the Advanced Study of Hominid Palaeobiology.
"It is during this 'arm-cocking' phase that humans stretch the tendons and ligaments crossing the shoulder and store elastic energy.
"When this energy is released, it accelerates the arm forward, generating the fastest motion the human body produces, resulting in a very fast throw".
The team analysed the biomechanics that produce this pent-up energy.
The main features in the shoulder, arm and torso first appeared in Homo erectus, a forerunner of Homo sapiens, two million years ago.
Humans and chimpanzees are believed to have gone their separate ways around six million years ago, some four million years after gorillas branched off from the primate evolutionary tree.
The chimpanzee, our closest primate relative, has only a fraction of a human child's throwing ability even though the ape has superior strength and athleticism in other areas.
Shoulder evolution would be another piece in the puzzle to explain how Homo erectus survived in a hostile environment where he lacked speed, strength and natural weapons such as claws or fangs.
Despite their physical disadvantages, our ancestors were eating meat at least 2.6 million years ago, and were probably hunting large prey 1.9 million years ago, if fossil evidence is correct.
"Although contemporary hunter-gatherers rarely rely on throwing to kill prey, earlier hominins probably needed to throw projectiles frequently to acquire and defend carcasses," said the study in the journal Nature.
Throwing a rock or pointed stick would be more effective and safer for killing prey than trying to capture it at close quarters. And added proteins would boost our energy-hungry brains.
"Eating more calorie-rich meat and fat would have allowed our ancestors to grow larger brains and bodies and expand into new regions of the world, all of which helped make us who we are today," said Roach.
Previous anatomical research into the rise of man has focused, among other things, on our ability to stand and walk upright, which would have boosted the chances of spotting prey or predators.
Another perceived gain came with our ability to form a fist, with the thumb providing rigidity and protection for the delicate hand bones and ligaments.
Individuals who could strike with a clenched fist would have a clear advantage in killing or taking food and in fighting for mates, thus giving themselves a better chance of reproductive success.