When the 2013 Little League World Series starts up next month, thousands of baseballs will be flung around the diamond as the best young players from across the globe faceoff.

But have you ever wondered how humans learned to throw in the firstplace. Our closest primate relative — the chimpanzee — is a terrible thrower whose fastest toss barely reaches a wimpy 20 miles per hour. In contrast, humans are powerful and accurate throwers, with some major league pitchers tossing baseballs over 100 mph.

"Chimpanzees are incredibly strong and athletic, yet adult male chimps can only throw about 20 miles per hour-one-third the speed of a 12-year-old little league pitcher," said Dr. Neil Roach, whose study on evolution on human throwing is the cover article for this week's Nature. Roach is currently a paleobiologist at George Washington University, but conducts research at Harvard while obtaining his Ph.D.

Roach and his colleagues invited baseball players from his school's collegiate team into the lab to study their throwing mechanics. Each subject was covered in motion sensors, the same ones used by Hollywood to create computer-enhanced movies.

The sensors allowed Roach's team to make the conclusion that human shoulders are built like slingshots. The tendons and muscles in the shoulders evolved to be tremendously elastic.

"When humans throw, we first rotate our arms backwards away from the target. It is during this 'arm-cocking' phase that humans stretch the tendons and ligaments crossing their shoulder and store elastic energy," Roach said. "When this energy is released, it accelerates the arm forward, generating the fastest motion the human body produces, resulting in a very fast throw."

Our throwing ability, relative to chimps, is also enhanced by the position of our shoulders. While chimps have high, shrugged shoulders, ours are lower and down to the side. This allows us to use our arms like whips, which yields speed in our throwing motion.

This is combined with taller and more mobile waists that, according to the authors, "decouple the hips from the thorax permitting more torso rotation". This allows for higher torque in our throwing motions, which equals faster pitches for baseball's elite.

Roach believes that our extinct ancestors Homo erectus evolved many of these traits about two million years ago.

"Taking into consideration archaeological evidence suggesting that hunting activity intensified around this time," wrote the authors, "we conclude that selection for throwing as a means to hunt probably had an important role in the evolution of the genus Homo."

"We think that throwing was probably most important early on in terms of hunting behavior, enabling our ancestors to effectively and safely kill big game," Roach said. "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."

However, baseball players may be pushing the limits of our human capacity for throwing, and the authors argue "repeated overuse of this motion can result in serious injuries in modern throwers."

(Courtesy of Nature Video)

(Courtesy of The George Washington University on Vimeo.)

Source: Roach NT, Venkadesan M, Rainbow MJ, Lieberman DE. Elastic energy storage in the shoulder and the evolution of high-speed throwing in Homo. Nature. 2013.