The kids at his elementary school played a game that involved tossing a tennis ball across the yard.

Whenever it was Chris Capuano's turn, everyone backed up a few steps. That's when he realized he was different.

"I threw a little bit harder, a little bit farther," he says. "I had a stronger arm."

Capuano eventually grew up to be one of the lucky few, a pitcher in the major leagues.

The 34-year-old left-hander belongs to a tiny percentage of the human population that possesses the ability to stand on a mound and throw with enough velocity and movement to earn a living at baseball.

This rarefied skill involves fitness and uncanny timing, a gift for sequencing the body's muscle groups in perfect coordination. According to a recent scientific paper, it might also reflect millions of years of evolution.

"If it were not for our ability to throw," says Neil Roach, a postdoctoral scientist at George Washington University, "we would not be who we are today."

Numerous factors contribute to the making of a top-notch pitcher. Genetics might play a role. Same with initial aptitude — when kids experience early success at something, they tend to spend more time and effort honing that skill.

Capuano's father was a minor league shortstop, and they played a lot of catch in the yard.

"I guess it's a great feeling when you have a good arm as a kid," Capuano said. "It's fun to strike people out."

Throwing was far more serious business for our early ancestors, according to a recent study in the journal Nature that was co-written by Roach and colleagues at Harvard.

Adult male chimpanzees served as the starting point for their evolutionary hypothesis. Though quite strong, the apes can throw objects only about 20 mph, or one-third as fast as a 12-year-old Little Leaguer might.

The researchers believe that, in the transition from ape to man, several anatomical changes endowed ancestors such as Homo erectus with significantly greater throwing abilities.

Whereas chimpanzees have shrugged shoulders, which allow them to hang from trees, humans have a lower, wider scapular anatomy. Just as important, humans can rotate their upper bodies independent of their legs and can twist their upper arms to create torque.

These developments might have allowed early man to hunt for big game and ward off scavengers. A calorie-rich diet of meat and fat would have facilitated the growth of larger brains, researchers say.

Though the research suggests that Homo erectus, whose origin has been traced to 1.8 million to 1.9 million years ago, could throw with accuracy, gaps in the fossil record make it difficult to say what might have been used for projectiles. Evidence of stone-tipped spears did not appear until about 500,000 years ago.

It is also unclear when throwing became a recreational activity among Homo sapiens, who came along about 200,000 years ago.

Baseball, which did not enter the picture until the 19th century, has seen its own mini-evolution in the art of throwing, with continually developing techniques.

Most modern pitchers fall into one of two categories. The "drop and drive" type stays low, pushing hard off the pitching rubber with his back leg. The "stand tall and fall" variety stays more upright, letting gravity pull him down the front slope of the mound.

As Capuano grew up in Massachusetts during the 1990s, attending camps and playing for his high school team, he began to focus on his form.