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Baseball, of course, is called America's pastime, and while the ability to pitch a ball is not unique to our country, it turns out that it is unique to our species. A new study in the journal Nature suggests that the ability to throw with speed and precision depends on key features of human anatomy, features that may have given our ancestors the evolutionary edge they needed to spread across the world. NPR's Rhitu Chatterjee takes a closer look into the findings of the study by first making a trip to a local baseball diamond.

RHITU CHATTERJEE, BYLINE: Pitching is hard work, and it's something that kids at the Homerun Baseball Camp in Washington, D.C. are learning.

JOHN MCCARTHY: Right here. Harry, on the line. Here, let's all do it together. Ready? Follow Coach Matt.

CHATTERJEE: As Coach John McCarthy prepares to pitch, he curls up his left leg and reaches back with his right hand, pulling his shoulder as far back as he can.

MCCARTHY: Look at my hand. Look at my hand.

CHATTERJEE: He swings his arm forward and sends the ball flying. Pitching involves the entire body, but anthropologist Neil Roach of George Washington University wanted to know which parts are most important. So he had 20 athletes throw a ball while he filmed them a 3-D video camera.

NEIL ROACH: And that allowed us to look at how the motion was generated in three dimensions.

CHATTERJEE: He used mathematical modeling to break down that motion into movements of individual body parts. As it turned out, the shoulder played the most important role. Dan Lieberman is an anthropologist at Harvard University and an author on the new study.

DAN LIEBERMAN: As you cock your arm back, you store up huge amounts of elastic energy in the muscles and the other tissues that cross the front of your shoulder.

CHATTERJEE: In other words, our shoulders work like slingshots. When we pull the arm back and behind us, we stretch the ligaments and tendons. When we move the arm forward, the tissues spring back, powering our throws with their elastic energy. The reason our shoulders can do this is because our shoulder blades are placed flat on our backs.

LIEBERMAN: If your shoulder were more vertically oriented, like in a chimpanzee, your muscles can't generate the same amount of power.

CHATTERJEE: Lieberman says the earliest human ancestors also had chimp-like shoulders. But that changed about two million years ago. That's when the ancestral species Homo erectus came into the picture.

LIEBERMAN: They widened and broadened their shoulders significantly in the genus Homo.

CHATTERJEE: Now, the authors were only able to study two fossils, so they can't say for certain exactly how human-like these Home erectus shoulders were. But being able to throw with power and precision, Lieberman says, must have been advantageous to our ancestors. If you consider humans, he says...

LIEBERMAN: We don't have natural weapons. We don't have fangs and claws and massive paws.

CHATTERJEE: So hurling things is about the only thing we can do to bring down our prey. Anthropologist John Shea of Stony Brook University, who wasn't part of the study, agrees. He says previous studies have suggested that Homo erectus was also a good runner.

JOHN SHEA: Now, you put these two things together, and basically you've got, you know, the primate equivalent of a fighter jet. You've got something that can run for a long, long time and has projectile weapons onboard.

CHATTERJEE: He says the new findings may explain why the species were so successful. They are thought to be the first human ancestor to spread all over the world.

SHEA: They are the first of these ancestral hominids that's found at the same time in Africa, Europe and Asia.

CHATTERJEE: No one knows what they were hunting with two million years ago, but Shea thinks they probably used stones about the size of a baseball. Rhitu Chatterjee, NPR News.

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