Next time you watch a baseball game, cast your mind back a few million years. A man steps back, cocks his arm behind his shoulder, then twists. His arm swings forward like a catapult, unleashing the fastest rotation in the human body. Is it just a pitcher’s windup, or an echo of a pivotal moment in evolution?
In modern life, throwing has been largely relegated to America’s favorite pastime, but scientists at Harvard and George Washington universities argue in a study published Wednesday that throwing has had a profound influence on the development of human bodies and behaviors.
Based on an analysis of the Harvard baseball team, chimpanzee anatomy, and fossils of early human ancestors, the scientists say that the capacity to throw a fastball began to emerge about 2 million years ago, enabling a surge in hunting. They suggest that the increase in hunting would have provided high quality food sources and helped spur the development of a more capable brain.
In short: I throw, therefore I am.
Human evolution “reconfigured our shoulders so we can store up massive amounts of elastic energy, which allows us to generate a lot of velocity,” said Daniel Lieberman, a professor of human evolutionary biology at Harvard and senior author of the paper published in the journal Nature. “We think the benefit includes throwing.”
Lieberman, best known for his work to understand the biomechanics of barefoot running, examines how the human body evolved and why.
It has long been known that brawny chimpanzees that can run up a tree or lift heavy weights have a weak throwing arm, even compared with a 12-year-old Little League player. But a few years ago, Neil T. Roach, a graduate student in Lieberman’s lab, began to wonder what evolutionary changes to muscles and skeletal structure allowed human ancestors to develop the unique ability to throw fast and accurately.
Roach, now a postdoctoral researcher at George Washington, recruited cricket bowlers, members of the Harvard baseball team, and other athletes to throw in the lab. He outfitted them with motion sensors similar to the ones used to generate animated characters in Hollywood movies.
He used braces to constrain the athletes’ throwing motions in ways that made them a bit more chimplike, measuring how different limitations in movement diminished their throwing ability.
He found three key differences. Unlike squat chimpanzees, whose rib cage is close to their pelvis, people have a long waist and the ability to swivel at the hips, allowing them to generate more torso rotation as they throw. A natural twist in the upper arm bone plays a role in allowing human throwers to cock their arms so that they can generate speed. People’s shoulders are configured at a different angle than those of chimps when throwing, allowing them to generate more force.
The team studied fossils of early human ancestors and found that throwing ability emerged gradually, but became more common about 2 million years ago, with the species Homo erectus. That date is significant, because it is believed to be when humans’ hunting activity intensified.
For years, the debate over how and when humans began to hunt has been contentious, and the new study will add to it. But Lieberman and Roach believe that the ability to throw fast and accurately could explain a lingering mystery: Early humans were hunting for about a million and a half years before they developed pointed stone spearheads. So how were they killing animals?
“It’s not easy to kill something with a sharpened wooden stick,” Lieberman said, unless you could throw it hard.
Roach is beginning experiments to see how much force would be necessary to hunt an animal with a sharpened stick, to test whether throwing hard could have enabled a bounty in hunting.
Glenn Fleisig, research director at the American Sports Medicine Institute, said the findings resonate with principles he uses helping athletes avoid injury or throw better.
The same anatomical differences that make chimpanzees poor throwers are the ones that might make some people better or worse at throwing. For example, the researchers found that chimpanzees cannot cock their arms back as far as people can. That ability varies naturally in humans, too. Generally, the farther back someone can get an arm, the faster the pitch.
“I don’t know chimps, but this again is consistent with what we’re finding,” Fleisig said. “When I have two pitchers come into my lab and one throws 60 miles per hour and one throws 80 miles per hour, the 60 miles-per-hour person often doesn’t get their arm cocked back a lot.”
Susan Larson, a professor of anatomical sciences at Stony Brook University School of Medicine, said the biomechanical analysis was sound and the paper was intriguing. But she disagreed with the authors’ evolutionary interpretation.
Her own study of the anatomy of early human ancestors suggests that the shoulders of Homo erectus would have been in a position that would have made them quite poor throwers.
A few years ago, she stumbled on a medical study that described a small group of people who were born with a shoulder configuration more like the one that she believes existed in those early humans.
“It’s not a particularly serious malady; it’s just they look a little funny,” Larson said. “Their shoulders are moved forward on their rib cage, sort of exactly how I’m describing would have happened, evolutionarily. They have a very limited range of motion.”Carolyn Y. Johnson can be reached at cjohnson@
globe.com. Follow her on Twitter@carolynyjohnson.