These two images show the muscular and skeletal differences in the position of the shoulder between chimpanzees (left on both) and humans (right on both).
Credit: Brian Roach/Neil Roach
(ISNS) -- It's completely ordinary to see today's athletes throw a javelin hundreds of feet in the air or fire baseballs accurately and in excess of 90 mph dozens of times during a game. However, not every close human relative has that ability to throw, despite the great strength that many possess. Researchers say they traced that ability back to three changes to the waist, shoulder and upper arm that happened about 2 million years ago in the human ancestor Homo erectus.
Making a strong, accurate throw requires the different parts of the body to work together in what biomechanics researchers call a kinetic chain -- the rapid and sequential activation of different muscles. The motion that launches a throw begins with the legs, moves through the hips, torso, shoulder, and through the arm to the hand. Throwing projectiles fast and with high accuracy requires coordination, and also the anatomical features that first appeared together in Homo erectus.
A team of researchers, reporting in Nature, found that the three key traits can be found in humans, but not our closest relatives, chimpanzees. Each feature allows the body to store more energy before a quick rotation that releases it: tall and mobile waists that permit torso rotation; the way the elbow and the bone in the upper arm, the humerus, join together and rotate; and the placement of the shoulders. Each trait has "a major role in storing and releasing elastic energy during throwing," the researchers wrote.
The change to the shoulder is crucial, explained Neil Roach, a biological anthropologist at George Washington University in Washington, D.C. While chimpanzee shoulders sit very high and close to the neck, almost as if the animal is permanently shrugging its shoulders, human shoulders are much more relaxed.
"That change in the shoulder really brings all of those things together and that didn't happen until 2 million years ago," said Roach. "That allows us to essentially use the arm like a catapult, to store energy as we cock our arm or rotate our arm away from the target before we rapidly rotate it toward the target."
The rotation of the humerus is the fastest motion the human body produces, said Roach, at over 9,000 degrees per second.
Sending Modern Baseball Players Back in Time
The researchers studied both the fossil record and Harvard University baseball players in order to develop their insights. They used motion capture technology to track the way experienced throwers launch the ball.
The researchers also studied restricted motion using braces, Roach said. They prevented subjects from relaxing their shoulders and restricted the motion of the arm.
"What that did was give us the ability to at least mimic what the ancestral anatomy would have been like," said Roach.
The resulting observations allowed the researchers to zero in on the most important features for throwing: the elbow, shoulder and waist. The fossil record showed that when Homo erectus developed these features together, it made them the first of our ancestors that could throw like modern humans.
William Hopkins, a neuroscientist at Georgia State University in Atlanta, said that most other research on the origin of throwing focused primarily on the hand and wrist. He studies chimpanzee behavior, including throwing.
The researchers, Hopkins said, "have really pushed the area forward in terms of describing exactly what changes biomechanically that allows for this enormous skill in humans."
Throwing and Hunting
When Homo erectus appeared in the fossil record about 2 million years ago, it coincided with an increasing amount of meat consumption, and probably more hunting. Roach thinks that the shifts in arm and shoulder structure probably made that easier.
"Given that this important change in terms of our throwing performance occurs at the time that we see a real intensification of hunting … we think there's a good possibility that that's the case," said Roach.
The researchers plan to investigate primitive spears and their effectiveness in injuring or killing animals, as well as how the throwing motion differs for launching different objects.
Throwing may have been connected to other behaviors as well, such as defense. Slinging rocks at a potential predator might have offered early humans protection. Scientists can look to chimpanzees for clues about early throwing.
With chimpanzees, Hopkins found that some of the animals threw and some didn't. He began investigating and noticed that chimpanzees mostly throw food or feces, often in defense, and that some of them are better at it than others.
"The more interesting forms of throwing are what we call aimed throwing," said Hopkins. "In many ways they look like a baseball pitcher."
But chimpanzees can only throw about 20 mph, despite their great strength, while baseball pitchers, cricket bowlers or even football quarterbacks can greatly exceed that figure with their respective projectiles.
Roach said that human ancestors prior to Homo erectus probably had better throwing performance than chimpanzees, but not nearly the same capacity that the anatomical changes made possible.
From the Mound to the Classroom
While the paper's information about throwing mechanics is generally well-known to professional baseball players, the link to evolution is particularly interesting, said Tim Layden, head baseball coach and evolutionary biology teacher at Florida's Montverde Academy. Incidentally, Layden was also a freshman All-American pitcher at Duke University and pitched in the Chicago Cubs’ minor league system.
"I'm probably going to use this next semester in my primate course, for sure," said Layden. "It makes perfect sense from an evolutionary standpoint that there would be a selective force for high velocity throwing and the buildup of energy within the shoulder joint."
Although there's no evidence of sports among Homo erectus, Roach said that play could have been crucial to learning to throw.
"Like any ability that requires incredible performance, play is an important mechanism for learning that behavior," said Roach.
Inside Science News Service is supported by the American Institute of Physics. Chris Gorski is an editor for Inside Science News Service.