The Evolution of Throwing: How Humans Gained Accuracy and Power

The ability to throw an object with great speed and accuracy is a talent held by one member of the animal kingdom alone: humans.

Sensing something significant was behind this unique skill, Neil Roach, who recently received his Ph.D. from Harvard, decided to take a closer look. As he and his fellow researchers did so, they discovered a suite of changes in humans' shoulders and arms that allowed early humans to hunt more efficiently by throwing projectiles, ultimately aiding our ancestors in becoming part-time carnivores.

"When we started this research, there were essentially two questions we asked - one of them was why are humans so uniquely good at throwing, while all other creatures including our chimpanzee cousins are not," Roach said in a press release. "The other question was: How do we do it? What is it about our body that enables this behavior, and can we identify those changes in the fossil record?"

To answer this question, the team started with chimpanzees, humans' closest relative.

Chimps are known to throw objects (often feces), though almost always underhand and with less accuracy and power than the average Little League pitcher, Roach said, adding that chimps throw as a part of display behavior and never when hunting.

Part of the reason for this, the researchers explain, has to do with their technique, which is limited by their anatomy.

"Chimps throw overhand using either a dart throwing motion, where the elbow is extended, or much like a cricket bowler, where their elbow is kept straight and they generate force by swinging their shoulder," Daniel Lieberman, a biology professor at Harvard, explained.

Seeing this, the team decided to study cricket bowlers as well as try to understand what happens when people keep their arms straight.

"Eventually, we began to think that changes in the way the shoulder is oriented with regards to the rest of the body could change the way you generate force when you're throwing," Roach said.

To explore those physical changes, the team created a complex model incorporating current research regarding the biomechanics of throwing. Using this model, they were able to explore how morphological changes to the body - wider shoulders, arms that are higher or lower on the body, the ability to twist the upper body independently of the hips and legs, and the anatomy of the humerus - effect performance.

Furthermore, Roach performed a series of experiments in Lieberman's Skeletal Biology Lab using members of the Harvard Baseball team and braces designed to limit their movements in a way that forced them into a more primitive condition. This gave Roach the chance to see how different anatomical shifts contribute to the mechanics of modern throwing.

Ultimately, Roach and his colleagues were able to quantify how restricting certain types of movements affected throwing performance as well as trace the effect to specific changes in the mechanics of each player.

"We [tried] to push these bits of anatomy back in time, if you will, to see how that affects performance," Roach said.

As a result, the researchers were able to point to key physical changes that helped to make fast, accurate throwing possible.

Evolutionary changes in the shoulder, for example, show that, as a pitcher cocks their arm back, they're starching the ligaments and tendons that run across the shoulder.

"Those tendons and ligaments get loaded up like the elastic bands on a slingshot, and late in the throw they release that energy rapidly and forcefully to rotate the upper arm with extraordinary speed and force," Roach explained.

This rotation, as it turns out, is the fastest motion the human body can produce.

Additionally, the team found that the twist in the bone of the upper arm and an expanded, mobile waist both gave humans the ability to store up and then release more of this elastic energy.

"The linchpin is really what's going on with the shoulder," Roach said. "When you see the shift from a chimpanzee shoulder to a more relaxed human-like shoulder, that enables this massive energy storage."

While many of the evolutionary changes they studied may predate Homo erectus, Roach said that it's the final change in the shoulder that "brings it all together."