The ability to throw a bottle of water to a friend, pitch a baseball or net a basketball is probably something most of us take for granted. But accurate throwing is something that evolutionary biologists and anthropologists have spent a perhaps surprising amount of time pondering. Are we the only species that can do it?
In 1975, PJ Darlington of Harvard University's Museum of Comparative Zoology claimed throwing with precision or accuracy is a uniquely human trait. "No other animal can throw as man does," he wrote. Darlington described a study in which wild chimpanzees threw 44 objects, but only successfully struck their target five times, and then only when they were within 2m (6.6ft). "Other primates do throw sticks and stones, but only awkwardly…Compare this with human throwing. A skillful man has a good chance to break the skull of another man with one stone at 30m (100ft)," he added.
Consider what it takes to play the game of darts. The spot where a thrown dart strikes the board is determined by the dart's position, velocity and direction of motion at the moment of release. And those variables depend entirely on the trajectory of the throwing arm's hand and on the timing of release. Whereas most of us might never hit the bullseye, we can at least come close. That is, the spots where the darts land are at least non-random. In a study published earlier this month, researchers demonstrated that dart throwing is a skill that can be learned. Compared to amateurs, expert dart throwers had less error in their release timing and had more efficient hand trajectory patterns.
While accuracy can improve over time, throwing itself is not something that humans must learn. That may not be the case with Japanese macaques. Several years ago, researchers from Kyoto University's Primate Research Institute looked for signs of throwing behaviour among 10 distinct macaque troops spread across six geographically isolated areas. They witnessed 83 throwing events in one troop, two each in two other troops, and none in the remaining six troops. The macaques' rock-throwing was mainly communicative in nature, since it usually occurred during aggressive interactions or when they were disturbed by an outside event (like loud Japanese military aircrafts that regularly flew over the Primate Research Institute). The pattern suggests that while Japanese macaques are physically capable of completing a throw (usually underhanded and with three limbs on the floor), throwing is actually cultural. Primatologists call it a "behavioural tradition." Humans retain their edge, though. The researchers found no evidence for “aimed throwing."
So why did humans acquire this talent? Throwing probably gave our early ancestors a better chance of acquiring a meal, and wouldn't have been possible without various skeletal and anatomical adaptations that allowed for the rotation of the arm and pelvis. But it is also possible that throwing was a communicative gesture, as it seems to be for Japanese macaques. That's perhaps why some have argued that more accurate throwing helped to usher in a host of cognitive advances, including language and music. After all, throwing requires a certain amount of psychological sophistication.
To better understand the evolution of throwing, psychologist Justin Wood wondered whether the he could separate the anatomical requirements for aimed throwing (that is, the "hardware") from the psychological operations that guide it (the "software"). To do so, he turned to rhesus macaques. These monkeys share the anatomy of Japanese macaques, so if they did throw, it would be fairly haphazard. But to what extent do they “understand” the act of throwing? Would they still retreat from a human experimenter that seemed to throw a rock at them? (The researchers never actually released the rock; the monkeys were never in any actual danger of being struck or injured. In truth, an angry rhesus monkey would be a bigger danger to a human.)
While 85% of monkeys ran away when the researchers completed the throwing motion, far fewer bothered to retreat when the trajectory, speed, or direction of the throwing arm were such that the throw would not be dangerous. They also didn't care about completed overhand throws if they were done with an empty hand or while holding a bit of soft food. In other words, despite being unable to throw accurately, they possessed the requisite mental machinery to accurately evaluate the possible trajectory of thrown objects, and to determine whether they were in danger of being struck.
That suggests that at least some of the psychological prerequisites for accurate throwing were present among our primate ancestors, long before the requisite skeletal and muscular anatomy evolved in our hominid ancestors to make it possible.
Our short fingers and other adaptations within the structure of our hands seem to have defined our species in the same way as a bipedal stance and enlarged skulls. Those changes in hand anatomy gave rise to more powerful and precise hand motions, according to hand surgeon Scott Wolfe. "The ubiquitous presence of the dart-thrower’s wrist motion in distinctively human activities involving tool use, throwing, and weaponry suggests that its development also may have played an important role in human evolution."
Oh, and in case you wondered, women can throw just as well as men, though perhaps a hair slower. In 2011, Kevin Lorson and colleagues compared overhand throwing among men and women of three different age brackets: adolescent 14-17 year olds, young adults aged 18-25, and older adults aged 35-55. While they reported that some differences in body mechanics between males and females in the two younger groups, those differences vanished by adulthood.
Neither sex may have an edge when it comes to throwing, but our species enjoys a clear advantage over our primate cousins. Still, I wouldn't want to play a game of catch with Santino, the most famous chimpanzee thrower. Each morning, Santino calmly strolls through his enclosure at a Swedish zoo, finding and modifying small concrete disks, and caches them in piles around the outer edges of his territory. He then chucks these missiles at visitors after the zoo opens. In that contest, my human ability to throw a dart with fine accuracy wouldn't prove all that useful – it’d be a better idea to duck.