Why Baboons Pitch Underarm

For the Australian anatomist Raymond Dart, the fossilized bones scattered among the caves of South Africa were testimonies to the murderous nature of early humans. The recovered skulls of baboons and our australopithecine relatives often looked as if they had been bashed in, and Dart believed the bones, teeth, and horns of slain game animals were the weapons hominins used to slaughter their prey. (He gave this sort of tool use the cumbersome name "osteodontokeratic culture.") Our origins had not been peaceful; Homo sapiens was a child of violence. In his infamous paper "The Predatory Transition From Ape to Man" Dart wrote;

The blood-bespattered, slaughter-gutted archives of human history from the earliest Sumerian records to the most recent atrocities of the Second World War accord with early universal cannibalism, with animal and human sacrificial practices or their substitutes in formalized religions and with the world-wide scalping, head-hunting, body-mutilating, and necrophilic practices of mankind in proclaiming this common bloodlust differentiator, this predaceous habit, this mark of Cain that separates man directly from his anthropoidal relatives and allies him directly with the deadliest of Carnivora.

Dart's vision of our origins was so brutal that even those who stressed the importance of meat-eating and hunting in human evolution shied away from it. Even so, the idea remained popular, and glimmerings of Dart's bleak vision could still be seen in pop culture (like the introduction to the film 2001: A Space Odyssey) and popular-audience books about evolution. In I.W. Cornwall's 1960 book The Making of Man, there is even an illustration of a group of Australopithecus chucking stones at a group of baboons.

Thanks to more recent studies of the same South African caves, however, we now know that the australopithecines that lived there between 2 and 3 million years ago were not "mighty hunters." Instead they often fell prey to leopards and other carnivores, just as the baboons did. The caves were not stashes of australopithecine leftovers but accumulations of bones that has been collected by predators or washed into the caves from outside, and these factors caused the damage that Dart had seen. Nor is there any evidence that the australopithecines attempted to stone baboons, but, strangely enough, some living baboons throw stones at humans.

While most accounts are anecdotal, baboons (as well as capuchins and macaques) have been known to throw stones at people. Their ability to do so, however, is somewhat hindered by their shoulderblades. Baboons are primarily terrestrial primates and their shoulderblades are oriented on the sides of their ribcages. They physically cannot bring their arms around to pitch overarm. That does not stop them from hurling projectiles at threats in an underarm fashion, though, and they can still throw with a good deal of force and accuracy.

This brings up an interesting instance of contingency in evolution. Our species can pitch overarm, and we owe this arrangement to our early hominin ancestors. As studies of early hominins like the recently-described Ardipithecus ramidus have suggested, the adaptations early hominins had for life in the trees were co-opted and evolutionarily tweaked for life on the ground, including placement of the shoulderblades on the back rather than the sides. We owe the range of movement we have in our arms to our arboreal ancestry. Without this historical quirk, the games of baseball and American football would not exist as we know them. The next time you see a pitcher strike someone out or a quarterback throw a pass, think of Ardipithecus and our other early relatives.