Throw backs

Natural History, March, 2004 by Stephen T. Palmer

In his review of two books about the evolution of bipedalism ["Stand and Deliver," 11/03], Ian Tattersall does not say whether either book addressed this question: How could small, ape-brained hominids have survived leaving the safety of the trees and going bipedal? Here's a theory to add to the others.

Chimpanzees are known to toss branches at predators to try to scare them off. It's not a stretch to imagine that human ancestors adopted the same tactic. As woodland gave way to grassland, objects readily available on the ground, such as sticks, clumps of sod, and stones, would also have been more effective throwing weapons than branches snapped off trees. The ability to aim projectiles is one advantage our ancestors might have enjoyed. And have you ever noticed that baseball pitchers (or javelin and discus throwers) have strong, well-developed legs? They provide a firm launching pad.

So bipedalism may have first rapidly developed not to run fast, or to free the arms for carrying, but to throw objects with force. One rock could startle a beast, and a shower of them, launched by a hominid group, would certainly send it fleeing.

Stephen T. Palmer

Plainfield, New Jersey

IAN TATTERSALL REPLIES: Stephen T. Palmer is not the first to suggest that these small-bodied and otherwise relatively defenseless primates might have discouraged potential predators by throwing objects at them. Such activities might have predisposed them to the hand-eye coordination that later hominids exploited in making the first stone tools. Unfortunately, it's hard to see how one could positively demonstrate which of its potential advantages was the crucial determinant of the success of bipedalism.