UNIVERSITY OF CHICAGO PRESS, OCTOBER 2013It happened years ago, but the event was so traumatic that I remember it as if it were yesterday: an elderly professor physically pinned me against a wall and berated me for rejecting his paper on why human ancestors got up on their hind legs and walked. “The reason,” frothed the empurpled sage, “was to make it easier for mothers to carry babies close to their chests.” See? So blindingly obvious that anyone, even I, could understand it.
Manuscripts seeking to explain the evolutionary roots of human bipedalism land in my in-box at Nature with monotonous regularity. We became bipeds so that we could carry food, or tools; so we could see farther; so we wouldn’t expose so much of our skin to the Sun; so we could wade better in rivers and lakes. They all make good stories, but they all share the same error—they are explanations after the fact, and, as such, betray a fundamental misunderstanding of how evolution works.
Natural selection, the mechanism of evolution, operates without memory or foresight. It has no intention. It is we who choose to interpret evolutionary purposes as such later on. The features of living things, therefore, do not evolve for any preconceived purpose ...
