"Oh what a piece of work is man," wrote Shakespeare, long before Darwin suggested just how little work went into us. Somehow, that same process that gave us reason, language and art also left us with hernias, flatulence and hiccups. One argument scientists often make against so-called intelligent design — the idea that evolution cannot by itself explain life — is that on closer inspection, we look like we've been put together by someone who didn't read the manual, or at least did a somewhat sloppy job of things. Viewed as products of evolution, however, our anatomical quirks start to...