Evolution has seemed to favor a large degree of distinctiveness in human mugs. Sheehan and a colleague reported last year in the journal Nature Communications that human faces differ in their dimensions more than, say, hands vary in their widths and lengths. In addition, the scientists found that more genes are known to be linked to looks than to other areas of human anatomy.  

“Human faces are more variable than we would expect them to be based on how variable other body parts are,” Sheehan said.

A plausible reason for the varied visages is that humans, just like other social animals, benefit from being able to distinguish individuals in their species. That way, those who serve as benefactors, or who act like jerks, can be approached or shunned accordingly. “You care who’s who,” Sheehan said.