Why do people have extra nipples?

Extra nipples were once thought to be a link to our evolutionary past, but the truth is a lot stranger, writes Jason G Goldman.What do Mark Wahlberg, Tilda Swinton, Lily Allen, and Bill Paxton have in common? For one thing they're all very, very famous, but so are lots of people, and you already knew that. Another thing these four individuals have in common is that they have extra nipples. A third one, to be exact, though there are humans who have had as many as six additional nipples.

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