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.

Third Nipple: Incidence, Types, Causes, and Removal

OK! or OMG?!: Read Our Review of Zac Efron and Seth Rogen's Neighbors

Harry Styles Confirms He Has Four Nipples, a Condition Called

From Paris to Pennsylvania: A Summer of Long Distance, Love, and Journals

Third (supernumerary) nipple: Types, causes, and removal

Supernumerary nipples

This Is Why Some People Have Extra Nipples

Why do people have extra nipples? - Quora

What is the cause of inverted nipple? - Quora

Supernumerary Nipple: Causes, Diagnosis, and Images — DermNet

The Hybrid Hypothesis: Additional evidence