An extra nipple – or ‘polythelia’ to give it its scientific name – is relatively common. As many as 1 in 18 people have a third nipple, although this can range from one that is invisible on the surface, to a full nipple with milk-producing tissue.
Most mammals have several pairs of nipples, but as humans evolved to have smaller litters than other mammals, we lost all but our last pair. However, the genes to produce them are merely inactivated. A mutation in one of these genes causes a signalling protein called NRG3 to be produced, which can cause cells to develop into a third nipple.