New Scientist, 13 January 2007:

EXTRA NIPPLES OR BREASTS

Most mammals have multiple pairs of nipples; like most primates we have only one. Reversions are common: around 1 in 20 people has at least one extra nipple and some have as many as six. These "supernumerary" nipples range from a patch of darker, aureolar skin to a complete nipple or even an entire breast that can produce milk.

In humans, extra nipples usually occur along the milk lines, two thickened areas stretching from the armpits to the groin that appear in the embryo in the fourth week or so. In other mammals this tissue gives rise to up to five pairs of nipples.

Extra nipples sometimes form off the milk lines, even on the face or legs. A 19th-century medical book describes a woman who could breastfeed from an extra breast on her leg. Such cases may be due to the failure of cells to migrate to the right place during development.

Extra nipples are often seen in other primates too. In one group of macaques in Taiwan, up to a third of females have extra nipples.

Atavism? When on the milk lines, yes

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