Question: is having a third nipple down to having faulty genetics or could it be a form of evolution?

Vera Weisbecker answered on 19 Jun 2011:

Well, it isn’t exactly faulty genetics in the sense that it impacts on your ability to survive or reproduce, as it is perfectly harmless. Extra nipples in humans are a fairly common return to an ancestral condition (a “default condition”, so to speak.. If you look at other mammals, most have more than one pair of nipples – e.g. cows have four, pigs have lots. So at some point, the lineage leading to apes (who I think have only 2 but not sure!) and humans lost the many nipples that are normal for mammals. However, since we all still have one pair of nipples, the mechanisms growing them are still there. Every now and then, these mechanisms accidentally don’t stop at 2 nipples, instead making three or more like they used to millions of years ago. So it’s a form of retro-evolution, if you want.