Why do man have nipples

Note: this question was posed a number of times by Creationists/ID proponents. We include this article from ilovebacterial.com for reference.

Original: http://ilovebacteria.com/nipples.htm

Men have nipples because they would look pretty silly without them (think about it, who else finds Action Man dolls strangely creepy?). But if that answer is not enough to satisfy then keep reading. In truth, men could have them surgically removed and their function on this Earth would be pretty much unaltered (although most women would find them a little strange and probably not want anything to do with them).

So why are male nipples still nipples? To answer this we need to begin with evolution. For something to be selected for in a population it needs to offer some sort of advantage to an individual that enables them to pass their genes to the next generation (i.e. helps them to survive long enough to have babies and makes them attractive to the opposite sex). But as odd as nipple-less men would look, I'm sure they would still manage to find mates so what advantage other than appearance do male nipples give men? In fact they give no advantage but importantly also give no disadvantage. For something to be selected against by evolution it needs to prevent someone from passing their genes on and nipples don't really get in the way of reproducing.

Lets think a bit more about men and women. You probably know that all of your genes come from your parents - half from your mother and half from your father. So everything about you is a combination of the traits found in your parents - eye color, height, blood group etc. So why are males and females different? Shouldn't we be half girl and half boy?

In actual fact all humans are based on the same default genes - e.g. those which give us our upright skeleton, eyes in the front of our heads, same arrangement of internal organs etc. The differences between males and females can be thought of as modifications of this genetic default state. Certain traits that differ between the sexes effectively become uncoupled from the basic blueprint that makes a person.

The differing characteristics of men and women are down to certain genes being located on sex chromosomes that differ between the sexes - females have two X chromosomes, and males have an X and a Y. In addition, the expression of lots of genes will differ depending on whether they find themselves in a female (XX) or male (XY) genome. So the same gene can be found in a man and a woman but how it is switched on can be different.

So what does this have to do with male nipples? Well, this uncoupling of males and female traits that I just talked about occurs when it is important to the reproductive success of the sexes to possess different versions of the same trait (e.g. you've probably noticed by now that the male and female genitals are significantly different - both versions are vital for reproduction to occur, so uncoupling from the normal blueprint occurs, allowing the male and female versions to be different). However, this uncoupling does not occur when either the same trait is required by both sexes or if a particular attribute is important to one sex but completely unimportant to the other.

This is the case with nipples. It goes without saying that they are pretty important for females. The fact that they aren't important to men means that this trait has never become separated between the sexes.

During the first few weeks of foetus development in the womb, the baby will be female and it isn't until about week 8 that the male Y chromosome kicks into action and starts to change the foetus to male. By this point, early versions of breasts have been formed in both male and female babies. Because it doesn't do a man any harm to have nipples, there is no reason why this would change. So men have nipples became women do, and evolution just hasn't bothered getting rid of them.