According to here:

http://www.thenakedscientists.com/forum/index.php?topic=4172

It seems that scientists aren’t very sure! One theory is that of ‘sexual selection’ – that is, something which came about by one sex choosing a mate for a particular feature – and causing a runaway ‘positive feedback loop’ of that feature becoming more and more prominent because the gene for selecting that feature becomes more and more common in the population. An example of this is the peacock’s bright tail – which we think came about by sexual selection.

Facial hair may be a feature of sexual selection. We used to be covered in hair (in our evolutionary past) but gradually lost it, to become the ‘naked apes’ that we are. Perhaps sexual selection resulted in males retaining facial hair for no other reason than females happen to have found it attractive. Imagine another example – men, generally, don’t like armpit hair in women (for no particular reason than they just don’t), and so the gene for armpit hair would, all other things being equal, slowly decrease in frequency in a population. My point is that this may have happened for facial hair on men – or, considering we used to be hairy all over, lack of facial hair on females.

Great question though, and apologies that I don’t have a concrete answer!

It’s all about hormones. When you go through puberty, testosterone is released in men and estrogen is released in women and these hormones are responsible for the appearance of secondary sexual characters, that is among others, facial hair for men and breast for women. If a woman has too much testosterone, she will have some facial hair, if a men has too much estrogen, he will get “moobs”

I am really not sure. Charles Darwin suggested that something called ‘sexual selection’ was in play, and I suspect that he might have been on to something. If males have, in most of our evolutionary history, preferred females with less hair, then the genes for less hair would eventually enter the male population, and men too would lose hair, but not to the same degree as women (which would make sense, because men have more hair in general). However, if women preferred men with some hair on their faces, then men could have evolved a counter-hair-loss mechanism. So, at least part of the explaination could be that women, for most of human history, prefer hairy men (although fashions have changed a lot over the short course of recorded human history).

Another explaination could be that it is a sign for females to know when males have the maturity to best feed or look after their children. There could be a ‘training’ phase in human life, childhood, in which hunting and social skills were developed. People who delay when they have children would be better at looking after their children because they will be more skilled, so these people would be more likely to pass on their genes. If genes that give a ‘maturity’ signal to the opposite sex, such as the development of breasts in females or the growth of beards in males, are passed on, then that could also explain why men have beards.

Hey. I didn’t know the answer to this – I just knew it was something to do with hormones. It’s great reading the answers that the other scientists have given – thanks for asking the question.

It’s all to do with hormones. Men have much higher levels of a hormone called testosterone, and this is what makes them grow hair on their faces (ironically, it’s also what makes them lose the hair on their heads!).
I’m sure you’ve come across older women who have hair on their chins. This happens because after the menopause, women have lower levels of the female hormones that ‘counteract’ testosterone, so they end up growing facial hair.