If your eye brows aren't growing it could be due to plucking/waxing. Doing these things damages the folicles over time and they just stop growing back. My poor sis now has to pencil hers in every morning after years of plucking to a trend when less was more. Not that I can talk. lol
I did JohnnyBoy, the question "Which facial feature was missing form the Mona Lisa?" came up when playing Trivial Pursuit once and my dad answered "Oh I don't know, nose?" I replied "(Laughter) Nose??" Dad- "Oh alright, chin??" Me- (laughing even more) "No it was her eye.." Dad- "Eyes!!" Cue much mirth from me and mum!
.....Maybe you had to be there!
Body and facial hair usually grows in places to protect the areas that are vulnerable to injury. The eyes = eyelashes - brows. underarm = glands. Ladygarden etc = gentillia. I have no real medical knowledge but I understand that is the reason for hair. Head = the brain. Can't explain bald men though.
Don't forget communications... the eyebrows are used in much non-verbal communication, such as surprise and "You talkin' to me!?"
Additionally, as previously suggested, with hypothyroidism, there is often an unusual loss of hair in the outer edge of the eyebrow. The brows tend not grow beyond a boundary equal to the outer edge of the eye... for some reason.... (hyperthyroidism doesn't cause this, however...)