Why Do We Have Eyebrows and Other Types of Facial Hair?

We humans seem to have an on-again, off-again relationship with facial hair. Prehistoric cave drawings reveal the myriad tools our ancient ancestors used to shave: shark’s teeth, sharpened flints and even clam shells. Nowadays, beards are back in style and people are taking a razor to their brows, instead. But is there a reason we evolved to have these hairy baubles in the first place? And, if so, what evolutionary advantage might we be throwing away for the sake of staying on trend?