On the topic of eyebrows, many mammals (maybe most?) also have some kind of special hair above their eyes. Those of cats are very noticeable, dogs also have muscles to move them (though it might have been influenced by humans), seals, otters, hares, all kinds of very diverse animals have something that is analogue to eyebrows, which makes it very unlikely that it would just be a random feature on humans.

But it also means it might not have appeared for a specific reason in humans. It might very well have just been there and useless (but not detrimental) at one point, then used for something.