Why did humans evolve eyebrows? There are reasons a-plenty.
Two of the most commonly cited are for ocular protection and non-verbal communication, as I explain in this handy BrainStuff video:
Their contributions to sexual dimorphism (his vs. hers eyebrows) and facial attractiveness also intensified the evolutionary selection pressure. Ultimately, it turns out these strips of hair -- or sometimes singular strip of hair in the case of the culturally maligned unibrow -- speak volumes for and about us. My routine trips to the threading salon are starting to make more sense now...
Even more fascinating, a 2003 study from MIT found that our eyebrows might play and even more crucial role in facial recognition than our eyes. Past research on facial recognition had established the eyes, nose and mouth as the top-down hierarchy of influential physical features, but none had tested eyebrows in isolation. The brows were always lumped together with the eyes. When the researchers showed participants a series of celebrity photos, some with their eyes Photoshopped out and others with the eyebrows erased, participants had a "significantly" harder time identifying the eyebrow-less stars.
Due to eyebrows' non-verbal communication skills, we actually tend to make more brow contact than direct eye contact. Their distinct color contrast also makes eyebrows easier to spot from a distance than, say, the color of someone's eyes. And so, without our eyebrows, our face becomes like a map with no oceans. We're not just eyebrow-less; we're nobody.