Sweat burns in the eyes and humans sweat whereas other mammals don’t—it’s what allows us to have unmatched physical endurance in the animal kingdom. I suspect this is a only one reason why we have eyebrows.
Another reason could be that we’re socially unparalleled in the animal kingdom. I imagine eyebrows allow better communication. To support this hypothesis, we’re the only species with a sclera (white of the eye) which also probably exists to facilitate communication.
There’s no way to know with absolute certainty. That is the nature of evolution: one can make educated guesses and build a hierarchy of hypotheses of varying degrees of certainty, but there’s always uncertainty such as with many of the queerer questions like this one.
To put this into better perspective, look up sexual dimorphism and sexually dimorphic birds; they’re *ridiculous*. We know sexual selection is a real mechanism. Bright blue color signals a chemical composition and abundance of y which implies good health, but *why*, for example, does this particular patch of feathers have this shape? The question of why eyebrows exist is easier to answer, but it’s far less clear in comparison to more fundamental features.
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