The robust brow ridge of archaic human species simply continues to be one of mysteries of evolution. One of my physical anthropology professors jokingly answered that question with “Don’t ask!” Nobody really knows, and there are very few (and very weakly supported) hypotheses.

Brow ridges might have served the purpose of protecting the eyes, or of shielding them from the sun or sweat–in which case the mystery is really why anatomically modern Homo sapiens don’t have them. If I had to speculate, I’d say it’s just another one of our neotenous features.