I remember reading somewhere that people might have evolved thick curly head hair to protect their heads from the savanna sun when they started going upright. This strikes me as implausible, since most savanna mammals manage to do that with shorter hair.
But “people who grew longer hair could groom it in ways that are attractive to their peers” seems like a trait that could easily be selected for, like the peacock’s tail. I suspect that long head hair evolved after hair cutting.