It’s just a feature like of the human face. Like why do we have less hair on our face than our head? Because that’s how we evolved there may be some evolutionary advantage to this or maybe early in our evolution when therere wasn’t very many of us someone got that trait and it’s dominant so they passed it on to a large percentage of the human race and if someone doesn’t have it your not likely to know unless you sequence their genome and figure out what gene that is
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