I think, it's sort of a side effect. The ancestor's taste buds were optimized for giving them better chances of survival, and one feature was better detecting o

I think, it's sort of a side effect. The ancestor's taste buds were optimized for giving them better chances of survival, and one feature was better detecting of sweetness. D2O didn't exist then and so couldn't influence survival, so detection of D2O is just an accident.