Which ancestors are you referring to? There's not much evidence about consumption of fruit unfortunately. We know that our distant ancestors left the trees where fruit was located to eat more meat however. Molecular biology seems to indicate that fruit in combination with other toxins are ok up to a point but for us it's much lower than other primates because they have larger livers and can detoxify a larger amount than us. That's why PHD recommends consuming up to 1.5 cups of fruit per day.