Answer:
Ancient Greeks.
Step-by-step explanation:
The human males and females are separated on the basis of their gender. The third gender might occur due to the mutation in the normal chromosomes of the humans.
Different cultures and the scientists have different thinking on the sexuality of the people. According to the Ancient Greeks, the heterosexuality and homosexuality might occur because according to them humans were used to be double creatures and homosexuality is common among them.
Thus, the answer is ancient Greeks.