Have you notice bitches have as many pups as they have tits?
Same for humans, women used to have two babies each pregnancy, one for each breast.
Then Hephaestus, working from his workshop in the Eden valley, Armenia, had the idea of building the golem, made of clay, to help him with his stuff.
He got annoyed with his creations, though, as they were too stupid, so he released them into the wild, and the golem breed with the daughters of men, mixing their blood with theirs. So women started to get sick during their pregnancies, with only one baby coming alive. And when both came, they were identical in appearance! As generations passed, women started to have only one kid, with the rare ocurrence of two being termed "twins".
By then the golem blood has already infected humanity, and the idiot became the norm. That's why you have those epic movies with huge armies of hundred thousand men. Before the golem you could muster only a hundred, because men were too individualistic to serve a distant lord.