Hitting children seems to be correlated to the rise of parents having possessions that they want to protect.

Jared Diamond studied hunter-gatherer societies to get a glimpse of what it might have been like for humans before the rise of society. He found that parents often did not use any corporal punishment on their children, but the more things the parents owned, the more likely they were to punish their kids. If someone has a goat, then he is going to protect that goat when a child wants to mess with it. But if someone doesn't have a goat, then there's nothing for his kids to mess with, so there is no punishment. Diamond wrote about this in his book The World Before Yesterday.

If this is true, then hitting children may very well be unnatural for humans to do. Perhaps humans in our most natural state, that which we evolved to fit into, do not hit their children. But then with the rise of society, hitting became commonplace; and only now with the rise of psychology, are humans turning back towards the rejection of hitting kids.