Well; I can see your point, but I have to disagree.
You see, in most modern cultures, people are, again, selected for their qualities rather than their part in reproduction. Of course it makes sense that most men would, in ancient days, be hunters, while most women would be gatherers. This is not only because of child carriage, but more because of physical qualities.
I've noticed that most women are far more clumsy than most men, for example. That is just my experience, though. Most men are better at moving unnoticed, and most men are physically more durable, have an easier time neglecting physical damage, and are simply more muscular.
However, some men are perfectly incapable of hunting. Some women are perfectly able. It's not as black'n'white as you come across.
So I think gender roles are social constructs based on averages.
An example of this is another species: Lions. The females do most of the hunting, while the males do most of the care and protection. The reason: Female lions are faster. The males are bulky, and that makes them less adept at hunting fast game. However, males, due to their bulk, make excellent fighters, which is, in a lion's pride, a very important attribute for their cub's survival.
In other words: I think that societies adapt; not the brains. At this point in time, the hunt is, for human survival, near to meaningless. We have farms, after all. A lot of people make their living in an environment where gender is simply not important. Because of this, society adapts gender-roles to be far more fluid.