Human nature may be crafted by evolutionary forces, but that doesn't mean that human behavious do is always in service of biological evolution.

The idea about people coming from Genghis Khan is unrepresentative in a couple ways. First, that a large number of people have a common ancestor does not mean that the common ancestor was in fact Genghis Khan. It could have been someone else associated with the Mongol conquest.

Second, six hundred years from now you or I may have a lot of ancestors, assuming you have kids (I already do). There is nothing remarkable about that.