We are predisposed to gather around people who are like us physically and mentally. As long as everyone is behaving in a similar way, we can get past the physical part and as long as there are enough of us with similar viewpoints we can just make it work I guess, but only temporarily. That's worked for a while and probably even introduced diversity that was positive, but it doesn't scale forever and it's not sustainable.
As far as I know, we're the only animal on this planet that regularly tries to convince ourselves that being different is good and we should just accept it to the degree that we intentionally mix with people who believe _completely_ different fundamental things about life. Even moderate islamic beliefs are very different than what we believe as Americans. Crazy religious nuts, regardless of which deity they're sworn to worship, are pretty much always going to be at odds and that's not really the only problem we're facing. It's a major one, but there are also plenty of people who simply don't want to integrate.
How often do you see hammerhead sharks hanging out with great whites? Yeah, it's not exactly the same and I know that, but the point remains that long ago groups of people developed different propensities for certain behaviors, beliefs, and cultural boundaries because they were with their own 'pack'. All of a sudden, we've become super modern and super transportable in a few hundred years while our biology is still lagging way behind. It's like my son when he was a baby - he learned to walk almost 4 months before a normal kid learns to walk and, as a result, his brain was lagging behind. He hurt himself constantly because he had no ability to understand anything around him yet. As soon as he got a little older and had the ability to understand his surroundings, he stopped being so stupid. The fact that he could walk prematurely before his brain had caught up was a huge problem and that feels similar to what all of us are doing now. Maybe mixing is the way of the future and maybe it's how we will ultimately succeed as a species, but our moronic behavior and completely irrational religious zealotry would have to go by the wayside. As it stands, we can't have both and I think recent events as well as most of our recorded history has proven that.
Ancient cultures that weren't brutalized by larger invading forces had no issues remaining practically homogenous for thousands of years. Life probably sucked for people at the bottom, but that's true right now as well. The difference is they got stomped on by invading forces and we probably won't. We're imploding. I don't claim to know exactly why, but I can't see past the cultural issues at the moment because they are real and significant and seem to be the main reason the shit is hitting the fan all over the planet. In the end, all I really want is for people to stop being fucking retarded and to stop killing each other, but I don't think that will ever be a reality.
I'm rambling and I know that. Sorry.