Stobex -- who is mere days away from needing a new name; suggestions, anyone? -- reads this blog. Only fair, and I'm thrilled that someone does! He also thinks about it (how radical, dude!), and has responded to my post recently about religion, tribalism, and evolution. I tried to explain about "Comments" particularly as it is embarrassing that there are so few here. Okay, none. (I'd like to think it's because I say it all so succinctly that no one need ever disagree, but I'm not that crazy.) But no, he's also smart enough to know no one will read his comment in Comments. So, here, then, are Stobex's comments:
1. Darwinian natural selection works on genes, not people. For this purpose, it does not matter whether the "gene" is DNA or anything else that is hereditable, such as sociobiological behavioral traits.
2. Thus, any hereditable characteristic that increases its own probability of propagation will tend to prosper. In a small tribe, which will tend to be fairly inbred both genetically and culturally, this applies to characteristics that advance the survival of the tribe, even at the expense of the individual. Remember Haldane's famous quip that he was willing to lay down his life for two brothers or eight cousins. The more inbred you are, the fewer cousins it needs for the sacrifice to be worthwhile.
3. Thus, a small tribe that survives is likely to have members with characteristics that encourage them to advance the interests of the tribe above their personal interests.
4. Evolution being rather clumsy, what we actually have is a hereditary predisposition to adhere blindly to seemingly irrational tribal rules of behavior, coupled with separate natural selection of those rules that favor the survival of the tribe in question. (Again, it does not matter whether the rules are hard-coded into our DNA or are inherited culturally, except that cultural rules can probably evolve faster.)
5. I think religion as we know it probably developed simply as a way of making this blind adherence to the rules seem to make sense to our gradually evolving self-awareness. Theistic religion did no harm, and so could attach itself to the inherited behavior, as long as it effectively just endorsed the naturally selected rules, and did not impose too much of an overhead.
6. That works, provided there are a large number of tribes with different tribal rules competing against one another. The competition selects the fittest sets of rules, and the large number of tribes allows random mutations in the rules so that the rules can evolve as conditions change.
7. However, the tribes will to some extent compete directly against one another. So, one of the rules that is likely to be successful is that in a conflict with a neighboring tribe You all combine against Them: because if They combine against You and You look after Your own individual interests, They are likely to win the war, by picking You off one at a time.
8. Unfortunately, the system has broken down, for several reasons. Writing preserves the existing rules too rigidly, preventing evolution. Large organized societies both increase that rigidity, and reduce the number of competing "tribes." As a result, the rules became fossilized about 3,000 years ago, when big kingdoms came into fashion. (Developing social structures were crucial, because without a formal government and economy, large tribes were unworkable,and would naturally break up into small tribes.) A surprisingly high proportion of the more famously bizarre rules of the world's religions can be explained in terms of late stone age economics in the home areas of the religions in question.
9. Thus, we have a highly technological society, with rules of behavior optimized for a small late stone age tribe in competition with other late stone age tribes. That includes the fundamental principle that if Your goats and Their goats are competing for the same grazing You unite against Them, get a lot of late stone age stones, and keep throwing stones at Them until You drive Them out of the area (or They drive You out of the area).
10. That is why we see this seemingly irrational division of the world into Us and Them: but without a proper tribal structure, deciding who is an Us and who is a Them can get a bit arbitrary. However, the idea that if one of Them attacks one of You, You come back and shoot up Their neighborhood makes sense, because You are instinctively assuming that Their neighborhood is full of Them.
11. Now start throwing bombs and bullets instead of stones, and a lot more people get killed.