I would like to add something here -- a doubt. There is no doubt that chimpanzees practice incest avoidance in similar ways as some human groups. But what is in doubt is why?
^^^^ CB: Actually, there is almost no doubt that they don't do it because of a set of rules codified in a symbolic kinship system. And that humans do do it for that reason.
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Actually in both apes and humans, there is evidence that there are chemical triggers between brothers and sisters who grow up together in the same group that aid in incest avoidance. But as mentioned incest avoidance among humans can at times involve complicated rules about "cousins" and distance of kin relation, etc.
^^^^^ CB; Demonstrating that human kin systems are qualitatively different from chimp social structure in that the latter is not based on a symbolic system and the former is.
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And of course there are also complicated exceptions among royal families at various times in history, and these exceptions are sanctioned by the gods, etc.. In what sense is there a difference that might called "causal" between these complicated taboos and incest avoidance is part of the crucial question of difference between humans and our ancestors and cousin species. Of course among chimpanzees there are no such complicated rules. The female when she comes of age wanders off to live with the neighboring troop and thus avoids mating with brothers and sisters.
Jerry Monaco
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CB: My position is that symbolling and kinship are invented together in one origin. The first critical symbols being names of people creating a kin system thereby.