O.K. I'll take the hook.
One reason why the incest taboo is relevant to the conversation is that the prohibition against incest is not exclusively a prohibition of homo sapiens. All apes observe, in practice, an avoidance of incest, to the extent that it is at least behaviorally prohibited. They do this through either female transfer to another troop or male transfer to another troop, depending on the species and sometimes depending on the density of the troop, and abundance or scarcity of resources. Of course an incest prohibition does not make an incest "taboo". The way the word "taboo" is used it implies the sense of symbolic representation or myth or symbolically represented "rules" and norms. (Thus the prohibition is represented by the "symbolic", the imaginary, and the real, and other (sometimes) interesting Lacanian absurdities.) These are the only differences between the prohibition of incest in a ape troop and the prohibition of incest among, say human hunter-gatherers. But if you define the incest prohibition as "symbolic" from the start, then you can define it as exclusively human and thus once again, by the kind of circular definition, as pointed out by brother Shane Mage, avoid the notion that you don't have to pay attention to kinship relations among chimpanzees or other apes. As a note I would like to add that what led the Marxist anthropologist Maurice Godelier to consider primate studies in the first place was precisely the similarity between incest avoidance as practiced among chimpanzees and bonobos and incest avoidance as practiced among humans.
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? 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. 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