"Exogamy lies far back in the history of man" wrote Edward Tylor long ago, "and perhaps no observer has ever seen it come into existence, nor have the precise conditions of its origin yet been inferred" (Tylor 1889, p 267). Tylor could hardly have foreseen that the answer to this enigma lay in our close relatives. From an evolutionary perspective, exogamy (primitive out-marriage) is simply the incidental by-product of the combination of two otherwise typical primate patterns: between-group transfer and pair-bonding. Female dispersal was presumably the rule in the ancestral male kin group. Upon the evolution of stable breeding bonds, females kept emigrating into a new group as before, but instead of mating promiscuously in it, they formed an enduring breeding relationship; they were "marrying-out" so to speak. Significantly, exogamy at that stage was deprived of any exchange dimension: females were transferring between groups on their own initiative, they were not part of transactions between males. That would come later.
We saw that for Levi-Strauss, female exchange was the primary and most binding form of reciprocity between men, an argument which he based on the empirical observation that throughout the world women are men's "most precious possession." Primatology vindicates this conclusion but through different arguments. Female transfer between groups was the ancestral condition. This helps explains why a female bias in dispersal - not a male one - is widespread cross-culturally. But another, even more basic factor was involved. Throughout the animal kingdom, including primates, females are certainly the most "precious possession" males may compete for, as has been overwhelmingly documented ever since Darwin (1871) 1981) first explained why this was so (for primates, see contributions in Kappeler and van Schaik 2004). In this sense, Levi-Strauss's assertion fits nicely with sexual selection theory.
Even as they were giving rise to a behavioral form of exogamy, stable breeding bonds were generating a primitive form of what anthropologists call postmarital residence. Chimpanzees and bonobos have a dual-phase residence pattern: females spend a prebreeding phase in their natal group, followed by a breeding phase elsewhere. Dual-phase residence is, thus, a phylogenetically primitive pattern. The evolution of pair-bonding transformed that pattern into one comprised of a prepair-bonding phase (or "premarital" phase) spent in the natal group, followed by a postpair-bonding (or "postmarital") phase spent in the new group. Like exogamy, "postmarital" residence emerged from the integration of pair-bonding to male philo-patry, a fusion that produced an embryonic form of patrilocality. To many sociocul-tural anthropologists, in contrast, prior to the invention of the incest taboo, residence had been a single phase spent in one's natal group (e.g., Murdock 1949, p 16).
Table 2.1 summarizes the state of the exogamy configuration immediately after the evolution of stable breeding bonds (Phase II). Compared with the previous stage, several new traits have emerged, but several others are still lacking. At this point in human evolution, hominid groups were independent entities like all other primate groups. Once individuals moved out of their natal group, they ceased to interact with the relatives they left behind. Social life was limited to one's local group. The remaining components of the exogamy configuration had to await the extension of social structure beyond the local group (phase III). They are attributes of between-group alliances, or supragroup social structures. For the sake of simplicity, I use the term tribe in a generic sense to refer to such entities.
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