The foregoing discussion points to the nature of the emerging tribe. At this stage in its evolution, the tribe was merely a set of interbreeding local groups exhibiting some levels of tolerance with one another, and the number of local groups forming a tribe was determined by the exact pattern of female transfer between groups. Simple models of female transfer make it possible to specify some of the conditions favoring male pacification through the formation of kinship and affinity bridges between groups (Chapais 2008). These models indicate (1) that male pacification ensues whether female transfer is unidirectional or bidirectional between groups, but that bilateral transfer promotes further congeniality, (2) that the pace of pacification between any two groups depends on the proportion of females in one group moving to the other group: the larger that proportion, the larger the number of kinship bridges between the two groups, and (3) that transfer between groups of substantially different size works to the disadvantage of the smaller group. Combining the three principles, one may infer that intergroup pacification was especially favored in situations where female circulation was bidirectional and concentrated among a small number of groups that were not too dissimilar in size.
If the composition of the primitive tribe reflected the exact pattern of female transfer between its constituent groups, that pattern was itself determined by the geographical distribution of groups. In chimpanzees, for example, female transfer was reportedly frequent between communities whose home ranges overlapped extensively, but infrequent between communities whose home ranges did not overlap (Kawanaka and Nishida 1974; Nishida 1979). Considering that the main factors affecting the geographical distribution of local groups are the presence of physical barriers between them and the distribution of food resources and predators, one may envision the first tribes as regional entities whose constituent local communities "exchanged" females and enjoyed various levels of peaceful relations with one another.
Importantly, as between-group hostility markedly decreased within the tribe, it remained at its prior level between tribes. This aspect of the present model helps resolve the discrepancy between chimpanzees and human foragers with regard to intergroup patterns of violence. Compared with chimpanzees, human hunter-gatherers are much more egalitarian and display substantially lower levels of intergroup competition and violence. So striking is the difference that some authors spoke of a phylogenetic discontinuity between chimpanzees and human foragers as far as patterns of violence were concerned (Knauft 1991; Kelly 2000). But as argued by Rodseth and Wrangham (2004), the local band of hunter-gatherers is not the right social unit for a meaningful comparison with chimpanzees—the tribe is (see also Crofoot and Wrangham, this volume).
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