As pointed out earlier, chimpanzees and bonobos are territorial: they avoid other groups and may even attack strangers. In chimpanzees, intergroup fights are initiated and conducted by adult males, and the targets include other adult males, infants, and, sometimes, mothers. The local community is, thus, the most inclusive level of social organization in our closest relatives. Assuming that early hominids were territorial, it follows that a necessary condition for the evolution of between-group alliances - the tribal level of organization - was the pacification of adult males living in distinct groups. The issue of the origin of the tribe brings Levi-Strauss's concept of the atom of kinship to the fore and, again, stable breeding bonds appear to hold the key to this major transition in human evolution.
Figure 2.9 pictures two hominid groups after the evolution of stable breeding bonds. The focus is placed on female Ego, born in group A and pair-bonded in
philopatric groups after the evolution of stable breeding bonds. The processes are illustrated by focusing on a single female from group A (black circle) after she has transferred to and pair-bonded in group B. The female has children in group B and is recognized by her paternal kin living in A. The female also acts as an intermediary between her kin living in A and her husband and his relatives philopatric groups after the evolution of stable breeding bonds. The processes are illustrated by focusing on a single female from group A (black circle) after she has transferred to and pair-bonded in group B. The female has children in group B and is recognized by her paternal kin living in A. The female also acts as an intermediary between her kin living in A and her husband and his relatives group B. Suppose the two groups were to meet at their common border in some nonaggressive way: intergroup meetings have been reported to occur from time to time in bonobos (Idani 1990), but have not been observed in chimpanzees. In the context of such meetings, female Ego would recognize, in addition to her mother and maternal siblings, her father, grandfather, and uncles living in group A, and she would be recognized by them. This could not be the case prior to the evolution of stable breeding bonds, Ego not having experienced a preferential bond with her father. Minimally, a male would be disinclined to attack his daughter, granddaughter, or sister, so Ego would have benefitted from some kind of immunity from her male relatives. The same principle applies to all transferred females and to both directions (group B females transferred in group A), hence to a significant fraction of individuals in both groups. Moreover, a female's immunity against aggression should extend to her own offspring: A male who refrained from attacking his daughter or sister should also refrain from attacking the individual that his daughter or sister carried on her back or belly. Male chimpanzees are known to attack and kill the infants of isolated mothers when they come across them at their common border. From the time males could recognize such individuals as their close kin, male infanticidal attacks should have dropped. In sum, owing to paternity recognition and its impact on agnatic kinship, group A males would be collectively inhibited from attacking several females living in group B and, reciprocally, group B males would be collectively inhibited from attacking several group A females. A state of mutual, though fragmentary, tolerance stemming from the existence of several "kinship bridges" between intermarrying groups would prevail.
Concurrently, another, distinct process would favor between-group pacification, this one involving the mediation of affines. In-laws are the relatives of one's spouse, or the spouses of one's relatives, depending on one's viewpoint. Cognitively speaking, the recognition of in-laws is similar to kin recognition. It requires no more than the ability to recognize preferential bonds between others - e.g., between one's daughter and the latter's husband. When groups A and B came into contact, ego's father could recognize his daughter's husband (his son-in-law) and, reciprocally, ego's husband could recognize his father-in-law. Importantly, from an evolutionary perspective, relationships between in-laws were bound to be, fundamentally, relationships between potential allies. Brothers-in-law, for instance, share a vested interest in the same female, one as a husband, the other as a brother. Both males derive benefits from the female's well-being, the husband through his own reproductive interests with his wife, the brother by virtue of his genetic relatedness with her - inclusive fitness benefits. Crucially, this shared interest is not impeded by sexual competition between the two males: owing to incest avoidance, a brother does not compete with his sister's "husband" for sexual access to his own sister. Minimally, therefore, brothers-in-law should refrain from attacking each other, as should, for that matter, fathers-in-law and sons-in-law and other affines. The importance of the affinity route in the pacification of intergroup relations can hardly be overstated because it is about peaceful relationships between adult males, the individuals directly responsible for intergroup conflicts.
The foregoing reasoning presupposes that interbreeding groups met sporadically at their common border in some nonaggressive way, in which case the structure of kinship and affinity bridges just described would be activated. But if interbreeding groups never came into contact in the first place, pacification could not start. In other words, pair-bonding and the expansion of kinship were a necessary condition for pacification, but not a sufficient one. For pacification to get going, some factors had to favor nonaggressive meetings between groups, such as those described for bonobos (Idani 1990). These factors might have operated through a reduction of the levels of feeding competition between groups, an increase in the opportunities for using the same resources simultaneously (food, water, or shelter), and/or an increase in the benefits of between-group cooperation against either other groups or other species. This point needs further investigation.
In the pacification processes envisioned here, the alliance between groups A and B hinges on female Ego who is simultaneously bonded to her male kin in group A and to her husband in group B; or, in the words of Edward Tylor's, on "the peacemaking of the women who hold to one clan as sisters and to another as wives" (Tylor 1889, p 267). The father-daughter/wife-husband triad is a dual-link chain, with female Ego acting as a swivel joint between the two groups. The same applies to the brother-sister/wife-husband triad. Each of these two Ego-centered chains embody simultaneously, the kinship basis and the affinity basis of between-group alliances. Taken together, they may be described as the atom of between-group alliances, the smallest social element involved in between-group social structures. This paraphrase of Levi-Strauss's "atom of kinship" is more than merely analogical. Levi-Strauss restricted the atom of kinship to the brother-sister-husband triad, neglecting the father-daughter-husband triad. He also erroneously ascribed the atom of kinship to brothers repressing a built-in drive for incest and renouncing marriage with their sisters, and he included the sister's children in it. Notwithstanding these differences, the atom of kinship and the atom of between-group alliances are basically the same thing, structurally speaking: a kinship bond connected to a pair-bond through the intermediary of ego. Levi-Strauss's atom of kinship, the hub of reciprocal exogamy, does have an evolutionary history.
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