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Conceiving man's genetic -- and social -- ancestors:

Fossil bones record the history of the human form but they say little about behavior. A richer source on the way human social behavior evolved may come from chimpanzees, with whom people shared a common ancestor as recently as five or six million years ago.

From knowledge of chimp behavior, biologists can plausibly infer the social behavior of the shared human-chimp ancestor, and from that reconstruct the evolutionary history of human social behavior ...

A major surprise has been that chimps turn out to live in territories whose borders are aggressively defended by roving parties of males. Jane Goodall, who pioneered long-term studies of chimps at Gombe, at first believed she was watching a single peaceful community. But as researchers started to follow animals throughout the day and watch their interaction with others, they found that groups of male chimps went out on border patrols, ready to attack and kill the males of neighboring communities.

The males in each community are related to one another because they spend their lives where they were born, whereas the females usually migrate to neighboring communities soon after reaching puberty, a practice that avoids inbreeding. This patrilocal system, of a community based on male kin bonding, is unusual, but familiar to anthropologists because it is practiced by most hunter-gatherer societies.

The males' operational strategy seems to be to defend a territory as large as possible so as to improve the community's food supply, which is principally fruit, and thereby their reproductive success. Dr. Anne Pusey of the University of Minnesota has found that the larger the female chimp's home feeding area, the shorter the interval between births.

In two known cases, a chimp community has wiped out all of a neighbor's males. Though the females may be absorbed into the victors' community, the basic goal seems to be getting rid of a rival rather than capturing females, since male chimps often attack strange females.

Within a community, there is a male hierarchy that is subject to what primatologists euphemistically call elections. Alpha males can lose elections when other males form alliances against them. Losing an election is a bad idea. The deposed male sometimes ends up with personal pieces torn off him and is left to die of his wounds.

Very few other species live in male-kin-bonded communities with female dispersal. And only two practice lethal raids into neighbors' territory to kill off vulnerable enemies. "This suite of behaviors in known only among chimpanzees and humans," Dr. Wrangham and Dale Peterson write in their book Demonic Males ...

A community size of 80 to 100 people, typical among chimps and hunter-gatherers, is one feature inherited from the common ancestor. Another is a society formed on the basis of male kin bonding ...