History Readings

That process is gaining speed. The railway company Deutsche Bahn promised earlier this year to change its signs after a former school principal complained to his local member of Parliament about the use of English-language terms like “hot line,” “service point” and “kiss and ride.” Between 2000 and 2009, the share of German-produced acts in the Top 100 album charts rose to 36.7 percent from 19.5 percent.

Each organism faces a trade-off between putting effort into raising its own offspring or helping its relatives. If the benefits of helping a relative outweigh the costs, Dr. Hamilton argued, altruism can evolve.

Standard natural selection, the scientists argue, explains everything inclusive fitness theory was supposed to, without these special conditions

Dr. Nowak and his colleagues argue that their analysis should free scientists to think of other ways that altruism and other kinds of social behavior might evolve.

It also created a new and more complex social structure. The bands who exchanged women with each other learned to cooperate, forming a group or tribe that would protect its territory from other tribes. Though cooperation became the norm within a tribe, tribes would wage warfare just as relentlessly as chimpanzee bands.

A system of cooperative bands “provides the kind of social infrastructure that can really get things going,” he said. In a series of experiments comparing human and chimpanzee infants, Dr. Tomasello has shown that very young children have an urge to help others. One of these skills is what he calls shared intentionality, the ability to form a plan with others for accomplishing a joint endeavor. Children, but not chimps, will point at things to convey information, they will intuit others’ intentions from the direction of their gaze, and they will help others achieve a goal.

Cooperation may have been forced on them as a condition of existence. “Humans were put under some kind of collective pressure to collaborate in their gathering of food — they became obligate collaborators — in a way that their closest primate relatives were not,”

Humans wear the mark of their shared intentionality, he notes, in a small but significant feature — the whites of their eyes, which are three times larger than those of any other primate, presumably to help others follow the direction of gaze.

How did a chimplike society ever give rise to the egalitarian, largely monogamous structure of hunter-gatherer groups?

Dr. Chapais sees the transition as a series of accidents, each of which let natural selection exploit new opportunities. Early humans began to walk on two legs because it was a more efficient way of getting around than knuckle-walking, the chimps’ method. But that happened to leave the hands free. Now they could gesture, or make tools.

It was a tool, in the form of a weapon, that made human society possible

As soon as all males were armed, the cost of monopolizing a large number of females became a lot higher. In the incipient hominid society, females became allocated to males more equally. General polygyny became the rule, then general monogamy.

With only one mate, for the most part, a male had an incentive to guard her from other males to protect his paternity.