Art is said to imitate life, or perhaps it is the other way round. In either case, the 50th anniversary of the movie version of "West Side Story" provided a convenient excuse to see it again, and this time it invoked thoughts of how unusual and interesting humans really are.

For those who have not seen it, "West Side Story" is the "Romeo and Juliet" romantic tragedy set in 1950s New York City, where two rival gangs fight over neighborhood turf. The Sharks are a gang of recent Puerto Rican immigrants who are beginning to encroach on the neighborhood controlled by the Jets, white kids born of European immigrants. Although violence provides the tension in the story, the qualities that define humanity occur between those scenes.

Scientists are very interested in understanding what it means to be human. One way to address this question is by studying the common chimpanzee, humanity's closest living relative, and determining how we differ from it.

Male chimpanzees belonging to the same social group occasionally travel into neighboring territory, looking for chimps that occupy that area. This is no boys' night out of light mischief. These patrols travel far, eat and socialize very little, and remain quiet. When the invading band finds a male, it is attacked and killed or mortally wounded. The term for this behavior is lethal intergroup aggression, and it interests anthropologists because it resembles human warfare.

To determine why chimps do this, researchers studied lethal intergroup aggression in a community at Ngogo in Kibale National Park, Uganda, over a 10-year period. They found that occasional lethal attacks by the Ngogo chimps over that period correlated with the annexation of land previously occupied by the victims' group. The invaders no longer patrol that territory, they occupy it. Like the Jets and the Sharks, it's all about turf. The territorial expansion gives the group more access to food, the females reproduce more often and the community grows in size.

Human warfare is also the acquisition of resources by lethal intergroup aggression. Access to resources in the new territory may be accomplished by displacement of the indigenous population or by co-existence with the vanquished group. The studies of chimps suggest that this lethal aggression is a successful evolutionary adaptation, and that the disposition to warfare is at least partly hard-wired in our genes. The Sharks and Jets may simply be acting out this genetic impulse. The chimp studies may help explain the evolution of human warfare, but they also make clear that war is not a defining human trait.

There may be some parallels between the patrolling Ngogo chimps and street gangs, but in fact they differ in a very important way. Chimps form relatively simple societal groups based primarily on kinship. Cooperation between unrelated individuals or groups is uncommon. By contrast, the Jets are a collection of unrelated young men of Irish, Polish and Italian descent. When they get together they socialize, make plans and care for each other, not to mention sing and dance. The Sharks probably would not have known each other in Puerto Rico, but here they work as an interdependent and cohesive unit. There is no counterpart to the Jets or Sharks in chimp society.

The Ngogo chimps do not enter neighboring territories in order to exchange goods and services or establish conduits of communication with the inhabitants. But humans do exactly that. What is uniquely human is the extensive interaction and cooperation of unrelated members to form complex social networks and societies despite the predisposition toward aggression.

Whether foraging for food or shopping at the supermarket, bartering or trading on Wall Street, humans interact most often with unrelated members of their species. This behavior is so universal that it is difficult to step back from it to observe how odd it really is. Cooperation among strangers, not war, is a defining behavior of humanity.

Scientists interested in understanding the evolution of human society recently identified common traits among 32 present-day hunter-gatherer groups. These populations are useful because they represent the predominant human social structure for 95 percent of our history. This study and others conclude that long-term and monogamous conjugal relationships are nearly universal in humans, and occur to an extent unprecedented in other primates. These unions create stable bonds between unrelated groups.

Had Tony and Maria, the lovers in "West Side Story," been successful, their union would have forged a bond between rival groups, contributing to mutual tolerance.

So, why does conjugal pairing in chimps fail to promote bonding of unrelated groups? In that species, one member of the conjugal pair, usually the female, leaves the natal group and loses contact with it permanently. Thus, intergroup ties are not established. Humans are unique in that both partners usually retain contact with parents, siblings and other members of the group. They join a new group while also remaining part of the old one.

This means that males recognize in-laws and others from outside of their group, resulting in tolerance of people that they would otherwise be hostile toward.

In early humans, this tolerance created opportunities to forge additional links between groups based on mutual interests rather than kinship. Cooperation is usually a better idea then aggression.

Although "West Side Story" ends tragically, the Jets and Sharks realize that their war is unsustainable, and work toward cooperation. They can do this because they are human.


Mark O'Brian, Ph.D., is a professor of biochemistry at the University at Buffalo.