As they see it, the roots of altruistic behavior far predate Homo sapiens, and that is why it seems to flow forth so readily once tapped. Recent studies that model group dynamics suggest that a spirit of cooperation will arise in nature under a wide variety of circumstances. "There's a general trend in evolutionary biology toward recognizing that very often the best way to compete is to cooperate," said Dr. Barbara Smuts, a professor of anthropology at the University of Michigan, who has published papers on the evolution of altruism. "And that, to me, is a source of some solace and comfort." Moreover, most biologists concur that the human capacity for language and memory allows altruistic behavior - the desire to give, and to sacrifice for the sake of others - to flourish in measure far beyond the cooperative spirit seen in other species.

With language, they say, people can learn of individuals they have never met and feel compassion for their suffering, and honor and even emulate their heroic deeds. They can also warn one another of any selfish cheaters or malign tricksters lurking in their midst. "In a large crowd, we know who the good guys are, and we can talk about, and ostracize, the bad ones," said Dr. Craig Packer, a professor of ecology and evolution at the University of Minnesota. "People are very concerned about their reputation, and that, too, can inspire us to be good." Oh, better than good. "There's a grandness in the human species that is so striking, and so profoundly different from what we see in other animals," he added. "We are an amalgamation of families working together. This is what civilization is derived from."

At the same time, said biologists, the very conditions that encourage heroics and selflessness can be the source of profound barbarism as well. "Moral behavior is often a within-group phenomenon," said Dr. David Sloan Wilson, a professor of biology at the State University of New York at Binghamton. "Altruism is practiced within your group, and often turned off toward members of other groups."

The desire to understand the nature of altruism has occupied evolutionary thinkers since Charles Darwin, who was fascinated by the apparent existence
of altruism among social insects. In ant and bee colonies, sterile female workers labor ceaselessly for their queen, and will even die for her when the nest is threatened. How could such seeming selflessness evolve, when it is exactly those individuals that are behaving altruistically that fail to breed and thereby pass their selfless genes along? By a similar token, human soldiers who go to war often are at the beginning of their reproductive potential, and many are killed before getting the chance to have children. Why don't the stay-at-homes simply outbreed the do-gooders and thus bury the altruistic impulse along with the casualties of combat?

The question of altruism was at least partly solved when the British evolutionary theorist William Hamilton formulated the idea of inclusive fitness: the notion that individuals can enhance their reproductive success not merely by having young of their own, but by caring for their genetic relatives as well. Among social bees and ants, it turns out, the sister workers are more closely related to one another than parents normally are to their offspring; thus it behooves the workers to care more about current and potential sisters than to fret over their sterile selves.

The concept of inclusive fitness explains many brave acts observed in nature. Dr. Richard Wrangham, a primatologist at Harvard, cites the example of the red colobus monkey. When they are being hunted by chimpanzees, the male monkeys are "amazingly brave," Dr. Wrangham said. "As the biggest and strongest members of their group, they undoubtedly could escape quicker than the others." Instead, the males jump to the front, confronting the chimpanzee hunters while the mothers and offspring jump to safety. Often, the much bigger chimpanzees pull the colobus soldiers off by their tails and slam them to their deaths. Their courageousness can be explained by the fact that colobus monkeys live in multimale, multifemale groups in which the males are almost always related. So in protecting the young monkeys, the adult males are defending their kin.

Yet, as biologists are learning, there is more to cooperation and generosity than an investment in one's nepotistic patch of DNA. Lately, they have accrued evidence that something like group selection encourages the evolution of traits beneficial to a group, even when members of the group are not related. In computer simulation studies, Dr. Smuts and her colleagues modeled two types of group-living agents that would behave like herbivores: one that would selfishly consume all the food in a given patch before moving on, and another that would consume resources modestly rather than greedily, thus allowing local plant food to regenerate. Researchers had assumed that cooperators could collaborate with genetically unrelated cooperators only if they had the cognitive capacity to know goodness when they saw it. But the data suggested otherwise. "These models showed that under a wide range of simulated environmental conditions you could get selection for prudent, cooperative behavior," Dr. Smuts said, even in the absence of cognition or kinship. "If you happened by chance to get good guys together, they remained together because they created a mutually beneficial environment."

This sort of win-win principle, she said, could explain all sorts of symbiotic arrangements, even among different species - like the tendency of baboons and impalas to associate together because they use each other's warning calls.

Add to this basic mechanistic selection for cooperation the human capacity to recognize and reward behaviors that strengthen the group - the tribe, the state, the church, the platoon - and selflessness thrives and multiplies. So, too, does the need for group identity. Classic so-called minimal group experiments have shown that when people are gathered together and assigned membership in arbitrary groups, called, say, the Greens and the Reds, before long the members begin expressing amity for their fellow Greens or Reds and animosity toward those of the wrong "color."

"Ancestral life frequently consisted of intergroup conflict," Dr. Wilson of SUNY said. "It's part of our mental heritage."

Yet he does not see conflict as inevitable. "It's been shown pretty well that where people place the boundary between us and them is extremely flexible and strategic," he said. "It's possible to widen the moral circle, and I'm optimistic enough to believe it can be done on a worldwide scale." Ultimately, though, scientists acknowledge that the evolutionary framework for self-sacrificing acts is overlaid by individual choice. And it is there, when individual fire-fighters or office workers or airplane passengers choose the altruistic path that science gives way to wonder.

Dr. James J. Moore, a professor of anthropology at the University of California at San Diego, said he had studied many species, including many different primates. "We're the nicest species I know," he said. "To see those guys risking their lives, climbing over rubble on the chance of finding one person alive, well, you wouldn't find baboons doing that." The horrors of last week notwithstanding, he said, "the overall picture to come out about human nature is wonderful."

"For every 50 people making bomb threats now to mosques," he said, "there are 500,000 people around the world behaving just the way we hoped they
would, with empathy and expressions of grief. We are amazingly civilized." True, death-defying acts of heroism may be the province of the few. For the
rest of us, simple humanity will do.