"The peacock's tail is a wild extravaganza," says Cronin. "It's a burden, unnecessarily bright and gaudy. The peacock could well be better off without it, in a way that you couldn't say the cheetah would be better off without its sprint or the wren without its camouflage. How do we explain this wild extravaganza that takes a lot of resources, doesn't produce anything, is heavy to tote around, and marks the bird as a target for predators?"

Why has nature designed something so useless? As useless as being nice to the other guy? As useless as sharing information? As useless as committing your life to pursuing an idea whose outcome you can't possibly know? Reputation, says Cronin, is a key element in competition. "Once you understand that sexual selection is displaying qualities like kindness or goodness, or is demonstrating that you can afford to give things away, then you understand the close connection between flamboyance and altruism. Altruism can be one of those evolved peacock feathers in our minds."

Physicists don't believe that in 100 years there will still be Einsteinian physicists. But in 100 years, biologists will still be Darwinians. "Once you understand that we are evolved animals, then everything has to be Darwinian," says Cronin. "That economics could treat us as pure, rational 'choice entities' is sadly mistaken. We're not; we're human beings." It's one of those rare instances in science where the founder of an idea continues to affect everything we desire: what we wish and what we don't wish. Darwin's radical finding is that ours is a world of "man from monkeys." For neo-Darwinists, there's an even more radical conclusion: It's also humans who make angels -- the symbol of altruism.

Harriet Rubin (hrubin@aol.com) is a Fast Company contributing editor. Her new book, "Soloing: Realizing Your Life's Ambition," will be published this month by HarperCollins. Her Web site (www.ivillage.com/thesoloist) also debuts this month. You can contact Helena Cronin by email (cronin@lse.ac.uk).

Sidebar: How Altruistic Are You?

The characteristics of altruism are of greatest value in a fast-changing environment, where people are frequently called on to trust strangers, invest in new companies, or make deals with people they have just met. To get a sense of your own evolutionary trajectory, ask yourself the following questions:

How much do you value high risk?

Taking risks is a primary characteristic of altruists. For example, altruists are apt to jump into a lake to save somebody they don't know. Other ways that risk and altruism intersect include camaraderie in battle -- that is, risking your life for someone who is not related to you or for an idea in which you believe.

Are you concerned with your own view of your reputation?

How do you behave when there is no one around to judge? For example, do you leave a tip in a town where you'll never be seen again? An altruist would behave appropriately, so she could think of herself as a person who does worthy and upright things.

How good are you at detecting when people are being kind for selfish reasons and when they are being altruistic?

Noticing altruism in others is a trigger for reciprocity, and it starts at the youngest ages: A very young baby can respond to a smile. The skill in adulthood is discerning between a real smile and a fake one.

Do you feel empathetic or sympathetic to others' situations?

There is a physical test for this: How much do you attempt to read other people's minds to learn their concerns? Eye gaze is the defining factor, says Helena Cronin. "You look toward someone else, and I follow your eyes and look at someone else. A baby develops eye gaze when he sees an adult stare away, and he follows that gaze. That baby can put himself into someone else's life. It's part of our psycho-logical machinery that natural selection has given us to be able to be good reciprocators."

If you notice these altruistic tendencies in yourself, you may worry that this type of behavior is dangerously unstrategic. Instead of worrying about being too altruistic, pretend you are a gene, Cronin suggests. Otherwise, economic behavior makes no sense. "Genes are the strategists," she says. "It's no good for nature to build a perfect bird if that bird won't sit on its nest and hatch its young. Behavior is strategies adapted by genes as they pass down the generations -- including genes for altruism."

Altruism, says Cronin, is dangerous to the individual -- and good for the species. We put ourselves in jeopardy to be altruistic, the same way many animals will act as sentries for their tribes. To think of ourselves as strategic, in Darwinian terms, is a mistake. Says Cronin: "Essentially, humans have no strategy. It's an illusion to think of ourselves as rational. We have animal brains in our bodies."

Sidebar: The Undiscovered Darwin