Suicide is ubiquitous.

Around the world, in just about every culture, some people decide to take their own lives. It happens far more often than most people realize, making news only in prominent cases, as with Deborah Jeane Palfrey, known as the "D.C. Madam," who apparently chose to kill herself last week rather than face up to 55 years in prison.

More interesting, just about everyone can imagine the urge to end it all, although most of us never get remotely close to acting on that thought.

And yet, suicide, even thoughts of suicide, makes no sense, at least from an evolutionary point of view.

Humans, like all animals, are designed to pass along genes to the next generation. But ending your own life means, in stark evolutionary terms, cutting off, or harming your future reproductive success.

When young people kill themselves, their genes are eliminated from the gene pool; when adults kill themselves they can no longer care for dependent children; when elderly people kill themselves, they, too, abdicate the role of caring parent for the next generations.

Why would such a negative behavior be part of human nature?

The answer is complicated by the fact that any number of emotions and experiences can push a person toward suicide. It might be loss, or loss of hope, or a change in life that makes life not worth living. Or it might be a lifetime of hardship topped by some final misery that makes suicide look more appealing than carrying on.

The answer is also complicated by the fact that the human mind is notoriously fickle. What is overwhelming to one person might be a seen as a temporary glitch to another, and our attitude about life changes over a lifetime. For example, we might be easily despondent in our teens, resilient at 20, and then unable to cope at 40.

Negative emotions also have deep evolutionary roots. Primatologist Frans de Waal of Emory University has shown that chimpanzees and other primates lead complex emotional lives, ones full of happiness as well as negativity. Chimps not only love and care for others, they also hate and become depressed. Although chimps can't talk about their feelings, researcher say these emotions are easily spotted by body language and behavior that mirrors the same emotions in people. Researchers have even watched as chimps die from what looks like a broken heart.

Obviously, sadness is part of life for animals with big brains. The capacity to feel presumably helps us solve problems and survive, and is essential for group living, and perhaps inconsolable depression is simply emotional baggage that tags along with the good stuff. Or maybe unhappiness and a tendency towards suicide is the product of the uncontrolled nature of our quicksilver minds. We think a lot, and our wondering minds are just as likely to think sad as happy.

It's also possible that deep sadness has, in some way, been selected for. Attempted suicide is much more frequent than "successful" suicide. Commonly called a cry for help, these acts do indeed change the life of a survivor as well as the people around them. In the best case scenario, the attempt is seen as a red flag that all is not well, and loved ones step in and make things right.

In an unexpected twist, the most negative of humans acts can become a life saver, and a way to keep genes where they belong — in the gene pool.

Meredith F. Small is an anthropologist at Cornell University. She is also the author of "Our Babies, Ourselves; How Biology and Culture Shape the Way We Parent" (link) and "The Culture of Our Discontent; Beyond the Medical Model of Mental Illness" (link).