The ancient roots of modern melancholy
Did depression help our ancestors survive?
The fifteen-year-old sits alone on the ground in a clearing, head sadly drooping, knees hugged to her chest. A tuft of brown hair and big brown eyes are barely visible as she curls into herself. Nearby, her friends are ignoring her, as they cavort on the lawn and share a snack of fruit. No matter that this solitary "teen" is actually a full-grown Indonesian monkey. Rocking herself slowly, she could be any lonely, depressed person.
Depression robs people of happiness, and sometimes of their lives. But according to University of Michigan psychiatrist Randolph Nesse, depressionlike symptoms in apes and monkeys, the closest living models of our early forebears, suggest that depression is not a uniquely human - or even a modern - illness. More important, some features of the illness, such as lack of motivation, may have actually helped our simian ancestors survive some tough situations. For this reason, these once adaptive features have been passed on to us. By understanding why this happened, Nesse writes in last month's Archives of General Psychiatry, we might better understand how a survival strategy turned into an illness that today ravages 19 million Americans. And we might learn better ways to treat it.
Disease across time. The idea starts with Charles Darwin's notions about the survival of the fittest. Indeed, this approach to sickness is called "Darwinian medicine." The 19th-century evolutionist argued that traits that help a creature survive to produce children - such as resistance to infections - get passed on to future generations. Traits that don't help eventually disappear from a population. So why, Nesse asks, does depression afflict so many millions of people? Why hasn't natural selection screened out genes that make us so susceptible, especially to a disease that strikes during the prime childbearing years?
His answer: because those same genes, once upon a time, did help us to survive. "Low energy, for instance, stops you from banging your head against a brick wall," says behavioral biologist Carol Shively of Wake Forest University School of Medicine in North Carolina. You can see how this works in the monkeys that she studies. They live in social groups that range from about five animals to about 20, and there's a pecking order. "As a subordinate male, trying to get access to food and mate with females, you find yourself fighting the dominant monkey a lot. And you risk getting hurt or killed," Shively says. "Also, there's constant low-level harassment from the rest of the group. Every other monkey redirects aggression at you."
This is, to say the least, a stressful way to go through life. And monkeys in the middle of it have high levels of the stress hormone cortisol. Over time, elevated cortisol actually kills off brain cells. "So your brain doesn't learn, and your emotions don't work properly," Shively says. "You get a real breakdown in brain function." Low-ranking females, her colleague Jay Kaplan has found, don't have normal estrogen production, and this leads to atherosclerosis and bone loss.
Monkeys in these groups seem to have hit upon a behavioral remedy for such chronic stress: strategic withdrawal. "The monkeys spend more time alone, out of physical contact with other monkeys," Shively says. "These animals look very much like depressed people. But by withdrawing, they have fewer chances to get beat up. It's actually an evolutionarily sound strategy. They get to stay in the group and function. Not optimally, but you survive. You still have a chance to reproduce." And monkey groups are very fluid: With a change in members, which can happen monthly or even weekly, a marginal monkey could very well end up on top again.
But people live longer than monkeys, Nesse says, and don't change jobs or family groups from month to month. That may be how these gray moods crossed the line from adaptation to disease. The genes are still there, but repeated traumatic events could produce a more exaggerated response from them, triggering a deeper, longer withdrawal from life into a black hole, from which it's hard to climb out.
This is, of course, speculation. And psychiatrist Peter Kramer, author of the bestselling antidepressant bible, Listening to Prozac, points out that depressed people often make impulsive decisions, such as bailing out of a job or a marriage, which don't seem to fit with the idea of a strategic, temporary pullback. He and Nesse do agree, though, that the idea highlights questions that therapists could ask more often. Is a depressed person trying to be perfect, trying to please a demanding parent, or trapped in some other futile situation? That could be triggering a withdrawal response. And people, unlike monkeys, can't count on their social world to change for the better every month or so.
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