Today is Charles Darwin's 204th birthday. Coincidently, Darwin was born the same day, same year, as Lincoln. Check this out: "The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick" he wrote.
If Darwin were alive today, he would probably say Fox News makes him sick, but that's another story. What was going on with Darwin was this: His work-in-progress theory of natural selection was based on the idea of adaptive advantage, such as strong wings to chase down prey and avoid predators. With enhanced survival came enhanced mating opportunities, which raised the odds of a particular trait (such as strong wings) being passed on from generation to generation.
But how did peacocks fit into this? Their showy tails were clearly a burden that worked against their survival. Natural selection should have favored the birds with the smaller tails. Then Darwin had his Aha! moment. The tails proved an advantage in procuring a mate, he reasoned. What happened to the poor bird after that was irrelevant. As Richard Dawkins, author of "The Selfish Gene," put it: "The chicken is only an egg's way of making another egg."
This brings us to the critical issue of trade-offs, the stock-in-trade of evolutionary biology. Without the ability to cough, for instance, we would all die of pneumonia. Understanding trade-offs is the key to understanding why depression, mania, anxiety, and other supposedly disadvantageous traits get passed on from generation to generation.
At the 2005 American Psychiatric Association annual meeting, I heard Randolph Nesse of the University of Michigan lecture precisely on this topic. Dr Nesse asked us to imagine a distant ancestor of ours at an ancient watering hole. The poor guy hears a sound behind him. A lion? A monkey? Even if it's just a mouse, panicking first and thinking later is not such a half-bad idea.
Dr Nesse compared the brain's limbic system to a smoke detector that is programmed to deliver 1000 false alarms for every genuine alert. The false alarms are the price of survival. Better to be too anxious than dead. There is a major catch, though. As Dr Nesse put it: "Human biology is designed for stone age conditions."
In other words, a cave man dose of anxiety may not be well-suited to our modern environment. A little bit of too much anxiety - fine. Too much of too much anxiety - not so fine.
As for the adaptive advantages of depression and mania, I urge you to do your own thought experiments. You may be surprised by what you come up with. Please share your insights by posting a comment.
In the meantime, Happy Birthday, Charles. Two centuries on, you are an inspiration.