As a teenager I suffered from hay fever, but not from rabbit's tic or neuralgia rosa. Why? Because only the first of these is a real disorder; the others are pure invention. In principle, though, they too could exist. We know how pollen triggers the cells of the hay fever sufferer's immune system into releasing a flood of histamine, the cause of all the undignified misery. It would not be hard to dream up some physiologically plausible chain of events to explain, say, why hearing a pure tone of a particular frequency might trigger a nervous twitch of the muscles around the nose (my rabbit's tic); or why prolonged exposure to intense red light could be followed by a sharp pain at the front of the skull (the imaginary neuralgia rosa). A priori such disorders are no more improbable than postulating a violent sneeze in response to sniffing certain types of pollen (hay fever); they just happen not to exist. How arbitrary are the ills to which flesh is heir.
Or are they? The Darwinian approach to an already lengthy list of ailments has revealed that bodily failures are anything but arbitrary. The many varieties of human disease are turning out to fit a pattern as sweet as the tidiest mind could hope for. My own first insight into what is now called Darwinian or evolutionary medicine came more than 20 years ago. I was at a conference in Davos organised by a drug company to celebrate an asthma product of which the firm's medical director was proud. In keeping with the "educational" nature of the event, much of the proceedings had nothing to do with the drug-including one paper which became, for me, the highlight of the event.
Its authors set out to explain why there is such a thing as hay fever. They explained that this and most other forms of allergy are the minor drawbacks of a branch of the immune system which originally protected us against parasitic worms. Throughout our evolution, such worms must have been ubiquitous, and seriously damaging to health. Natural selection would therefore have favoured the emergence of effective defences against them. The drawbacks of a system sensitive enough to be triggered from time to time by things which are not parasites (pollen, strawberries and so on) would have been outweighed-so the argument goes-by the advantages of a guaranteed response when the genuine article really had smuggled itself into the body.
But that was then and this is now. In an age when most humans are no longer troubled by parasites, and so cannot reap the benefits of this defence system, we are none the less stuck with its drawbacks. Although this is a very neat explanation for allergy, no one can be sure whether it is correct. But it does give the flavour of Darwinian medicine. One of its recurring themes is that a physiology and psychology adapted through millions of years of natural selection to a hunter-gatherer life cannot necessarily be expected to cope well with the circumstances of 20th century industrial living.
One of the tribunes of evolutionary medicine, Michigan psychiatrist Randolph Nesse, visited Britain recently to take part in a Darwin seminar organised by the Centre for Philosophy of Natural and Social Sciences at the LSE. These seminars have been running for a couple of years now, invigorating the LSE with an influx of real scientists, all eager to show how human evolution can provide a framework for understanding ourselves, our culture, even our future.
On this occasion Nesse asked: how far is it wise to block all mental pain? Another tenet of evolutionary medicine is that disease symptoms are not all of a piece. Some, such as fits or seizures, are real defects; but others-such as coughs, fevers and vomiting-represent the body's defences in action. These serve a purpose; and we should be circumspect in our efforts to suppress them. To what extent, asks Nesse, is mental illness, depression in particular, also adaptive? In what way might a low mood-sadness-be useful?
Depression is so common that keen Darwinians are convinced that it must have evolved for a reason. Nesse says that you can see some logic in sadness if you try. Following the accidental loss of a child, say, this emotion can stimulate attempts to understand how this occurred, to change behaviour in ways which will avoid the accident happening again. The explanations may seem primitive-even risible. They often are. But then we have scarcely begun to think about them.
There is more to this than intellectual curiosity. Anti-depressant drugs are big business. Consumption is rising. Nesse is convinced that within a decade or two depression will be optional: an experience, like physical pain, which we may choose to avoid. This worries him. It is not that he is puritanical or masochistic; rather that he sees us as hell-bent on eliminating low mood states without any clear idea of what unrecognised purpose they might be serving. A further parallel with physical pain points out the potential danger. We know that pain warns us when something is wrong. Abolish the capacity to feel pain and you lose essential information: the hot plate has time to burn before you drop it; frostbite sets in before you realise. Likewise, only when we have eliminated sadness may we realise why we needed to feel it in the first place.
