Marlene Zuk

RIDDLED WITH LIFE

Friendly worms, ladybug sex, and the parasites that make us who we are

This is a book about parasites in the light of Darwinian medicine. It starts with an excellent chapter on "Why doctors need Darwin". Here Zuk makes it clear that, as evolved organisms, our bodies work 'well enough' but not perfectly. Contrary to what a widespread but sentimental view of our nature assumes, we are not primarily designed to be healthy; natural selection maximises reproduction, not survival or health. This basic truth underlies much of the book.

Many of us today seem to think that to live free from parasites is an achievable goal, but Marlene Zuk's view is that this is an unrealizable ambition. Parasites have evolved along with us and are an inescapable feature of our world, and the best we can hope for is to live with them. Zuk dislikes the militaristic language that many writers use to describe our 'battle' with parasites.

In fact, a parasite-free existence is perhaps not even desirable. As Zuk points out, there is some evidence to show that exposure to parasites early in life protects us against the subsequent development of allergies. It seems possible that the remarkable increase in allergies that has occurred in many industrialized countries in recent times is partly due to excessive hygiene. 'Parasites' here includes viruses and bacteria; young children who encounter a lot of viruses as a result of contact with other children are less likely to develop allergies later.

Zuk discusses the theory that sex has arisen in evolution as an adaptation to parasites and thinks it is correct. But that does not mean that sex is wholly a good thing. Sexual intercourse provides many parasites with the opportunity to infect a new host. And in many species males seem to incur particular risks and costs. They may fight with one another for females, sometimes being injured or killed in the process, and high levels of testosterone, though they often make a male more attractive to females, can also have deleterious effects. As a rule females tend to live longer than males.

There is an optimistic view of parasites which holds that they will always evolve to become less virulent because it is not in the parasite's interest to kill its host. As Zuk makes clear, this happy scenario is by no means universal. All we can say is that the parasite will evolve in a way that favours its own ability to pass on its genes. Sometimes this will result in a decline in virulence, but by no means always; in some cases it is advantageous to kill the host.

Perhaps the most interesting part of the book is Zuk's account of how parasites sometimes modify a host's behaviour. As she remarks, this is an illustration of Richard Dawkins's 'extended phenotype'. She provides many examples of this, but of course the really interesting question is whether the same thing happens in humans. It is difficult to be certain, but the possibility definitely exists. The most likely candidate is toxoplasmosis.

Toxoplasmosis primarily occurs in cats, the intermediate host being rodents. In rats, infection with toxoplasma makes them indifferent to the smell of cats and hence more likely to be eaten, thus infecting the cats. Humans who have been infected from cats or cat faeces seem to have subtle personality changes. They are more accident-prone. Men who are infected are more reserved, less trusting, and more likely to break rules. Women, in contrast, are more out-going, trusting, and self-assured.

It is still not clear whether these differences are due to the toxoplasmosis or are personality features that make certain people more liable to infection. If toxoplamosis is indeed responsible it suggests that humans, thought not the 'intended' target, can show the same personality changes as do infected rats. As Zuk points out, such ideas have profound implications for how we think of ourselves.

Again, few would argue that the pathogen alone dictates our essence, but then few would argue that we are solely the product of any one factor, whether it be our genes, our hormones, our birth order, or our early experiences with toilet training. Personality is the sum of all these things, and it seems artificial to disregard parasites' influence.

30 July 2008