A Queasy Feeling

Why other things cause nausea and vomiting is more difficult to explain, but scientists are beginning to see some sense in nature's design. You'd think that pregnancy sickness, for example, would be evolutionarily disadvantageous, since a growing embryo needs nutrition. In a famous 1992 paper, however, the evolutionary biologist Margie Profet made a compelling case that pregnancy sickness is actually protective. She pointed out that natural foods that are safe for adults commonly turn out to be unsafe for embryos. All plants produce toxins, and in order to be able to eat them we have evolved elaborate detoxification systems. But these systems don't eliminate harmful chemicals completely, and embryos can be sensitive to even tiny amounts. (For example, toxins in potatoes have been found to cause neural malformations in animal fetuses, even at levels that are nontoxic to their mothers; indeed, Ireland's heavy potato consumption may account for its having the world's highest rate of neural defects, such a spina bifida.)

Pregnancy sickness, Profet suggested, may have evolved to reduce an embryo's exposure to natural toxins. She pointed out that women with pregnancy sickness strongly prefer bland foods that do not spoil easily (like breads and cereals) and are particularly averse to foods associated with high levels of natural toxins, such as bitter or pungent foods and animal products that are not extremely fresh. The theory also explains why sickness occurs mainly during the first trimester. That is when the embryo develops organs and is most sensitive to toxins; at the same time, it is small and its calorie needs are easily supplied by the mother's fat stores. Overall, women with moderate to severe morning sickness have a lower rate of miscarriages than women with mild nausea or none at all.
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Researchers have now established that motion sickness occurs when there is a conflict between the motion we experience and the motion we expect to experience. Merely to balance our heads on our shoulders, our bodies on our hips and feet, we require an incredibly fine-tuned "body sense" - a system that learns to anticipate motion based on input from vision, muscles, and especially, the inner ear. Nausea arises when the brain receives unanticipated sensory inputs - for someone new to boats, say, feeling the ground beneath him pitch up and down, or, for someone in a virtual-reality helmet, seeing oneself move through the world while one's body knows its is standing still. (Taking the wheel of a vehicle helps, because one can have more control and feel for how one is moving.) To put it simply, motion sickness is really sickness from unfamiliar motion.

But why does unfamiliar motion make us feel so miserable? A leading explanation returns to the notion of nausea and vomiting as something that protects against toxins. During the Pleistocene epoch, when our species evolved, people had no occasion to experience sustained passive motion, as they do today, on a boat or in a car. Much the same sensation can occur with the ingestion of many hallucinatory toxins, however - as anyone who had drunk too much alcohol can attest. So the nausea and vomiting that comes with motion sickness may be a modern by-product of our standard system for expelling poisons and nurturing avoidance of them. This theory is not nearly as well examined as the explanation for pregnancy sickness, however. And we still don't have a convincing explanation of why anxiety or the sight of blood or of vomit itself should make people sick.