"If you ever cook meat in this house again, I'm leaving." Those were not the words of a vegetarian but of a woman eight weeks pregnant with a whiff of steak up her nose--my nose. Usually, I was happy to chow down on a hunk of meat, and I hadn't lost my appetite for most other foods. But suddenly, the smell of broiling cow had me heading for the toilet. Of all the things I could have hated, why meats?

New mothers, guided by their doctors, have attributed such aversions to the hormonal storm brewing inside them, or to the roller-coaster emotions that attend pregnancy. But in the late 1980s, biologists such as Margie Profet, of the University of California at Berkeley, offered a new explanation: Morning sickness, she said, could be an evolutionary adaptation, one that protects a vulnerable fetus from natural toxins. By avoiding certain foods during pregnancy, women may improve their chances of having healthy children--and so pass on the same aversion to future generations.

This summer, that idea became science when Cornell University graduate student Samuel Flaxman and his adviser, neurobiologist Paul Sherman, published the results of a cross-cultural study in The Quarterly Review of Biology. Their data confirmed the outlines of the evolutionary theory, but the details came as a surprise. Whereas Profet and others suggested that primarily vegetables and spices trigger morning sickness, the new study fingered meat, fish, poultry, and eggs. Many women avoid these foods early in pregnancy when fetal organs are forming, Flaxman and Sherman found. And the evolutionary reason seems clear: Meat was once the food most likely to carry parasites and pathogens that could harm a fetus, as well as put a mother at risk.

"Morning sickness is a misnomer," Paul Sherman explained recently, over lunch at a Vietnamese restaurant on the edge of the Cornell campus. "It's not always in the morning but happens all day long for most women. And it's not a real sickness, but really something positive." Getting sick makes sense early in pregnancy, Sherman says. When the body is invaded by something--be it a virus or a ball of foreign cells called an embryo--nausea often results. Estrogen, progestin, and other hormones clearly mediate this response, and studies show that women who have some form of sickness are less likely to miscarry. But contrary to conventional wisdom, high hormone levels don't necessarily translate into severe morning sickness. In fact, women with morning sickness tend to have no more estrogen and progestin than women without it.Flaxman and Sherman looked at the results of 56 morning sickness studies covering 79,000 pregnancies in 16 countries. Sixty-six percent of pregnant women, they found, reported some sort of illness in weeks six through 14 of their pregnancy. More interesting, about 65 percent of the women had an aversion to at least one food--far too many for the aversions to be merely whimsical. Among those women, 16 percent avoided caffeinated drinks, 8 percent avoided such strong-tasting vegetables as broccoli and cabbage, and 4 percent avoided spicy ethnic food--all of which contain natural toxins, called secondary compounds, which protect plants from pests and pathogens. A full 28 percent, however, couldn't stomach animal products, especially meat, poultry, eggs, or fish.

The Cornell team found further proof of this pattern by comparing anthropological reports from 27 societies where the early symptoms of pregnancy were discussed. In seven of those societies, morning sickness was all but unknown. In 20 others, it was common. Societies in the first group rarely ate animal products, relying instead on corn, rice, tubers, and other plants. Most in the second group drank milk or ate fish or meat.

Pregnant women in meat-eating societies seem to be caught in a double bind. Their fetuses could use the protein in meat, but the pathogens it might carry are too dangerous to ingest--hence morning sickness. In a previous study, another Cornell graduate student, Jennifer Billing, worked with Sherman to show that spices such as chili pepper, allspice, and oregano kill parasites and pathogens in food. "When you go to the local grocery store and look at the spice rack, it's an apothecary," Sherman says. "We're using those compounds to protect ourselves." Especially in hot climates, where meat spoils quickly, people traditionally use spices to disinfect their food. Unfortunately, pregnant mothers don't have that option: Spices contain natural toxins too. If spices are out, then pregnant women are better off avoiding foods that spoil altogether.

To most women, morning sickness seems to begin and end arbitrarily, yet its timing makes sense: A fetus's major organs develop between six and 14 weeks after conception. That's also when a mother's immune response temporarily weakens, giving the embryo time to burrow into the uterine wall. As a result, pregnant women are especially susceptible to bacteria, viruses, and tumor cells during those weeks. For example, spoiled food commonly contains toxoplasma, a protozoan parasite. Toxoplasma is usually harmless, but early in pregnancy it can cause maternal infection and possible miscarriage. Once the fetus is less vulnerable--after the first trimester, say--the nutritional value of meat outweighs the risk, and the aversion usually subsides.

All of which underscores the hazards of eating in general. "Everything we do in food preparation--boiling, drying, spicing, cooking--is aimed at avoiding our competitors, that is, parasites and pathogens," Sherman says. "These are things that people put in their mouths, and it's not trivial or unimportant." Across the generations, he suggests, the kitchen has been a battlefield marked by constantly shifting weapons and strategies. One strain of bacteria moves in only to be beaten back by a powerful spice or boiling water; then the strain mutates and spreads again, until another kind of spice can control it. "There is an evolution in cooking and recipes, and this arms race against harmful parasites and pathogens might explain it," Sherman says. "One person tries a spice, and it also protects against gastrointestinal distress. That recipe is quickly passed on to others." Pregnant women are merely the pickiest cooks: Their bodies are designed to protect the bundle of genes growing inside them, even if it means having to puke their guts out.

Not long ago, paleontologists in Ethiopia found some 21Ú2-million-year-old animal bones near the village of Bouri, in the Middle Awash region. They were the remains, it seems, of an australopithecine meal of three-toed-horse steak and antelope tongue. A million years later, our ancestors were hunting on a regular basis, but they were still a long way from inventing refrigerators to keep that meat in Grade-A condition. My guess is that pregnant hominids took one look at the day's kill and ran screaming from the bloody carcasses, just as I ran from my kitchen trying to escape the sight and smell of meat on the stove.

My recent meal with Sherman followed a different path. With neither of us currently pregnant, we were free to dig into a very spicy pork dish and then linger over shrimp and noodles topped by a blistering red pepper paste. By the end of lunch, our eyes were watering and our noses were running, but we were smiling: If either of us happened to be carrying a lot of parasites in our stomachs, they were surely dead by then.

To find tips from the American Academy of Family Physicians on relieving morning sickness, see home.aafp.org/patientinfo/mornsick.html.