Morning Sickness is an Adaptation, not a ... Sickness

There is new information from an older idea (from about 2000) by Paul Sherman and colleagues. The idea underlying this research is simple: Symptoms of illnesses may be adaptive. Indeed, this may be true to the extent that we should not call certain things illnesses. Like "morning sickness."

Broadly speaking, there are two different kinds of reasons that a woman may experience nausea in association with pregnancy. 1) This pregnancy thing is a complicated mess with all kinds of hormonal (and other) things going on, so you puke; or 2) a woman who is pregnant feels nauseous for good evolutionary reasons.

(a repost)

From an article in American Naturalist by Flaxman and Sherman:

"Morning sickness" is the common term for nausea and vomiting in early human pregnancy (NVP). Recent interest in why NVP occurs-that is, in the evolutionary costs and benefits of NVP-has spurred the development of two alternative hypotheses. The "prophylaxis," or "maternal and embryonic protection," hypothesis suggests that NVP serves a beneficial function by expelling foods that may contain harmful toxins and microorganisms and triggering aversions to such foods throughout pregnancy. The alternative "by-product" hypothesis suggests that NVP is a nonfunctional by-product of conflict-over resource allocation-between the pregnant woman and the embryo. The critical predictions of the prophylaxis hypothesis have been developed and tested, whereas the by-product hypothesis has not been subjected to similar scrutiny. To address this gap, we developed a graphical model and used it to derive predictions from the by-product hypothesis under two different assumptions, namely, that NVP is either (i) a by-product of current conflict between a pregnant woman and an embryo or (ii) a by-product of honest signals of viability produced by the embryo. Neither version of the by-product hypothesis is fully consistent with available data. By contrast, the timing of NVP, its variation among societies, and associated patterns of food cravings and aversions are consistent with the prophylaxis hypothesis.

In the prophylaxis, or more easily pronounced, "adaptive" hypothesis, a pregnant woman becomes hypersensitive to certain inputs (smell and taste of certain foods) that she should be avoiding for the health of her fetus. There are lots of things that typical adult humans can eat without causing harm that can do bad things to a fetus, especially during the first several weeks of development when organs are forming. Many plant products, for instance, are suspect.

After testing the two dominant theories (one adaptive and the other non-adaptive) for why two-thirds of women around the world -- but seemingly no other mammals -- experience nausea and vomiting in pregnancy, only one holds water, says Paul Sherman, Cornell professor of neurobiology and behavior and a Weiss Presidential Fellow.

"Our study, which tested theories and predictions about the nature of parent-offspring conflict in human pregnancy, shows that nausea and vomiting in pregnancy is beneficial by expelling such foods as meat and strong-tasting vegetables that historically and still may contain harmful toxins and microorganisms that could potentially sicken the woman and damage her fetus just when its organs are developing and are most vulnerable to chemicals," said Sherman, who is an expert in Darwinian medicine -- viewing diseases from an evolutionary perspective.

"Morning sickness" is rare after the first 18 weeks ... the organ forming weeks ... of pregnancy. Severe morning sickness is associated with lower rates of spontaneous abortion. The most pathogen-containing foods (historically, as in, pre-refrigerator) are those that tend to induce nausea in these women. Morning sickness is found more often in societies with risky foods as part of their day to day diet. And, this phenomenon ... morning sickness ... is only known in humans. Humans have a broad and, in terms of toxins, relatively dangerous diet.

This last part could be investigated further, I would think. There must be enough variation out there among omnivores to find some other species with morning sickness. Keep looking guys!

(I would add that slow development is another feature of humans that could be important.)

If the alternative theory that morning sickness is a non-adaptive outcome of an evolutionary tug-of-war between the mother and fetus for resources were correct, then the nausea should peak in the final trimester, when the fetus requires more nutrients and blood than at any other time. But it doesn't. Neither does it occur with every pregnancy. If morning sickness were the result of the fetus signaling its viability to the mother, then all humans and other mammals should experience it.

"All this leads us to suggest that morning sickness is a misnomer," Sherman said. "It doesn't occur just in the morning, and it's not an illness. It can occur any time of day and it appears to be beneficial -- we could call it a form of evolutionary wellness insurance."

Sources:

Flaxman, S., Sherman, P. (2008). Morning Sickness: Adaptive Cause or Nonadaptive Consequence of Embryo Viability?. The American Naturalist, 172(1), 54-62. DOI: 10.1086/588081

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