I’m talking about the nausea, being unable to keep food down, etc. ISTM that it can potentially have some harmful consequences in depriving both mother and fetus of nutrients at a pretty crucial time. And yet, it’s pretty common. Are there any health benefits that make it a beneficial trait? Or is it solely a consequence of something which has beneficial aspects in other situations?
Wikipedia claims (linking to two papers) that:
Prior to modern food preservation and safety standards, being really picky about what you eat was more existentially relevant.
Plus the calcium and salt expecting moms need from all that ice cream and pickles.
Speculation, and confusing cause with correlation.
In particular, I had always heard - I don’t recall where - that the reason for the correlation was thought to be that pregnancy-related hormone changes are related to morning sickness. Women who are low on these hormones have less morning sickness and a higher likelihood of miscarriage.
The usual adaptive explanation is that having a sensitive stomach prevents the ingestion of toxic substances that could harm the fetus. This would be more compelling if there was much evidence for morning sickness in non-human primates and other mammals. The evidence is very slight.
I would think that if morning sickness were as common in other mammals as it is in humans there would be more evidence for it.
Not all traits are adaptive, and traits that are adaptive (such as hormone changes during pregnancy) may have side effects that are not adaptive (morning sickness).
Good answers here. Yes, I’ve always heard the morning sickness is coming from the fetus itself, not the mother’s body. Most believe this is the fetus attempting to protect itself by discouraging the mother from eating toxic foods.
That’s why it said ‘may.’
Motion sickness may be another case in which an adaptive response may be accidentally elicited in a situation where it is not adaptive. A common symptom of the ingestion of toxins is disorientation and dizziness, and nausea and vomiting is an adaptive response. Motion sickness is the result of disorientation and dizziness for other reasons. (Exactly what factors are the cause of it is controversial.) The body thinks it has ingested toxins and vomiting results as a response, even though there is no adaptive reason to in this case.
Correlation might or might not indicate causation but a link is a link.
For example, nobody’s saying creased earlobes cause heart disease but there is some evidence for (and some against) getting things checked out of you have them.
It may be that the underlying cause of morning sickness confers a beneficial effect on the fetus and the sickness itself is rarely so detrimental that is overrides that benefit.
If it were commonly so severe that nutrition was greatly reduced then evolutionary pressure would select against it, as it is, humans manage well enough for it not to matter on a species wide basis.
Motion sickness may be another case in which an adaptive response may be accidentally elicited in a situation where it is not adaptive.
Could be. This type of dynamic is probably very common. (Not just in evolution of living creatures - all sorts of endeavors like engineering and such involve tradeoffs between different designs, some of which have flaws that could be avoided with other designs, but are overcome by the advantages of the overall design.)
But the thing is that you don’t need to look for this type of explanation in the case of things like motion sickness, which are relatively uncommon. Such things could simply be illness-like imperfections that haven’t been weeded out. So the question doesn’t really start. It’s when you have very widespread phenomena like pregnancy illness that you really need to find alternative explanations.
The stomach has a very limited vocabulary. All it can say is that it is hungry, it hurts, or nausea. That is a small set of responses for a large range of feelings or responses to stimuli. So many illnesses and injuries and the progression of good old shock create nausea, sometimes of a productive nature. Convenience bags are a great invention for us prehospital care providers.
Or all’s well.
That would be my guess as well.
The optimal hormonal balance evolved because it gave the women the best chance of carrying a healthy child to term, a child that would grow to adulthood and have children.
The fact that it also made the women a little uncomfortable at times is largely irrelevant, as long as the discomfort wasn’t so severe that it caused women to avoid pregnancy.
While everything evolved, every single trait doesn’t have to have a proven benefit. Sometimes a trait will survive simply because there’s not a strong disadvantage, or it’s closely enough connected to something else that it just rides with it.
That’s a perfectly good explanation. I don’t have much in details but it doesn’t seem unusual that the unique aspects of human pregnancies overall are handled better by an usual level of hormone changes that cause morning sickness, and also may simply make morning sickness more obvious. Pregnant bears may feel just as bad but not tend to vomit or express signs of sickness. I’m sure other animals have been studied extensively and wouldn’t be surprised if they encounter plenty of other unique pregnancy related effects not seen in humans.
Other points to consider - pregnancy is much more risky for humans, and each pregnancy longer and a bigger investment than for, say, chimps. I would think this would make the body more sensitive to pregnancy issues. Without sounding too sexist - evolutionarily, unlike bears or wolves or hamsters or antelope or chimps, evolving humans seem to have had a division of labor where men did the heavy lifting of big-game hunting, fighting, etc. (Hard to chase down gazelles when 8 months along). As a result, morning sickness would not be the impediment it would be for less species with less sexually differentiated lifestyles, and less likely to be selected against.
Could it be a recently-occurring adaptation? If the theory about it being due to picky eating to protect against tainted food, the evolutionary value of it would be greatly influenced by agriculture. We haven’t been an agricultural species that long, but increased food-borne illnesses that went along with it exerted a lot of selection pressure until very recently.
Agriculture promoted the acquisition of pathogens from domesticated animals, but I don’t see any reason why it would increase the level of toxins in the diet. Hunter-gatherers would probably be using a wider variety of plant foods with potential toxins, and also perhaps be more likely to encounter spoiled meat.