High school mystery

I had a teacher, Mr. B, in high school who taught psychology and sociology. He spoke on many subjects and one of the subjects was preterm births. This was in 1996. He said: "At some point in our evolution there was an advantage to giving birth preterm, but no one knows what that advantage is." I thought to rephrase this as: "There are many disadvantages to giving birth preterm, but there is a single advantage to being born premature."

If I make five assumptions, a singular advantage presents itself. First, I assume that he was speaking of early humans. Second, these humans form a large group, say, thousands of individuals. Third, the group is cut off from contact with other humans. Fourth, the group survived a famine. Fifth, the famine lasted longer than a generation, say, 30 years.

The advantage then is: the preterm child needs less food.

Some might say, "In such a famine, individuals that are shorter of stature or of smaller build would be more likely to live long enough to reproduce." My reply would be, "This does not hold to the third assumption. Humans in the isolated population would be roughly the same in stature and build."

The isolated population would eventually make contact with other groups of early humans. Then there were low numbers of preterm births for a long time. (That concludes my attempt to support Mr. B's statement. In my opinion he was describing our prehistory, but I wish to apply his theory to our modern era.) It's possible some women who give birth preterm carry a recessive allele causing the condition. If so, the recessive allele occurred with low frequency.

I ask, "Did something happen recently to cause that recessive allele to occur with higher frequency?" Women in the 19th and early 20th centuries were pressured to have very slender waists. You could argue that there remains a preference for slender waists on women. Has there been selectivity in favor of women with that trait? Could that trait be associated with giving birth preterm?


Mr. B was one of my more memorable teachers. Other things he said:
(To his wife:) "I don't want to go to school today." (She made him go.)
"Teaching is a very loud job."
"It's the job of the older generation to irk the shit out of the younger generation."
"Just because you could do something doesn't mean you should."