How could embryonic diapause have naturally developed in humans?

Question

embryonic diapause is a reproductive strategy that is used by a number of mammals. In embryonic diapause, the embryo  does not immediately implant in the uterus after sexual reproduction has created the zygote, but is maintained in a state of dormancy. Little to no development takes place while the embryo remains unattached to the uterine wall. As a result, the normal gestation period is extended for a species-specific time.

Mammals do this to time the birth of their offspring and to avoid risking their lives in unfavorable conditions. Suppose this ability was natural in humans. How could it have developed? How could it serve as an evolutionary or social advantage?

Answer 1

First we need to ask why humans don't already do this? Mainly because modern humans have social and technological solutions that make it unnecessary. So we would need circumstances where those solutions don't function to make breeding relatively safe year-round. Longer seasons could have forced and maintained such a switch, if winters and springs, the hungriest parts of the hunter gatherers' and subsistence farmers' year, were extended (and summer and autumn too but they won't make the same changes) then the relative advantage of summer and autumn births would be so high that a timing mechanism would have to emerge to ensure all births happened in season.

In reality any such a change would have to be made and permanently fixed far back in our evolution, before we starting tool using and social living, otherwise the timing mechanisms will be social/technological rather than biological.

Answer 2

The practice of infanticide by unrelated males has already influenced human evolution. Embryonic diapause can be part of a female's defense against having her offspring get killed by unrelated males.

Embryonic diapause facilitates early pregnancy termination. Early pregnancy termination prevents wasting resources on offspring that will be killed by unfamiliar males.

Sometimes a male will show up who wants to mate with a mother. Killing her pups improves the chance that she will become pregnant with those of the new male. The male's fitness is improved at the expense of the female, who has wasted resources on the prior pregnancy. This happens in lots of different mammal types. In mice, one function of embryonal diapause is to facilitate the Bruce effect - termination of a pregnancy when a new male is detected in the region.

https://en.wikipedia.org/wiki/Bruce_effect

The Bruce effect, or pregnancy block,[1][2] is the tendency for female rodents to terminate their pregnancies following exposure to the scent of an unfamiliar male.[3] The effect was first noted in 1959 by Hilda M. Bruce,[4] and has primarily been studied in laboratory mice (Mus musculus).[1] In mice, pregnancy can only be terminated prior to embryo implantation...In many rodent species, males kill unrelated young; pregnancy block may avoid the wasted investment of gestating offspring likely to be killed at birth.[5][27] The Bruce effect is most common in polygynous rodent species, for which the risk of infanticide is highest.[28]

A new male will probably kill any pups not his, so no reason to carry them to term. Recycle those resources and start fresh.

Infanticide by strange males is still a major cause for infant mortality in humans - stepdads and new boyfriends are dangerous to infants and young children. There is reason to believe that regular infanticide by strange males has driven human evolution: the ability for females to have sex at any time in their cycle, concealed ovulation and breasts that always simulate lactating breasts are all parts of the human phenotype evolved to confuse potentially infanticidal males as to the reproductive state of a given female. If it is possible an infant might be his, he should not kill it. This benefits the fitness of the female at the expense of that of the male, who will waste his efforts and resources protecting offspring that is not his.

https://en.wikipedia.org/wiki/Infanticide_in_primates#Paternity_Confusion

A Bruce Effect type phenomenon in humans could have easily been produced by these same selective pressures, as it was in mice. It could even have been produced in tandem with the above - the Bruce effect works on embryos in diapause but not once a pregnancy is underway. A human might keep a pregnancy in diapause if it were conceived under iffy circumstances with a questionable father. Perhaps the father would need to stay present in the vicinity for some time before the pregnancy would launch in earnest. Once underway, if the father disappeared and a new male arrived, tactics from the existing human repertoire would be used to confuse the issue of paternity such that the new male might think the newborn might be his, and so not kill it.

Answer 3

Evolutionarily speaking, it is not obvious if that reproductive strategy makes sense for us. It could undermine the part about having been impregnated and thus possibly being in a tribe or society in that given time frame, where protection and dependency on others (ideally male partner) becomes a necessity and is likely more available. Thus the child has to be shipped as soon as possible, and 9 month is quite a lot of time anyway. If child birth could be voluntarily delayed, it would require 1. sufficient intellectual capacities in order to make that decision given that planning would become a major part in all of that, 2. undermine reproduction rates given that the choice can be simply not to bear the child due to facing difficult circumstances (which may exist all the time), 3. may require additional resources (higher energy consumption, different body parts) in order to have that function in the first place.

Also why "carry" around the DNA of the male if the impregnation could occur at a better suited time. It is not that relevant which of the successful males are reproducing with the female, so "saving" the DNA of one in the past is unlikely worth it. Therefore it did not evolve because it was neither necessary nor useful.

The societal effects as of today would be a decrease in reproduction rates as stated above. Having such an easy option to delay child birth or possibly abort it, will cause a decline in reproduction. Our instincts actively drive us towards "irrational" reproduction, which is needed for any species' survival.

How it would be perceived in everyday situations... well, I leave that to others to answer. It may not be too different from what we have today, given our modern means of pregnancy prevention. "Birth control" as of abortion would also be perceived differently in societal context. Religions may rather go for the reproduction aspect instead of the morality aspect.