I'm working on a story that is set in the future where human females have evolved to have a fertile period once a year for two months. Is such an evolution scientifically feasible?
The classic explanations for species wide human infertility usually involve genetic mutation caused by either pollution or biological warfare.
Another, less traveled path, involves the Gaia Theory in which the planetary ecology is a self balancing system with built-in safeguards against destructive species. When humanity starts threatening the health of the entire ecology, Gaia turns off our ability to procreate and thus solves the problem.
But all these explanations get thrown out when you through in the magic word, "evolution".
Is such evolution biologically possible?
As we currently understand it, evolution is a rather short sighted process, favoring traits which increase an organism's chance of survival only within a single lifespan. If a trait helps a creature survive to breeding age then there is an increased chance that that creature will breed, passing on the trait and thus increasing the chance that that trait will survive within the creature's prodigy.
Imagine, however, that evolution is more than we currently believe. Imagine that it somehow favors traits which will increase the chance of survival of the trait-holders over the span of multiple lifespans. Imagine that it favors such traits, even when they do not contribute to the survival of their holders within a single lifespan. Although extremely unlikely, such a redefinition of evolution changes everything...
This new evolution might be intelligent enough to lower our fertility rate to keep us from over populating the planet and destroying the ecosystem. It might be smart enough to help us avoid such an extinction even though that extinction is still several generations away.
What I am proposing is something like the Gaia theory (because it requires a steering intelligence greater than our own) but it is a Personal Gaia Theory with dominion only over our singular species, a species which has already achieved the milestone of sentience. Since we have already achieved the impossible in obtaining our ability for independent cognition, is it too much to believe that we might also have covertly and under subconscious cover, achieved an ability for collective thought. If we have, then within that collective thought lies the ability to avoid our coming extinction...
... by decreasing our women's fertility to a couple of months out of every year and thus significantly decreasing the burden which we put upon this planet's ecology.
Before all the scientists flame me for this very unscientific hypothesis, please understand that what the O.P. is requesting would otherwise simply be impossible. As I have already acknowledged, evolution does not work this way. Any trait which annihilates its ability to propagate itself through procreation (which is by definition what infertility does), cannot be spread by evolution. Only by embracing the unlikely can I give this question a positive answer.
We have already evolved to this point, because we have evolved to be able to control our own hormones pharmacologically. There are birth control pills that result in 4 periods a year, or no periods a year. They are available and safe. An added benefit of these pills is decreased lifetime cancer risk.
If it's set in the future, rather than the past, have this accomplished with genetic engineering. That seems like it will become technologically trivial, since we already have the tools we need (an understanding of how fertility works, and the ability to do genetic engineering). Personally, I suspect that even within our lifetimes, fertility could become "opt in" rather than "opt out." It would solve an awful lot of societal problems.
For us to have evolved this way on our own, that's harder to justify. Lower fertility is not normally an evolutionary advantage, it's a disadvantage. Perhaps resources were so scarce that it was be advantageous to invest more resources per child into fewer children?
The reason for being receptive to mating all year long and not just few days in a year is to be found in the evolutionary advantages behind it:
- the male, having the possibility to mate, is more likely to be around and help raising the proles, which for humans is a heavy effort (for the first few years babies do nothing more than sleeping and processing food to fertylizers)
- the female can lower the chances of pregnancy (being homo one of the few mammal species suffering from heavy delivery related deaths, due to the standing posture)
In principle it is possible, nowadays we have pathologic cases in which the female skips some cycles. Some scientists even postulate that removal of the cycle would be beneficial for the woman (I read it some years ago). It strongly depends on how this would affect the sexual receptivity of the female.
If it would make her indifferent to mating except than in the fertile period, it could be evolutionary unfit, unless there are other means to support the growing baby. Else it would be perfectly plausible and sustainable.
Think in a reverse way than the main stream - whether they are scientific-wise, evolution-wise, biological-wise... these are merely doctrines adopted since we don't know (and we will never know with the same level/type of intelligence) the Ultimate Answer to everything hence all these doctrines will be revised from time to time; that intrinsically menstrual cycle is the result of sexual desire, the cycle is for making the by-product the offspring. Then this cycle will be re-regulated if the sexual desire changed.
The monthly cycle is the result of daily, hourly, minutely sexual desire... it's possible in any day there will be sexual activities hence impregnation, the most accommodating cycle will be monthly.
Now if there are sex-robots to feed the sexual needs, over time the body will learn that sexual activity doesn't lead to impregnation, thus it will alter the cycle to conserve energy (menstrual is lost of blood hence lost of energy).
If, another scenario, some humans develop certain self-cultivation therefore their pleasure don't derive from sex hence don't have the tail of impregnation, then this cycle may even be controlled, i.e., only if the woman decided to have pregnancy.
Last, if the human society evolves to completely under control and planned, then new humans will be "hatched" from incubators and female role of reproduction is unnecessary therefore the menstrual cycle will be obsolete too.
From the above understanding, setting the cycle to whatever period of time is as if it's free for anyone's wish, or the author's wish like this OP.
The only logical, with scientific support purport to set the cycle to twice a year is to change the orbit of the moon. Like the tide, the female cycle is regulated by the moon. The moon is orbiting the earth in a monthly cycle, if for any reason this orbit changed to yearly, then the menstrual cycle will be altered too.
Why the moon regulates the female cycle? This new question will take another big page to explain...