There is an interesting principle in biology, named Haeckel’s Recapitulation Theory: ontogeny recapitulates phylogeny. In simple terms, that means that you can read the whole evolutionary tree leading to a species just by looking at successive development stages of the fetus. We start looking like single-cell individuals, “evolve” into bunches of cells, eventually “turn” into fish, then lizards, then monkeys, etc.
It’s not as simple as that, and the principle/theory has been refuted/rejected. But the idea has wider applications than just in evolutionary biology. In general, I’d say, the notion is that the behavior of the (averaged) individual allows reconstruction of the group’s creation/formation/development.
Humans are a weird bunch. We have all sorts of odd characteristics that make us stand out even with respect to our closest relatives, The size of our brains is what we are most proud of, but there is hairlessness, too; upright walking and long-distance running; our opportunistic digestive tract, not specialized for anything, and unable to digest anything that requires complexity; and more.
The picture that emerges is fairly clear: we evolved in a temperate climate with little seasonal variation, with a diverse food supply, none of whose components could be counted on in the long term. The ability to run indicates that we were group hunters, since we are not fast enough to outrun prey or hunters.
One of the things that is strangest about humans, though, is our sexuality. You see, life usually has two strategies: continual mating, where offspring is generated indiscriminately throughout the year; and seasonal mating, where sex happens at a specific time of the year.
Typically, the strategy picked indicates the environment in which the life form lives. If there is strong seasonal change, it is advisable to time pregnancy and birth so that it matches availability of food and environmental protection. If there isn’t, then it is smarter to have any sexual encounter be a chance for reproduction, since you don’t know whether there is going to be another day.
Humans, though, have the strangest method: our females are fertile for a few days every month, and infertile otherwise. That seems to be the stupidest combination possible: it limits the chance to reproduce without giving any obvious advantage in timing of offspring.
It is true that human females have only a limited supply of eggs, and that making them always fertile would mean the eggs would be depleted faster. But the fact is that the limited supply of eggs is artificial. That’s in the sense that there is nothing preventing a woman from generating more eggs, she just doesn’t do it. By that I mean there is no resource limit, or organic limit. Eggs are not hard to generate, certainly not more so than sperm, which consumes a lot more resources since it needs to be “launched” with nutrients galore to survive outside the host vessel.
A hint comes in the form of timing. Women’s cycle seems to be lunar, since the cycles are of about the same length. What is in a lunar cycle, though, that makes it relevant for reproductive purposes?
The obvious answer is tides. When the moon aligns with the earth and sun tides are larger than when they are not aligned. But that’s unlikely to be the real reason, since humans evolved on a highland plateau far from any measurable tide, and since the alignment actually happens twice a month, on new moon and full moon.
The less obvious answer is light. Every month the moon goes from full to new and back. The moon, of course, is the single major light source at night, outshining all stars by a factor of magnitude.
Women are fertile for a few days a month: “You’re most fertile two to three days before ovulation, and 12 to 24 hours after ovulating.” The thing is that the variation in luminosity of the moon around the full phase is minimal: from quarter moon to full moon and back to quarter moon, it takes half a month and there is plenty light. But around new moon the amount of light reflected changes dramatically. On new moon night, there are only stars to guide us; every subsequent night, there is a doubling of light. The short period of fertility is likely to be determined by the shorter period of waning light.
It stands to reason that women become fertile around the time of new moon. They actually don’t in today’s society (every woman becomes fertile at her own time). Which is when something else happens: women that live in close quarters coordinate their cycle. That’s really interesting for two reasons: first, it indicates that women used to live in close quarters; second, because it means that the moon itself isn’t regulating the cycle (there is an independent mechanism).
So, if women live in close quarters and ovulate around the time of the new moon, here is a question: why? According to the New Moon Theory, light matters. Which is when we add someone else to the mix: men.
Unlike women, who are complex, multi-faceted, and in all respects the better people, men are simpletons. We (I am one of the type) don’t have any complex regulatory mechanisms for our seed. We just produce way too much of it and spread it far and wide – nowadays mostly in the form of inseminated tissue papers. In particular, there is no discernible pattern in our production. Certainly no monthly variability.
I find that really odd. So odd, in fact, that I have to wonder whether the sexes use sex in the same way. Probably not, but that’s a topic for a different post.
Night light could matter for a variety of reasons. Most of them, though, either involve hunting or getting hunted. We might need light to move about and chase; or we might need light to be able to see predators. Hard to tell which is the case, and it might well be a combination of the two. In any case, making fertility coincide with lack of light means that males and females of the human kind would mate when they couldn’t move at night.
A plausible scenario (you add your own if you’d like) is that human males spent much of the month far from home, hunting and gathering. I mean, what’s the point of being able to run long distance if you stay close to home, right? Then, when the light in the night sky would start shrinking, our ancestors would run home. There would be much debauchery and generation of pregnancy. Then the males would leave home as soon as the light in the sky would be big enough.
That’s a great scenario, but it runs into a big problem. Why would we need the night light? The day is plenty long and dependable, with only very little seasonal variation in the region of earth where humans were born. To make things worse, all adaptations indicate we are day beasts: our vision works best in bright light; we need sunlight for production of vitamins; sunlight gets us in a better mood; our natural sleep cycle is diurnal.
Interestingly, our threshold for night vision is much higher (we need more light) than in other mammals. A cat will be able to see clear as day when it’s just pitch dark to us. But we retain night vision – we just need a certain amount of light. Measuring the amount of light we require for night vision indicates the amount of moon light we need to see. I haven’t had any luck figuring that out yet, but I would think it’s about the same amount that “makes women fertile”. (It doesn’t, as noted above.)
I have this image in my mind of roaming bands of thuggish males (I just added the “thuggish” for effect) making their way at night after doing something during the day that required them to stay away from home. They must have been carrying something with them, since they must have brought something back to the women. If there had been enough to feed them all back at home, why would they have roamed, right?
Some people will note that humans have not evolved satchels of any kind. That’s a good point: I have no idea how males would have carried whatever they got back to the hearth. Most of the stuff we eat perishes relatively quickly, certainly before a month is over, so the theory that men stayed away for a whole month and only came back for the new moon is not going to work.
The alternative is that sex happened only during full moon, which gets us into a completely different direction. Namely that humans are tragically bad at detecting their own offspring.
You’ve heard of the lion that eats other males’ cubs. That’s a smart strategy when there is a relative lack of resources and little fear of predation. The lion can eat the cubs of another male because he can create his own offspring.
Humans would love to do that (and we invented paternity lawsuits for that purpose), but we can’t. We can’t tell in any meaningful way whether a child is ours or not. Especially not in the early human case, when all humans looked very much alike. We do know the mother, of course. And women report that they have some special sensory apparatus to tell their own children in a mass of others.
So, maybe the New Moon Theory still works. Only it speaks of what men were not doing during the new moon, namely roaming around at night, presumably hunting for night prey. That makes sense if a considerable number of predators hunt during the day and a considerable number of prey animals show up only at night. It’s a good niche to live in, because you have the place to yourself.
This wrinkle would also explain why we have such poor night vision: improving it means losing on nooky-time, which means less offspring.
There are a few additional facts about human sexuality that come into play. For instance, lactating women tend to be infertile. Interestingly, it is not lactation that makes infertile, but the use of the milk. Only if a baby is actually sucking on the teats does the woman remain infertile, not just because she produces milk.
This effect lasts for about 6 months. At that point, both the baby requires new and different food, and the mother starts automatically ovulating again. Consider that this adds to a nine month cycle, and you realize humans are not seasonal animals at all: one of our pregnancies lasts 15 months from ovulation to ovulation! (Of course, I was born just shy of 12 months after my older brother, so there is that).
There is the fact that males are sexual all the time, but only for a short period. By that I mean that we want to do it all the time, but once we have done it, we are done for a while. We (males, again) are spent and tired. Women, on the other hand, can and want to have multiple orgasms, which indicates that they (women, again) would have sex with more than one man on those famous new moon night orgies. The seeds would battle it out in the womb, and whoever won would not be notified, since there was no way to figure out who was who. (Dan Savage presents all that beautifully in his book, Skipping Towards Gomorrah.)
To summarize: the New Moon Theory of sexual reproduction states that humans used to have mass sex during the new moon days of the month, and that this was not just a preference but an evolutionary adaptation.