Or put differently, 8-10 thousand years ago, the number of men reproducing dropped significantly. That makes sense to me given that this is the period in human history where society begins to form into agrarian communities [1]. The beginnings of social stratification would likely impact reproductive trends.
Saying that the development of society caused more sexual competition among men isn’t crazy. This study basically uses Y-chromosomal tracing to support that point.
Showing total # of male ancestors (left) and female (right)
I guess writing, agriculture, and fixed settlements allowed for dominance on a far greater scale. There's a natural limit to how much power you can project without technology.
Millions ? Lucy isn't an human and yet 3M years old. Humankind is about 300K year for Homo sapiens and half of that about what can be called modern human.
Is the suggestion elites were controlling access to procreation? Maybe men were also just killing each other extra fierce in the land grab. Uncompletely uneducated speculating on my part.
Definitely a factor. As agrarian societies expanded into hunter gatherer territory they would kill the men and take the women. Agriculture did not spread because it was a better deal for the individual at the time, it spread because it could support higher population density and specialization, including a warrior caste. And thus was more succesful at conquering and holding land.
8-10k years ago puts us at the near end of the last ice age.
I know it's poo poo'd in academic circles but if there was a great flood that destroyed most of human civilization that would also describe why this occurred.
> if there was a great flood that destroyed most of human civilization that would also describe why this occurred
Do floods kill significantly more men than women?
How much area would need to be flooded to affect most of human civilization 8-10k years ago? By that point we were spread out throughout the entire planet (except what, Polynesia and New Zealand?)
It's one of the key results from the paper. From the abstract: "In contrast to demographic reconstructions based on mtDNA, we infer a second strong bottleneck in Y-chromosome lineages dating to the last 10 ky."
From the body: "However, the Y chromosome plot suggested a reduction at around 8–4 kya (Supplemental Fig. S4B; Supplemental Table S4) when the female Ne is up to 17-fold higher than the male Ne (Supplemental Fig. S5)."
Indeed, the title does not match the title of the paper, but hopefully makes it more approachable.
What's the number now? I'm assuming that the falling popularity of monogamy means it's headed in the same direction again, albeit not in quite as extreme a way.
Having multiple partners doesn’t mean men are having children with multiple women.
At least in my community, many of the men having kids with multiple women get married, have a kid, then divorce, remarry and have another kid. And even that is extremely rare compared to the men and women that only have children with a single partner.
My guess is that the number of babies born out of wedlock is much, much lower than ever before. Probably due to contraception, child support, abortion and DNA testing.
The falling popularity of reproduction probably intersects with the falling popularity of monogamy, albeit in unclear ways. All these trends are linked to reliable contraception, which disconnects sex from reproduction; however, I suspect that contraception merely facilitates / accelerates the cultural trends rather than being their primary cause.
> probably intersects with the falling popularity of monogamy
this sounds like a bias looking for data to suit it
birth rates generally decline as countries become more developed, there are numerous suspected causes (not related to monogamy)... generally the rule of thumb is... as education and income go up, birth rates decline: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4255510/
My use of intersect was not fully articulated, thus unclear. My focus is genetic, not sociological. That is, I'm not saying that the rate of monogamy and rate of reproduction are causing each other. Rather, the intersection that I have in mind is whether the two things would have synergistic impact on allele frequencies (i.e., demographic structure).
In the extreme, if a population stops reproducing, sexual selection doesn't occur (genetically) even if all sort of interesting sociological phenomena occur.
A name isn't quite what it used to be. Monogamy and large families no longer have the social power they once did. It's all been replaced by money and new forms of communication.
People may call this unnatural, but I wonder if other animals have similar behaviors that limit reproduction.
Experimental studies have shown that signals of high density (e.g., playing squirrel calls on loudspeakers) reduce reproduction even when density is not actually high. I would love to test the hypothesis that Instagram exposure of rural populations reduces reproduction independent of education and actual population density.
im not sure instagram is the signal; plenty of mommy influencers, and small but strong set of rural folks embracing the "quiverfull" movement. maybe if we were talking MLMs or something.
test the hypothesis on student debt and housing costs.
unless they're showing up with tons of coke, and i ain't talking soda, i find that debatable. ain't no 18 year olds trying to mate with 42 year old divorces.
I had never heard of "cumulative Bayesian skyline plots". A search on Google gave one hit, which was a dissertation quoting this study.
I assume they mean "coalescent Bayesian skyline plots".
Why invent a new name for something which already has a more informative name?
I have great difficulty imagining a culture which could socially sustain an N_e of 1:17 (male to female). Just do a few sample calculations. The priviledged males would be massacred.
A few things could explain it: (1) large scale male infanticide or male child castration (2) A disease which kills mainly male children.
Both of these alternatives suffer from one of the same problems as their proposal: How would the cause (whatever it was) spread so relatively quickly among distantly seperated populations ?
I think it far more likely they have made mistakes somewhere.
I agree. 10,000 years ago it would have been difficult for majors trends to spread across different continents.
And I have a guess (though unsupported) that every civilization's mythology would tell similar stories of this event if it really were occurring globally.
Today there is a correlation between women’s rights and reproductive rate. I wonder if that correlation existed during the era represented in the study. If so, it must have been particularly brutal.
Probably not. Birth rate depends on a lot of variables, including how wealthy a society is. If birth rates dropped to a level that was making us poorer, people would start reproducing more to compensate. Also, wealthy societies can manage the birth rate through tax cuts and other incentives.
The correlation is complex and im not sure its well understood though. You need to be rich to raise kids, but people in poor countries have more children. Also, Japan (and parts of Europe). They don't seem to control birth rate although quite rich.
Japan gives an allowance [1] to families with children under a certain age, in addition to other tax incentives, parental leave policies, and subsidized child care. Similar policies exist in many countries in Europe.
If fertility rates remain below replacement levels, then eventually we will go extinct. How far below replacement determines how quickly this will happen.
The low fertility rate is not very uniform across cultures. Some pockets of society have high fertility rates which would keep things going. Also, fertility rate cultural norms can easily reverse a few generations down the line.
Yes, and at current rates it would take many, many generations for extinction to occur. Do the math with whatever birth rate you want to use. It’s not like birth rate is likely to stay constant over such a long period, either.
There's been at least a couple points in history when the homo sapiens population was <10,000... the current population is 8.1 billion. Seems unlikely, and certainly not something we'll personally have to worry about.
The overall population numbers might drop, but not in our lifetime (it is forecasted to continue to go up until at least 2100). Once you are dead you will cease to exist, so whether humanity goes extinct 1 minute after you die or 1 million years after you die, as far as you are concerned it is the same.
I have children that I hope live far beyond my death, and hopefully grandchildren after that, so I actually do want the future to be there for them, regardless whether I'm around.
Any debts I have are ones I incurred. The moral thing to do is to pay off my own debts. If I don’t want to pay a debt, I shouldn’t have incurred the debt in the first place. Though, when I say debt I am talking about things like loans and credit card debt and not medical debt that might arise through an accident.
I don't understand a moral framework that's concerned with the posthumous status of one's financial obligations but is unconcerned with the future continuation of the human race.
I don't see the distinction between paying my debts in advance of my death, versus establishing in my will that my debts will be repaid from my remaining assets.
Nor do I see a major moral distinction between, say, setting off a nuclear bomb timed to explode one microsecond after your death, versus one microsecond before it.
It won't remain at low levels once population has shrunk a lot. The greater availability of space and the collapse of complex society will likely trigger a population boom before we get to 100 million.
Perhaps if half a generation didn't feel locked out of traditional family things like home ownership they wouldn't feel so adverse to traditional family things like having kids.
Having more female than male babies might result in more offspring in total for the species, but it also creates a strong genetic "advantage" (or, you could say a gradient in the fitness landscape) for the few individuals who themselves have more male children. This creates an evolutionary feedback pressure that results in the number of reproducing-age men and women to be approximately equal.
For example, imagine just for argument that the probability of the offspring being male or female is proportional to the number of sperm that carry either X or Y chromosomes. And imagine that this ratio is controlled by a gene, and that initially, the gene is such that across the whole population, 17 sperm carry an X chromosome for each that carries a Y chromosome. In this population, any male who has a mutation in this gene resulting in them producing a closer-to-equal number of X and Y sperm will have a reproductive fitness advantage, because they will have more male children amidst a majority-female population. Those mutations will therefore spread until the advantage disappears.
In order for 17-to-1 female:male ratio to be stable, you'd basically need to have a period of thousands of years in which, for whatever reason, 16/17ths of women do not reproduce. That's the exact opposite of the condition described in this article.
If there is a lot of "polygamy" (Genghis Khan levels), wouldn't it be more advantageous for upper-class parents to have male offspring (who will each have many, many children) and for lower-class parents to have female offspring (who will at least have children, unlike lower-class males)?
I mean, only if the upper-class and lower-class are separate gene pools, which I don't think is the scenario you are describing, at least not over the course of multiple generations. Not with offspring sex being randomly determined, anyway. If a "lower-class" female is mating with an "upper-class" male, then in which direction does the genetic fitness gradient point for the children of such a couple? Try working through it.
Remember that (mitochondria and Y-chromosomes aside) it's not the case that sons inherit their genes from their fathers and daughters from their mothers. The children of an upper-class father and a lower-class mother will have half of their genes from each parent regardless of sex or class.
Thankfully, that's no longer the case! The miracle of modern medicine means that we can take the DNA of the billionaire man, and the DNA of the billionaire man's wife, combine them in an egg from a second woman, and implant it in a third. The third brings the baby to term, a baby that has none of her disgusting lower-class DNA.
Okay, but now this is mixing up levels of abstraction. The sex ratio is determined based on the fitness gradient of the genes, but this is almost totally independent of the incentives that drive the individual. Most people, let alone billionaires, are actually not consciously motivated to pursue technological strategies to maximize their genetic fitness, even though mutations that induce those drives might theoretically be selected-for. Perhaps that's why most men don't become sperm donors, and most women don't urge their sons to become sperm donors. Billionaires in particular tend to be more concerned about their legacy, estate, and inheritance, and having a large number of heirs divides the pile they've worked so hard to accumulate. But to each their own, I guess?
Being upper class and lower class apparently doesn't have significant biological indicator to affect strategies such as these.
The poorest among our society are still mostly drawing in the same amount of calories, and feel happiness and sadness, age mostly in the same way.
There are no indications that billionaires are significantly happier/healthier than us - in fact these indicators are mostly tangential with wealth past a certain point.
The blunt answer is that evolution benefits from males killing other males for breeding rights much more than it would from every male getting a harem.
If you think this is inconceivably lopsided, wait until you see Elephant Seals mating on the shores of Point Reyes. Those bulls have one hell of a job come mating season.
But I'm pretty sure evolution still wants heaps of genetically diverse males to come out and compete in the reproductive lottery. Even if only a few of them win, they're the .. well, winners, genetically speaking.
With fewer participating at an existence level, you'd have would-be losers succeeding for lack of competition.
I think we cannot compare humans with worker bees as these bees cannot reproduce. Basically, only the beehive is an complete organism.
For the latter part, I'd argue along the lines of the selfish genes (Dawkins): Not an organism wants to reproduce but rather the individual genes. They use the body as a vehicle for that. So it can still make sense for an individual to not reproduce and increase the chances of the same genes to be passed on (e.g., when an uncle without kids supports the kids of his siblings).
An acknowledged offspring. How many unknown kids do they father with mistresses? How many pregnancies get terminated? More darkly, how many wives of millionaires terminate pregnancies before 12 weeks when the fetus is female, but not male?
And why leave anything to chance? IVF makes it possible to pick the gender of the child, among other attributes. What's impossibly expensive for you and me is a Tuesday medical appointment at the wing of the hospital the billionaire bought.
Saying that the development of society caused more sexual competition among men isn’t crazy. This study basically uses Y-chromosomal tracing to support that point.
[1]: https://en.wikipedia.org/wiki/Timeline_of_prehistory#cite_no...