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Tomorrow, a sudden airborne plague comes around that kills nearly all human males - women are unaffected. The UN and different nations try their hardest but fail to find a cure. There are riots and some breakdowns, but on the whole global society manages to stay somewhat stable through the process. A lot of knowledge, especially technical knowledge, is lost but most governments either stay in power (with some replacements) or are replaced with new but similar ones. Everyone in the whole world is infected, and those that are not immune all die off within a few months. Everyone else are still carriers of the disease.

The only men that survive are to be those that were treated for and cured from a certain rare disease - as it turns out, 5 men in total. All new male babies made from frozen sperm die quickly in the womb. The reason these men are immune is assumed to be genetic, since male babies conceived by them survive and are perfectly healthy. Using the cure on women makes no difference. Since the treatment for the rare illness is rather expensive, these men all live in developed nations.

The question is this: can humanity keep enough genetic diversity for a long-term sustainable population or are we on the road to inevitable extinction?

(I got the basics of this premise from a manga that clearly didn't put much thought into its premise, but it got me thinking of the practicality of it. In that world technology was more advanced but not very well defined, which is why I'm setting this in the modern world.)

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    $\begingroup$ The term you are looking for is genetic bottleneck. A real world example would be the cheetah. See for example Dating the genetic bottleneck of the African cheetah, M Menotti-Raymond and S J O'Brien, Proc Natl Acad Sci U S A. 1993 Apr 15; 90(8): 3172–3176. $\endgroup$ – a CVn Jun 9 '16 at 11:21
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    $\begingroup$ Possible duplicate of What is the minimum human population necessary for a sustainable colony? $\endgroup$ – a CVn Jun 9 '16 at 13:05
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    $\begingroup$ This is a different question. Note that there are (by pretty much any measure) plenty of females. It's just males that are limited. The two questions are related certainly, but by no means duplicates. $\endgroup$ – Brythan Jun 9 '16 at 13:56
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    $\begingroup$ Even if the final verdict were the same, the first consideration here has to be if it changes things. And the fact is that it the final verdict is not (IMO) the same. Further, that is also the opinion of three out of four answers. The problems that arise with limited genetic material are mostly relieved by the high number of females. The only remaining problem is the Y chromosome, and obviously the Y chromosomes are survivable since the five men are surviving. Now, they may be subject to problems with certain XY combinations but there is plenty of X diversity to cover for that. $\endgroup$ – Brythan Jun 9 '16 at 14:09
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    $\begingroup$ As a male, I'm torn between the upsides and downsides of such a situation ... $\endgroup$ – Peter - Reinstate Monica Jun 9 '16 at 16:24

13 Answers 13

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I don't think that, even with artificial insemination techniques, you could successfully repopulate with only 5 men. Check out the math here https://en.wikipedia.org/wiki/Sperm_donation. It takes a man 2-3 days to produce a good sperm sample, but even with IVF you are only going to get a few successful conceptions from each sample. So those 5 men can only impregnate a few hundred women a year. Even assuming a large increase in success rate, the large gap from when new males would be able to reproduce (12-13 years or so) would mean there would be only a few thousand children at that time. Plus you are assuming the 5 men are relatively young with healthy sperm and don't die from other causes in those 13 years.

The issue then becomes sustaining modern technology with such a small population. Even if the existing women can cover down on all aspects you will only have a few decades before tasks and knowledge will have to be passed on and I don't think there will be enough capable young people to do it. As technology drops, the ability to sustain fertilization beyond natural methods will also drop, slowing the birth rate further. Not to mention that these new children, or at least the males, suffer from this rare disease that requires first world technology to treat, if that is lost then the babies will die from that, not just this killer disease.

So a best hope scenario is a few tribes of low tech survivors that are able to at least treat this genetic disease that leads to males surviving the virus, or that the virus dies out on it's own and new males without the genetic disease can then survive. Humanity has bounced back from VERY low numbers in the past (70K years ago a supervolcano dropped humanity down to a few thousand individuals https://en.wikipedia.org/wiki/Toba_catastrophe_theory ) so while the species may survive, I don't think our technology and society will survive with us.

One possible bit of salvation is that FEMALE children from existing banked sperm would still be immune to the virus, so you could probably keep on using existing sperm banks and IVF techniques to give lots of women daughters. This could prevent a HUGE hourglass population curve and allow for a more seamless transition of knowledge until such time as there were enough males to return to more normal procreating methods.

I did some reading on the Y chromosome. It is very gene poor (obviously since women don't even need it), doesn't seem to be shrinking in size anymore (ever since we diverged from our common primate ancestor 6 million years ago), and has a high mutation rate (due to the sperm environment), so even just 5 copies will probably not spell inbred doom for humanity down the road provided those 5 men don't harbor one of the fertility limiting diseases (which, given the assistive reproductive tech future of humanity anyway, may not matter [or it could even be a DESIRED trait since it may help maintain the new political structure that will arise in a women-led world that, for at least a few decades, has no males to mate or work with. Keeping men from being able to naturally impregnate women could prevent men from taking over again since the numbers of males could be better controlled]).

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    $\begingroup$ You touched key points in the scenario. 1) the man's offspring would probably carry that rare disease they were treated for 2) you could use sperm banks to make more women 3) success rate of IVF 4) tech decay. $\endgroup$ – Mindwin Jun 9 '16 at 16:02
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    $\begingroup$ 250 children/year is plenty. If we were to assume that all males started being able to reproduce at 13 and have just one surviving male. 13 years after the incident, he will have 3250 children including 1625 sons. After 26 years total, this original survivor has 6500 children and 1625*3250 (5 million) grandchildren (2.5M grandsons). After 39 years, he will have 9750 children, 15 million grandchildren and 625 million great grand children. To educate these 625 million great grand children, there will be women who were 21 at the time of the incident and are 60 now, not even retirement age. $\endgroup$ – Scott Jun 10 '16 at 3:35
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    $\begingroup$ Good link but IVF is typically for couples that are having a hard time reproducing. So the quoted rates are wrong for actually fertile people. There would also be sudden fervent research into improving yields. One can imagine a sample being added to a huge batch of eggs at a time as an example. $\endgroup$ – Dirk Bester Jun 10 '16 at 7:02
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    $\begingroup$ I think you underestimate the amount of women you can inseminate with one sperm sample. Just look at how they do it with cows. This particular bull produced 500,000 + offspring in just a few years: wsj.com/articles/… people may have smalller quantities of sample but they live much much longer. $\endgroup$ – Pieter B Jun 10 '16 at 9:26
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    $\begingroup$ You begin your answer saying humans could not successfully repopulate, and then spend the rest of your answer explaining how with a low tech level or with IVF we would not go extinct. Seems like you changed your mind half way through. $\endgroup$ – Mike Nichols Jun 10 '16 at 13:53
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One of the first things I would be concerned with is how the people of the world would be made aware that these five men are even alive, much less the only viable sperm donators.

We are facing a world that would have lost half of its population and most where most people in high level executive positions would have died. There would most probably be a lot of administrative chaos.

If nobody knows the significance of these men humanity would most probably be out of luck. The remaining scientific community would probably spend years trying to find ways to make babies from frozen sperm samples survive before the special abilities of the men were discovered. In which time they might even have died in the chaos. If the men are randomly distributed in the world the chances of them being found at all by anybody who would understand the importance of setting up a sperm bank, would be relatively small.

And even if after a few years they are discovered and a sperm bank with world wide distribution is set up we would probably face a massively depopulated world after women in most areas age out.

Then there is the option where people know already at the onset that these men are special and that their sperm must be milked and conserved. Then these men would probably be quite busy wanking for the next ten to twenty years.

If trade routes are still working most of the areas around major trade hubs in the world might be able to keep their populations up if there is a steady supply of sperm samples to impregnate the women.

As far as genetic diversity the only thing I would be concerned with is whether the men have Y chromosomes that are resistant to mutation and decay. As long as a large enough population of women are impregnated genetic variety would not be a problem as the children would get their diversity from their mothers. Only the genes on the Y chromosome would be in danger.

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    $\begingroup$ I think the significance would be clear and there would be a concerted hunt for any men that survived. It wouldn't take long to find at least some of them. Still, good answer - +1 :) $\endgroup$ – Tim B Jun 9 '16 at 12:24
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    $\begingroup$ The genetic diversity is a problem not for the first generation of kids, but for the subsequent ones. If there are 5 men who equally share their sperm across the world then in the F1 generation, 20% of the kids will be descended from one guy, and got half their chromosomes from him (not just the Y). Let's call him Tom. Tom's kids very carefully only mate with Dick's kids. So a big chunk of F2 are descended from Tom and Dick. It doesn't take very long before everyone on the planet is descended from ALL of those first 5 guys in the founder generation. $\endgroup$ – DrBob Jun 9 '16 at 12:42
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    $\begingroup$ With a careful sperm distribtion system most problems would be avoided, again save for the Y chromosome. The first generation of kids would necessarily have 50% genetic material from one of the dads, but second generation kids will have a random selection of genes from both parents. In time most the genes from the original guys will wash out in competition with the genes from the millions of women. Save for Y that is. $\endgroup$ – Martine Votvik Jun 9 '16 at 12:57
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    $\begingroup$ "Save for Y that is." Exactly. I'm pretty sure that's what DrBob was getting at. $\endgroup$ – a CVn Jun 9 '16 at 13:00
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    $\begingroup$ One thing though is that there is relatively little genetic material in the Y chromosone even compared to other chromosones and this is why I doubt that it would impact genetic variation much unless the specific Y chromosones in this case were damaged or vulnerable to mutation or decay. $\endgroup$ – Martine Votvik Jun 9 '16 at 13:06
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They wouldn't need to rely upon the men. Scientists have already been able to create almost sperm from female stem cells. I imagine, given that this was now a matter of species survival, the world would invest heavily in advancing the techniques already pioneered, and within a few years they'd have a solution. In the same way that ebola was considered impossible to cure... until 9/11 happened, and the US government reallocated top scientists to finding a cure, for fear of Al-Qaeda using ebola as a weapon. Then a decade later they had a prototype which they were able to test when the outbreak happened in west Africa.

http://www.telegraph.co.uk/news/science/science-news/3323846/Sperm-cells-created-from-female-embryo.html

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    $\begingroup$ Of course, sperm created from female stem cells can only produce female children. Thus it is similar to the "use frozen sperm to make girls" answer from InstantMuffin, except that it could be continued as long as civilization lasted instead of being just until the stores of sperm ran out. $\endgroup$ – Ross Presser Jun 9 '16 at 15:42
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    $\begingroup$ @RossPresser which is consistent with the original claim "They wouldn't need to rely upon the men" :P $\endgroup$ – djechlin Jun 9 '16 at 16:34
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    $\begingroup$ Gah! Someone on the internet is WRONG! scientists...coaxed male bone marrow cells to develop into primitive sperm cells, emphasis on primitive, meaning that they have something like a normal sperm, but which doesn't actually work yet. They're on the right track, yes, but it's still several leaps away from being able to create an actual, viable, healthy human. $\endgroup$ – Martin Carney Jun 9 '16 at 17:47
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    $\begingroup$ Edited. Still a lot closer than many would presume! We can get there! $\endgroup$ – inappropriateCode Jun 9 '16 at 19:13
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    $\begingroup$ @MartinCarney Something tells me they would have the top researches in the world putting down all other projects until they got this one solved. $\endgroup$ – corsiKa Jun 9 '16 at 19:44
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Edit: I have updated my answer with improved analysis. My old answer was flawed in its analysis

The question of population survival depends on enough genetic material being preserved to maintain a healthy population. The first thing is that there will only be at most 5 Y-Chromosomes in the world after the disaster, and some places may have only one for quite some time (for example if one of the men is in Australia or New Zealand his descendents might be very isolated). However provided that Y-Chromosome is generally healthy that won't be a major problem - it will create higher vulnerability in men to particular illnesses but that's not enough to guarantee the population will go extinct especially as the Y-Chromosome will be paired with different X-Chromosomes in different men.

Now it worth looking at breeding strategies and I'll consider a bit of a worst case scenario where a man can only produce 100 children before dying of "over-exertion" (this is only for simplification of the analysis). So the man successfully produces 100 healthy offspring (that is children who reach reproductive age) before dying. He should focus on mating with older women who are still fertile (i.e. those in their 30's) as this will be their very last chance to pass on genetic material.

First generation

This generation of 100 children will all be half-siblings, having half their genetic material from the proto-man. If these half-siblings were to breed with each other it would be a problematic level of in-breeding and applicable to incest laws, but there is no need for this as after approximately 13 years the ~50 sons will reach reproductive age and at this point women who were 20 years old will still be fertile and the sons should focus on mating with those women and not mate with their half-sisters or other girls their own age. Let's say that all of them also produce 100 offspring before dying of over-exertion.

Second generation

This generation of 50,000 children all have a common grandfather so 25% of their DNA comes from the original man while the other 75% comes from random women. This is already a much improved level of genetic diversity and some countries incest laws would not prohibit pairings between individuals with this level of genetic similarity, although it's still somewhat less than ideal.

After ~26 years the grandsons will reach 13 years old, they should mate with the women who were infants at the time of the dieoff, these women will now be ~26 years old. Once again let's assume each grandson produces 100 succesful offspring before dying of over-exertion.

Third generation

This generation of 2,500,000 children (yes, 2.5 million, the wonders of exponential growth!) all have a common great-grand-father and so 12.5% of their genetic material comes from the original man, and the other 87.5% comes from random women. At this level of genetic similarity incest laws are not applicable anymore and inbreeding will generally not be problematic. This is the first generation where unmated "original" women might realistically run out - that's not a problem though as each woman can bear multiple offspring.

Fourth Generation

It may even be possible to get one final generation with unrelated women. The fourth generation will come of age at a time when the girls who were babies at the time of the great dieoff are 40 years old, and girls made from frozen embryos/sperm would be a bit younger, at least some of these women should still be fertile. This fourth generation would have a great-great-grandfather in common and only 6.25% of their genetic material would come from the original man.

Overall if the women successful hold society together and utilize such a breeding strategy then even with only a single man who dies quite quickly the population would be in no risk of extinction whatsoever. Even if the numbers are significantly poorer, say only 10,000 women survive 26 years, the population would not be in serious risk of extinction and there would still be plenty of genetic diversity.

Ideally the daughters from each generation would mate with male offspring from one of the other 4 original men, this would greatly improve the diversity of Y-Chromosomes and eliminate inbreeding problems. The best strategy would probably be to have most the daughters migrate to another population as the first generation of sons would too valuable to risk losing on a long journey in a post-apocalyptic world. If migration to another population is not possible the daughters should mate with the youngest generation available as the youngest generation will have the most genetic material from random women.

The real question is whether a society without male workers would hold itself together or catastrophically implode, however it would seem reasonable that at least millions of women would survive and even if such a breeding strategy is not perfectly employed the population would not generally go through a bottleneck (i.e. the population would never fall below 10,000 genetically distinct individuals and quite possibly wouldn't even fall below 10 million).

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  • $\begingroup$ Hm, I'm not sure how you get to the assumption that a man will pass on more of his genetic material the more children he has. It is true that there is a statistical chance for this, but the chance is completely random and with much overlap. A guy could have tousands of children and still not pass on all of his genetic material to the next generation.. $\endgroup$ – Martine Votvik Jun 9 '16 at 13:33
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    $\begingroup$ @MartineVotvik yes there is a chance but even if a tiny percentage isn't passed on it doesn't mean doom for the species as there are also factors which increase genetic diversity (i.e. random mutation and funny businesses with retroviruses and such), it is not the case that a population strictly loses genetic diversity over time, if it were the case there would be no life on earth as everything would die out. $\endgroup$ – Blake Walsh Jun 9 '16 at 13:36
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    $\begingroup$ @MartineVotvik - Consider the Birthday Paradox. If a man conceives 100 children, the probability that he has only passed on 50% of his genes is about 10^-30 (a decimal point followed by 29 zeroes and a 1). $\endgroup$ – WhatRoughBeast Jun 9 '16 at 13:46
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    $\begingroup$ @MartineVotvik But it's statistically correct. If a man has 10000 offspring the chance of any of his genetic material not being passed on is literally infinitesimal. If I had said "If he mates with one woman, he passes on 50% of his DNA, and if he mates with a second woman, he has passed on 100% of his DNA because 50%+50% = 100%", then you would have a valid point. But what I have said in my answer is simply correct. $\endgroup$ – Blake Walsh Jun 9 '16 at 14:40
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    $\begingroup$ @BlakeWalsh, :) after you edited it it looks better man :) but claiming that a man that has impregnated two women has passed on approximately 75% genetic material seems like going out on a stretch. When the genes in the sperm is random, it could be anything from still just 50% to all the way up to 100%. My statement of a tousand should be read as a clear case of exaggeration to make a point, but it is still not entirely implausible due to the amount of variations and the random combinations. Nowhere did I claim non of his material would be passed on, just that he might not pass all of it. $\endgroup$ – Martine Votvik Jun 9 '16 at 14:54
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Nobody has mentioned that it takes only one sperm to fertilize an egg. It involves injecting the sperm into the egg. It is not normally done because of a mix of ethical and medical safety issues. But it is not really hard to do given modern technology.

In this plague scenario these issues fall away and I see no problem for one male to be parent to hundreds of millions of children. There will be rapid training of an army of IVF technicians. There will be many individual tragedies as a consequence of breeding from a damaged sperm that in nature could not have fertilized an egg. But need dictates. The human race will survive in sufficient numbers to maintain a C21 civilisation on at least one continent.

The first generation of children will need to be very careful not to choose a partner who had the same father. Thereafter there is no problem with genetic diversity provided all five Y chromosome sources are good. There are good biological reasons why a Y chromosome contributes almost nothing of note to a complete genome other than sex determination.

I strongly suspect that if all males went extinct it would be possible to create new males using a Y chromosome from another primate species and/or synthetic DNA. In ethical terms imminent extinction is the only thing that could justify this sort of research!

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Those last few men have an immunity to the whatever-it-was which killed billions. That immunity can’t be anything widespread like simply possessing a Y chromosome or having blood group A, otherwise there would be a lot more male survivors. (How on earth ALL the women survived, I shall leave for others to puzzle over). The initial post said they were treated for a rare disease

So the factor has to be something specific to those men. That could be:

  • Genetics alone. It would have to be an incredibly rare gene or complex of genes to explain why there are only 5, and that makes it likely that the men are all related – a couple of brothers and their sons, for instance.

  • Genetics plus environment. Lots more folk have the gene/s, but the immunity only gets switched on in very specific circumstances – like say if your mum smoked like a chimney while pregnant, you lived at high altitudes as a child, and you have a vegan diet as an adult.

  • The initial post said they were treated for a rare disease. Unless that’s a genetic disease, then that’s a purely environmental factor. Someone in the maternity ward had better be poised to dose every baby boy with the treatment before the midwife has even cut the cord!

If it is genetics alone, then any sons the men father must also inherit the immunity gene/s or they will die.

If the gene is on the Y chromosome then (as others have pointed out) hurrah – no loss of genetic diversity, because the billions of mothers can supply the rest of the genes to the billions of offspring (assuming the guys and/or their sperm go on a world tour). There will, however, be noticeable shifts in phenotypic expression of that diversity, because the frequencies of the alleles has been radically changed. For instance, if all the surviving men are blood group O (genotype OO), then there are no blood group AB in the F1 generation – all their kids are OO, AO or BO. The ABs return in the F2 generation, since when AO kids mate with BO kids a quarter of their grandkids will be AB. But compared to now, AB will be much rarer, simply because the system is now swamped with O alleles. Ditto any other genes those men carried.

Whether this matters to long-term human survival depends on what genes the men are carrying – and IIRC everyone carries an estimated 12 to 20 lethal recessive genes. In a small group of related guys the probability of them all having the same lethal recessives goes up. Lots of blood group O alleles means resistance to smallpox and tons of universal blood donors, so no problem. But lots of genetic disease alleles which kill people in childhood will be a problem.

If the gene is on the autosomes (the non sex chromosomes) all the above “founder effect” of the small number of men still applies. Plus the sons who don’t inherit that chunk of autosome die. Genes that sit very close to the “you don’t die” genes on the autosome get a free ride, as they are less likely to be separated from it by crossover during chromosome replication. So for instance, if the genes for adult inability to digest lactose and for knobbly knees are snuggling up to the immunity gene, then the world soon fills up with people with lactose intolerance and knobbly knees.

If a son needs 2 copies of the immunity gene – one from each parent – then the situation is even worse. Women who don’t possess the gene can only have daughters. If whole populations don’t have that immunity gene (very likely given that only 5 men in the world did), then the internal genetic diversity of that group is slowly eroded, because each generation they are breeding with an outgroup. Silly example – the women are from Island of Blood Group B, where the whole population is BB. The OO men give them BO daughters. Next generation more OO men arrive from outside to give them OO and BO granddaughters. Next generation another influx of O alleles and the frequency of B drops further. (yes I know this is a gross simplification).

If it is genetics plus environment, then these guys and their sperm going on a world tour is not going to solve the problem of lack of males until people figure out the environmental factors.

Telling the difference between "My baby died because he didn’t inherit the right genes" and "My baby died because I live at sea level and don’t smoke" may take an awful long time. In that time, you have a very reduced pool of mothers who can produce sons (e.g. chain-smoking vegans of the Andean altiplano). This means reduced genetic diversity on the female side as well as the male, since only these Andean chain-smoking vegans are actually reproducing until you work out what’s happening.

Imagine if it took 20 years to work it out. A whole bunch of genetic diversity lost, because millions of women outside the Andean Altiplano have died or are now beyond child-bearing age.

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I remember reading articles that a group would need to have a specific minimum size to not have inbreeding issues for the following generation. I think this number was around 10000, but that would be easy to look up. Now the issue at hand would be the very small sample size of y chromosomes. Female offspring would also carry the male's x chromosome. Either way you basically end up with 5 samples of half-siblings, I don't think this is enough.

There is a different approach though. Recently there has been some news about creating sperm cells from tissue samples. If I recall correctly it has been proven to work with mice. Now what would happen in a case like this is that males would die out in 2-3 generations, and women would fertilize themselves using this technology, whoever due to missing Y-chromosomes in their markup, could only produce female offspring.

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    $\begingroup$ Or just simple cloning of the women. $\endgroup$ – PCSgtL Jun 9 '16 at 13:37
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    $\begingroup$ Cloning at present stage of science would start of clones with reduced lifespan, with cell age starting at the original's age. This also leaves out the purpose of mating to increase the gen pool variation and therefore evolution and survival. But to follow along with your point, they could keep cloning newborns over and over again. Assuming both our options will be done, this would leave for interesting cultural conflicts between "male survivors", "women", "in vitro conceived women", "old clones", "newborn clones", "legitimate, rare offspring", "clones of said offspring" $\endgroup$ – InstantMuffin Jun 9 '16 at 13:44
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This topic is actually discussed at considerable length in Neal Stephenson's novel Seveneves.

In his story, rather than a largely intact human female population, and only 5 men, humanity is reduced to 7 women.

Due to loss of all viable sperm samples and male DNA, the women reproduce by Parthenogenesis, while making some specific alterations to their own genomes in order to cultivate certain traits.

Parthenogenesis of human embryos has already occurred, so it is clearly a potential line for maintaining humanity's population, and avoiding extinction.

From the Wikipedia article mentioned above:

On August 2, 2007, after much independent investigation, it was revealed that discredited South Korean scientist Hwang Woo-Suk unknowingly produced the first human embryos resulting from parthenogenesis. Initially, Hwang claimed he and his team had extracted stem cells from cloned human embryos, a result later found to be fabricated. Further examination of the chromosomes of these cells show indicators of parthenogenesis in those extracted stem cells, similar to those found in the mice created by Tokyo scientists in 2004. Although Hwang deceived the world about being the first to create artificially cloned human embryos, he did contribute a major breakthrough to stem cell research by creating human embryos using parthenogenesis.[91] The truth was discovered in 2007, long after the embryos were created by him and his team in February 2004. This made Hwang the first, unknowingly, to successfully perform the process of parthenogenesis to create a human embryon and, ultimately, a human parthenogenetic stem cell line.

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Chances are good the men would be placed into medically induced comas and their sperm would be harvested on a continuous basis, thousands of sperm per second per male. This would happen immediately, and their samples frozen until IVF picked up the pace.

IVF efforts would transition to more stringent IVF - rather than exposing eggs to a lot of sperm, sperm would be separated and only a few would be available for each egg. If the egg was successfully fertilized it would be frozen in the fertilized state and sent elsewhere in the world to be gestated.

It would take some time, but essentially a whole system would be set up where each country could maintain a reasonable birth rate for the next 20 years using this method, while new baby boys would essentially be raised as precious valuables - even though they'd be as regular as baby girls. After 20-40 years, the balance between male and female would be well on its way to restoration.

The genetic bottleneck would be significant. You would be related to 20% of the world's population, and there would have to be controls to prevent second and probably third generation incest. However, the diversity of the female contributions would be enough to prevent that bottleneck from destroying the human race, and after several generations the only thing you'd notice is that society becomes a lot more homogeneous, but not dangerously so.

After 100 years you wouldn't have the imbalance at all, and the human race, though changed, would continue apace.

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I'm going to focus solely on the demographic aspects of this apocalypse.

Assume that everyone lives to the age of 64. Females are fertile from 16 to 32 and can have a baby every other year (for a total of 8). Males are fertile from 16 to 64 (death) and with IVF can have 256 children per year.

For the first few years, you are limited by the number of men, but after 16 years, the population of fertile women starts to dwindle, and you become limited by that.

I ran a simulation. Starting with a population of 4,160,750,208 women, evenly distributed between 0 and 63 years old, and 5 16-year-old boys, the population declines until it bottoms out (not surprisingly) 63 years after the disaster with a world-wide population of

129,972,480, a distressingly small number, equivalent to the global population in 300BC, but not an extinction-level event.

The Python code is below, if anyone is interested (or wanted to catch my fence-post errors). You could try experimenting with different fertility numbers, or assume that the baby's sex could be controlled: should we make more males or more female?

import locale
locale.setlocale(locale.LC_ALL, 'en_US')

femaleBirths = [2**26] * 64
maleBirths = [0] * 64
maleBirths[-16] = 5

def generation():
    fertileMales = sum(maleBirths[-63:-15])
    fertileFemales = sum(femaleBirths[-31:-15])
    newBirthsOfEachGender = min(128 * fertileMales, fertileFemales / 4)
    femaleBirths.append(newBirthsOfEachGender)
    maleBirths.append(newBirthsOfEachGender)

def pcomma(x):
    return locale.format("%15d", x, grouping=True)

for i in range(128):
    generation()
    livingMales = sum(maleBirths[-63:])
    livingFemales = sum(femaleBirths[-63:])
    print i+1, pcomma(livingFemales), pcomma(livingMales), pcomma(livingFemales + livingMales)
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Although I am not a geneticist, I would have to say that the answer is "no". Eventually the 5 living males would succumb to "death by snu-snu" (in layman's terms).

Please see the Futurama Episode ""Amazon Women in the Mood" (Season 3, Episode 1) for a more thorough analysis on why this is not feasible.

See - https://www.youtube.com/watch?v=eCdrUW--Jic

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  • $\begingroup$ This is what we refer to as a "link only answer". Although the linked material may answer the question, it is a good idea to include some of the vital information to make the answer work on its own. $\endgroup$ – Hohmannfan Jun 11 '16 at 8:32
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Women wanting to have children in this world would have a few options.

  1. The "ideal" option is to get sperm from one of the 5 living men left in the world. This gives you a 50% chance of a son that's immune to the plague, and in either case you get a viable child.

  2. Next, you can go to a sperm bank and get fertilized there. Without any filtering, you have a +50% chance of miscarriage, but you can at least have daughters.

    Eventually the sperm banks will dry up - that is, run out of viable sperm. This will happen either because the sperm all get used, or because they go bad. A quick search indicates that frozen sperm samples aren't reliably viable after about 12 years. If this is like the "best by" dates on food, the sperm don't suddenly stop working, they just provide lower and lower success rates dropping off after 12 years or so.

  3. Go to a country where human cloning has been legalized and researched. In light of a serious threat to the human race's existence, at least some 1st world countries would look into human cloning once again. There's some problems with the process, but it's an option after sperm banks dry up. Add genetic manipulation, and you might be able to get some male clones with immunity to the plague whose genes aren't from your 5 remaining men.

  4. If scientists find a cure, but it's too late to take advantage of sperm banks, then track down all the tribal groups* which have no contact outside their community. Immunize them, and bring them into the developed world. Even if you have to lock them up, the men will be able to provide sperm for viable male children, increasing the genetic diversity available in the world.

  5. There's also a good chance that space agencies (NASA, etc) would be wise enough to keep the ISS quarantined, and any men on board would still be alive. Find a cure, and you can add them to the gene pool, too. Even without a cure, they could contribute sperm which still produce viable daughters, even after sperm banks dry up.

    As a commentor suggested, there are also submarines which are routinely isolated for a few months at a time while on patrol. These will also provide some men to add to the gene pool. There may be other groups which escape infection through isolation when the plague strikes.

* Unless the airborne disease was a weapon and was applied intentionally to the whole world, or animals can be carriers, then at least some of these groups would escape infection simply because they live too far from any other humans to catch it from them. If scientists never find a cure, leave those groups alone. Even a few minutes' contact would extinct the group in 1 generation. (See Uncontacted peoples.)

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  • $\begingroup$ About #5, the ISS can really only go maybe 4-5 months without resupply missions, and 125-128 days was really pushing the limits in 2003. Not sure if that's good enough for the OP. $\endgroup$ – a CVn Jun 9 '16 at 20:18
  • $\begingroup$ @MichaelKjörling True, but it's possible to resupply them while maintaining quarantine. Just make sure everything that enters the living space is sterilized, and that the crew is never exposed to pilots of the resupply ship. Granted, you still have the problems with long-term microgravity. Maybe they could bring them down to Earth while maintaining quarantine. $\endgroup$ – Martin Carney Jun 9 '16 at 23:23
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    $\begingroup$ If the plague breaks out suddenly, you would have uncontaminated ships at sea, especially submarines (they patrol between two and three months IIRC). $\endgroup$ – Law29 Jun 10 '16 at 23:51
  • $\begingroup$ @Law29 good point, I'll add that in. $\endgroup$ – Martin Carney Jun 13 '16 at 17:43
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Existing sperm banks represent a huge pool of existing genetic diversity.

I think you could pick up a lot of genetic diversity by:

  • Finding women who are immune, or curing them.
  • Isolating them (bubble boy style).
  • Fertilize them.
  • Girls can just leave.
  • Boys need to be cured/immunized prior to leaving isolation.

Further, women are capable of having children well past age 32. Doctors suggest not having children past age 40, because birth defect rates take off at that point. According to this web site, 35-39 year old mothers will have an additional 1 to 4 babies with congenital heart defects per 1000. At age 40, it jumps to 30 per 1000... and that just the heart defects... Those numbers don't take into account other defects like club foot or diaphragmatic hernia.

Many of these problems could be corrected given a large enough medical budget, and perhaps some research.

OTOH, there's research (at that same link) showing that mothers under 20 are more likely to have children with specific defects, though it appears the rates are much lower compared to older mothers.

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