# What should three men and three women do if everyone else is dead?

Recently, I asked what could kill all humans except for those in low orbit. Out of the answers that popped up (which included the Ragnarök), I picked a plague as being the most desirable for my particular needs.

Some background

A virus, purposely engineered to target and kill humans was (deliberately or accidentally) released and successfully killed all human beings within the span of a few months. Well, not quite all humans, the 6 people in space were not infected. When it was clear that this virus might very well kill all humans, they were instructed to stay in space for as long as they can last and then attempt reentry and hopefully restart society.

The people in space are 3 women and 3 men, they are all relatively young, able-bodied and intelligent, as well as educated.

The question

Assuming these men and women return 8 months after the virus got released and thus spent between 8 and 14 months in space, what could these people do in order to maximize humanity's chances of survival? (and what are some less obvious problems they will face?)

You may assume that apart from humans, other life on the planet has not been directly affected and that people on Earth could have made some minor preparation in order to help the 6 remaining humans with their task.

Also keep in mind that they cannot know for sure if there might be survivors. (There are none, but our astronauts don't know for sure.)

• Starting with 3 couples, it is not clear that inbreeding would be a substantial problem. Statistically, first cousin couplings produce the same number of birth defects as non-cousin couplings where the mother is 40 years-old (both about 3%). Interestingly, both Charles Darwin and Albert Einstein married their first cousins. It is estimated that 10% of worldwide marriages today are first cousin couplings. – Epsilon Oct 27 '14 at 14:05
• "people on earth could have made some minor preparation in order to help the 6 remaining humans with their task" -- Might this include some form of sperm-bank, or frozen embryo bank? (And if so, can such a facility remain operational, despite being unmanned for a few months) – Caleb Hines Oct 27 '14 at 15:11
• As far as inbreeding goes, you can look to the Pitcairn Islands for a real-life analog. It was mostly populated from four families, the survivors of the mutineers of the Bounty in 1789. In the 1930's it had a population of over 200 (though it has since been in decline). Apparently there aren't yet any noticeable signs of genetic defects. Although there are... issues: telegraph.co.uk/news/worldnews/australiaandthepacific/… – Caleb Hines Oct 27 '14 at 15:50
• Something else to consider: What assurances (if any) do the astronauts have that they will not be susceptible to the same plague that wiped out everyone else? – Caleb Hines Oct 27 '14 at 18:11
• I'm still struggling with the premise that this virus kills every single human on Earth. Everybody in Antarctica? isolated tribes in New Guinea and S. America? Military personnel in bunkers, Minuteman silos, nuclear subs, ...? All those Americans who keep a year's worth of food in a shelter? It would actually take a lot of things going just 'right' in literally millions of separate cases, to exterminate all 7+ billion of us in a few months. Guess I'm saying I'd need to be sold on the premise. – Spike0xff Oct 28 '14 at 21:28

The existing answer covers the idea of egg and sperm banks, if you've got access to these then great. For the sake of variety I'm going to assume there are no other sources of new DNA available and try to focus on the bigger picture rather than the unavoidable genetic issues.

## Survival

First and foremost everyone MUST survive, inbreeding is going to be a massive problem for future generations however if people die then there will be no chance for humanity to reach future generations!

The survivors must ensure they are safe, they must (if possible back up their own DNA through egg/sperm banks) but they must also ensure their own health. Whatever wiped out the rest of humanity must be handled, medical supplies, knowledge, sterile surgery must all be preserved.

## Repopulation

As I mentioned above if there are sources of other human DNA around that's by far the best option, in vitro fertilisation from dead donors would an ideal solution because it rebuilds the population from a wider variety of "parents". However assuming that this isn't an option it's likely that sexual partners will be designated by some authority in order to maximise the genetic variations produced. I'm not going to speculate on the details of which pairings will produce the best results as it would depend largely on the six people in question.

Second (and I imagine this is going to be very obvious), each of the women must spend as much of the rest of their lives as possible pregnant with as many permutations of genetics as possible. It's not going to be much fun for the women involved (being permanently pregnant) but the more children the six people can have (each man with each women) the better. However this needs to refer back to my previous point, if a 1st generation woman dies in childbirth this is worse than not having the child at all. With her the survivors lose her knowledge!

Family trees must be mapped in excruciating detail, it's impossible that inbreeding is going to be avoidable but at least everything can be documented for future generations.

I expect that sex will lose its taboo status very quickly... after all everyone is doing it with lots of different people!

After the first few generations the issues of inbreeding are likely to become an issue, the six people in question will have a very limited amount of DNA to work with (although on the bright side a massive proportion of genetic defects will be eliminated overnight!).

The colony will be faced with some serious issues such as whether people who are born with genetic defects should be permitted to breed (and I do suspect it will be a permission/licence based society) or whether their "faulty" DNA would be removed from the gene pool.

After a few generations I expect DNA profiling will have become a major part of every day life, people with "good DNA" will be highly sought after as parents. The family structure as we know it will have been all but forgotten and children will be raised by the state.

In short I don't believe it looks good for our colony until the population develops gene manipulation technology, however in the early days their first priority will be survival of the human race rather than genetic anomalies which will appear in a few decades/centuries time.

## Knowledge, Learning and Jobs

Everything must be written down and backed up! The original astronauts are going to die and they must have everything they know backed up as soon as possible!

People are going to have to put real value in education and training. The onus will be on learning skills yourself. Hopefully records of humanity have survived - people will need to re-learn everything from maths, to science, through cookery, sport, healthcare... everything. The original survivors are going to be highly educated but likely in a niche field. It's very unlikely they're going to be able to be able to quote much Shakespeare for example!

## Labour

With women being pretty much permanently pregnant and their unborn children too valuable to the men are going to have to take on a lot of responsibility for the physical labour. Feeding and powering the colony, scouting and exploring the post apocalyptic world.

I suspect it's likely that women are going to take on much more of a planning/learning/teaching roles (which they can do while pregnant) and the men will be the hard labouring.

## in summary

• Lots of babies
• Inbreeding is going to happen, there's nothing they can do about it. However it's not going to be an issue for a generation or two.
• Planned breeding and breakdown of traditional families
• Women will take on academic roles
• Men will largely take on physical roles
• Am I going crazy or does this society sound kind of... nice? – overactor Oct 27 '14 at 13:38
• As long as they have access to StackExchange! – Stephan Oct 28 '14 at 5:49
• A key issue that would need some planning is the sustainable level of technology. The equipment involved in e.g. manufacturing a tractor requires more knowledge and skills than a small population could maintain, even if they had books telling them how to do it. Perhaps the last humans on earth could build a predator proof corral, with automatic feeders, and stock it with horses, mules, and draft oxen. That way the astronauts could start with trained animals, instead of having to catch and train their own draft and riding animals. – Patricia Shanahan Oct 28 '14 at 21:43
• Note that inbreeding doesn't exactly cause genetic defects; rather it allows defects that might be present and problematic to be passed along. They might not be passed or they might be passed and paired with a dominant counterpart. Inbreeding definitely increases the risks, though. – user2338816 Oct 29 '14 at 10:56
• Fantastic answer, but I don't understand your final statement at all. In societies facing many hardships, children are expected to begin working and pitching in at a very young age. Modern "child labor" sensibilities are just that: very modern. So I'd expect a lot of menial and hard labor jobs to be taken by children aged 5-15, maximizing use of educated adults in knowledge fields. Perhaps what you meant was that dangerous jobs would be handled by adult men? – Nicholas Oct 30 '14 at 14:21

Everybody mentions sex and inbreeding, but first these astronauts have to land without assistance, and recover from months in zero-g. Bone loss and muscle atrophy, as well as the impact on your sense of balance, require months of rehabilitation, and without medical personnel to help them, that's going to be tough.

What also matters is where and how they land. I believe the Space Shuttle generally lands on the runway where it's supposed to, but other capsules tend to land in the middle of nowhere, sometimes in the ocean. With nobody to pick them up, humanity's future could be very short indeed.

If they do manage to make it to the rehabilitation center, I hope it comes fully stocked with food and other supplies for a couple of months, because it'll be a while before the astronauts will be able to hunt or scavenge their own food.

If the earthbound people who died made some effort to help the astronauts to survive, they hopefully put the rehabilitation center close to the landing strip (or vice versa), and filled it with all the supplies, tools, equipment and information they could. This center will be the start of the new civilization. It might even include equipment to screen for genetic defects caused by inbreeding. Our astronauts are probably intelligent and educated; inbreeding is probably not going to be their biggest problem. The dead earthbounds may even have transferred the contents of a sperm bank here.

• I like this answer - I've focused very much on what would happen if there's no assistance from the people killed off (I assumed they were too busy dying). But I think it's much more likely that this would have been worked out in advance and the last humans would help to prepare the astronauts! – Liath Oct 27 '14 at 15:31
• This is the best and most hopeful option for survival. But if the astronauts land in a simple capsule in the middle of nowhere, help from the last humans is unlikely to matter much, since it's hard to predict exactly where they'll land. And that will mean the astronauts won't have a safe place to recover, and have to scrounge for food and supplies with atrophied muscles and dizziness. I hope they land in a place with few wild animals and lots of easy to reach food. – mcv Oct 27 '14 at 15:38
• various fiction works consider that landing on a rather specific place is not going to be a problem, if you can decide when you leave orbit and have the required equipement for computing stuff. – njzk2 Oct 27 '14 at 20:37
• Who says they were in zero-g? – BlueRaja - Danny Pflughoeft Oct 27 '14 at 22:50
• In Michael Collins' (third man on Apollo 11) biography, he describes being given survival training in case of having to land in a remote area. I can't remember whether this was specific to NASA or general USAF training, but "you have gone off course into a remote area and have to survive for 24 hours until we find you" is definitely something that would be in mission planning somewhere. – pjc50 Oct 28 '14 at 12:28

Unfortunately, despite human intelligence, despite sperm banks, and despite all the environmental factors being kind to the last remaining 6 - there is no way the population could survive.

Our best estimates for how many genetically different individuals you need to keep a population going with 95% confidence for 1000 years is around 4169 individuals: http://www.sciencedirect.com/science/article/pii/S0006320707002534

If at any time the population dips below that number, the population is considered extinct without human intervention, as the genetic diversity in the population is so prone to genetic drift that all alleles will become fixed or dropped given enough time - irrespective of planned mating arrangements - and recessive mutations kill everyone off.

This is called the Minimum Viable Population (an important statistic when dealing with endangered animals): http://en.wikipedia.org/wiki/Minimum_viable_population

So you've got 3 women. Forget the men, if they can't make babies they are junk, because frozen sperm lasts forever. With the current technology, the only thing that can make a human baby is a human female womb. There are no ex-vivo births. There are no animal surrogates. The technology isn't there, and it's impossible for 6 astronauts working their entire lives to change that.

The most births (not children, births) a human has ever had in their lifetime is 27.

Frozen eggs for IVF last at most 10 years, so the astronauts would never be able to hit 27, only 15, with a baby every 8 months.

Which means the astronauts, could create AT MOST 45 totally unique individuals to get the ball rolling. Those F1 children, assuming they are all girls because male sperm was selected out, would have to use frozen sperm to make their life-time goal of 27 children a reality. Remember, we don't need 4169 individuals, but 4169 UNIQUE individuals, or rather, 8338 unique chromosomes.

So, those 45 use frozen sperm to make 27 half-unique kids: 45 * 27 = 1269 individuals, but only 1314 unique chromosomes!

So picture this.... we are 60 years down the line now, with 1314 people who have about as much genetic diversity as 657 people. Everyone is female. The original 6 survivors are dead. No one remembers why, but they know they have to spend their lives cranking out frozen-sperm babies, because their grandparents said so. They spend their lives crawling around the wreckage (pregnant) looking for signs of frozen sperm, whilst trying to control their 27 kids - 11 of which are also pregnant. They know their needs to be at least 7024 births before the last drop of sperm thaws out and they only have each other for company.

It's a pretty bleak future :P

• There are a few assumptions made in the paper whose result you quote that make it not necessarily applicable to a scenario like the one presented. The first is that the minimum viable population they present is intended to be the minimum population guaranteeing a 99% probability of survival over 40 generations accounting for the possibility of catastrophic events causing further large scale die-offs. – Periata Breatta Oct 28 '14 at 3:43
• Using the inverse of their normalisation model and adjusting for 95% probability of survival over 20 generations without catastrophic events yields a much more manageable minimum viable population of 45. Taking probability of survival to 50% breaks the model -- even a single individual is apparently enough for that. – Periata Breatta Oct 28 '14 at 3:44
• The other is that the model they're working with is animals facing anthropogenic environmental pressure, i.e. it has as its background a human race that is quite good at destroying habitats, hunting populations to extinction, and generally screwing with the environment in ways that are quite likely to kill small populations. – Periata Breatta Oct 28 '14 at 3:46
• Actually we've had the technology to make human babies (and not just clones) outside of the womb for quite a while. They've already successfully done it for pigs and the process outlined is exceedingly simple. The only barrier for humans is the ethical one. I imagine the results would have occurred as early as Dolly if bioethics hadn't roped us back so much. – Black Oct 28 '14 at 10:07
• Even if childbirth did require a human womb, we could still do way better that 27 per woman. Make each pregnancy a multiple and continue way past menopause. Developing the technology to raise a fetus outside the womb needs to be the number one research project. – Jolenealaska Oct 28 '14 at 16:37

With a founder population of three pairs, you can't avoid first cousin pairings. But with four pairs, all of which produce at least one fertile male and female, they can ensure that all descendants will be no closer than second cousins by breeding as shown in this schematic:

M   F   F   M   F   M   M   F   # Morse-Thue sequence of sexes
a   b   c   d   e   f   g   h   # older generation
-+-(---(---(---'   |   |   |
| ---(-+-(-------'   |   |
|     -(-(-----+-----'   |
|       |  ----(-------+-'
,-+-.   ,-+-.   ,-+-.   ,-+-.
a'  b'  c'  d'  e'  f'  g'  h'  # younger generation


This could be implemented in a population as something analogous to the "skin group" system of Australian Aboriginal kinship.

• A male of skin group ab and a female of skin group ef are an ae pairing, which produces children in ab.
• A female of skin group ab and a male of skin group ef are a bf pairing, which produces children in cd.
• A male of skin group cd and a female of skin group gh are a cg pairing, which produces children in ef.
• A female of skin group cd and a male of skin group gh are a dh pairing, which produces children in gh.
• Other pairings are forbidden as incest until the population rebounds enough.

"Inbreeding" on Pin Eight shows a diagram of this pattern continuing through three generations, with a proof that all great-grandchildren are second cousins (and no closer).

And with breeding taken care of, they'd also need some counterpart to the Survivor Library, a project to collect all information needed to restart civilization.

This is one of the most fascinating questions I have seen in some time on this site because there are so many dimensions to the question.

(I am being rather America-centric here but considering the needs of your astronauts - a highly developed space agency, civic infrastructure such as roads, instruction manuals that every astronaut regardless of their origin can read, ease of access to a wealth of resources, safety in terms of wild animals, clean air and water - The USA and to a lesser extent Canada would be a very good choice. Russia would be excellent as well though, and I think most everything I mention here would still apply.)

Specialization NASA astronauts, and I assume current spacefarers from the rest of the world, are chosen for a given mission based upon the experiments and repairs that we intend to execute during that mission. So, you need to consider what these six are good at. You must have at least one pilot and one engineer, probably more of each. Chances are you have a biologist/geneticist who may be an MD. This would be very good for your scenario. You probably have a chemist. Pretty much everyone will be a decent level computer programmer. You may also have a botanist if you are lucky. A lot of prolonged space-based experiments involve growing various plants in zero-G. Whatever you have, they are six individuals of high intelligence in extremely good condition and with very advanced, but fairly specialized knowledge. The practical end result of this is that there will be significant gaps in their knowledge. Let's discuss later where they will be important.

Reproduction This has been covered well. An immediate birthing program would need to be put in place, but I think chances are, the astro-men would have a rather limited role here. Their sperm would remain viable for many years to come, whereas they may be in a hurry to find viable donors of the greatest genetic diversity possible, both male and female. I don't see a whole lot of issue here worth noting. The biggest source of conflict is likely to be that we have a viable Mongolian woman's egg, and just the right South African father's sperm, but the blood types don't match any of the mothers so you have to trek to find the correct anti-rejection drugs. Also, the first several generations of girls will need to start getting pregnant at or near the age of 13. Pregnancy and sex may become vastly separated concepts.

Disease Here's the good news. The plague killed off almost all sources of infectious disease your hero's are going to run across. Both viruses and bacteria tend to be highly specialized and those that will harm humans tend to live only in humans. I say 'tend' on purpose. There are still many that will cross species boundaries, and mammals are going to be your biggest danger with birds close behind. Parasites can be a problem, with malaria being a bigger issue than the common cold to your heroes. Forget the flu and pneumonia, lyme disease and rabies is your killer.

Education This one is tricky. What good will a sociology or pharmaceutical (linguistics?) degree be to generation 0-10 humans? The knowledge would be preserved through books and video lectures, etc., but at what point would you decide that this is knowledge worth having? At such a point, who would be around to teach, or can people have enough general knowledge that they can self-teach? Would such pursuits be planned out by Gen-0 so that Gen-10 has a job assigned to them at birth/first-appearance-of-aptitude? Are they simply lost and rediscovered 1000 years hence?

Some things may seem unimportant at first, and in fact will rely upon skills our astronauts do not have, such as farming. There will be tons of food laying around, and a lot of it would survive for decades, though your choices get fewer and less interesting as time goes on. And six non-experts working full time do not have the ability to raise the biodiversity necessary to make a good lasagna from seed. They would still have to. There are seed banks out there, but they need working refrigeration to last more than a few years.

More on Food Since mammals and birds will be the biggest bio-threat to your heroes, fish would be the best initial source of sustainable protein. It would be a good idea to husband/domesticate a few reptile species as well, though choosing them would have to be done carefully. Iguanas come to mind, and constrictor type snakes that don't get too big would be good. Even boas and pythons that are slaughtered before they get to be around one meter long would be ideal as long as they procreate before that. Wild versions of big-agro plants would be abundant for generations, though the American food-belt would face drought conditions for generations without anyone to maintain irrigation systems.

Security Let's face it. All animals are wild animals now. At the point where people realize there is no bunkering down and surviving this, they will let their pets loose. After an entire year, they are pretty much feral. But after only a year, some will return to domestication with little effort. Dogs and cats may become as important as they were to the cavemen. I ascribe all malicious intent to humans, and we can consider that gone at this point. A solid, well-maintained chain-link fence three meters above ground and one meter below completely surrounding your new home should be fine. The materials are readily available and it wouldn't take more than a week or two to secure a sizable area for several generations.

Keeping the power up That oil/gas/petroleum we were worried wouldn't last more than a few decades? We now have enough for several hundred years. The electrical grid will be hugely unreliable though. A storm could take down a few lines, and if the option is between finding the central office, turning off the proper switches, getting a cherry-picker to the right location, repairing the line yourself and switching the power back on with all fingers crossed, or simply moving to an unaffected area, your heroes are probably moving (Or just living without a power grid). I think localized power sources will dominate. Wind and solar. After a few generations are out the shoot, I wouldn't be surprised to see someone develop a hydro-dynamo in the small-scale. Gas would probably only be used to power cars and for heating and cooking. Viable seed and reproductive cells would have to be moved to where the heroes could guarantee continuance of power. Maintaining this system would be a constant concern, if not a full time job.

I think the interesting part would come in 20 generations into it when two-thirds of the population are forced to found new population centers, or 30 generations down the line when the all-knowing computer program which the progenitors set up says it is time for governance and law enforcement. Or 50 generations in when someone approaches the machine to receive their job and 'Archeologist' comes up, and no one knows what it is.

• Petroleum products has a shelf life. So don't bank on having fuel or gas to burn in ten years. – Taemyr Oct 30 '14 at 14:36
• Looked around a bit, and the conclusion is that even banking on fuel to burn next year can be a risky proposition. mechanics.stackexchange.com/questions/119/… – Taemyr Oct 30 '14 at 14:42
• @Taemyr A good point, but your reference specifies gas stored in a car's tank for all that time. I don't have the expertise to say what other storage solutions and other petroleum products (or unrefined source) would be like. My conjecture discounts most use of gasoline in favor of solar, wind and water. Heating would be natural gas, which is a different beast entirely, and gas only used for long distance trips if necessary. – IchabodE Oct 30 '14 at 17:29

One of the biggest problems reestablishing humanity with a group that small is a lack of genetic diversity. People make jokes about inbreeding because it can be a serious problem, leading to birth defects and genetic diseases. Read up on haemophilia among European Royalty.

So if it is possible for society to store disease free genetic material (sperm and eggs) or at least use existing (hopefully clean) stores of such material, then that would be a huge help. It would probably be helpful if those stores could be moved to one central location where the would be easier to maintain and access. Eventually, this material would make it possible to reestablish a fair range of human genetic diversity. If the machinery can be kept working, the genetic material could be gradually as the population base expands.

On the other hand, if everyone is infected and it is not possible to get or save any disease free material, the survivors are going to have to do the best they can and accept that there may be some problems. The best thing they can do is for each of the three women to have at least one child with each of the three men. The more, the better. Their children should be encouraged to mate only with those who don't share a parent, and in general they should have as many children as possible by as many different people as possible. This will likely result in a society with very different moral and ethical standards from our current society.

• I don't think "disease free" can be emphasized enough here. Note that you describe the cause of your population die-off as a virus, and many (albeit not all) viruses are able to insert their own DNA into the host's genome -- unless great care was taken to ensure this hadn't happened, this would be a quite likely vector for reinfection. – Periata Breatta Oct 28 '14 at 3:20
• Just so we're clear - it is the original poster who specified a virus. And yes, "disease free" is crucial (I did put it in italics after all). – Donald.McLean Oct 28 '14 at 3:54

Newbie here, just stumbled in and am already hooked.

Reading and re-reading the question, I think I understand how I'm supposed to address it . . . . but I can't. Partly due to the impressive knowledge of genetics shown already, partly because I think something needs to be addressed before that. A couple or three modest (ahem) questions:

What if they don't care about propagating the race?

What if even one of the women says, "No way am I going to be impregnated even once, Buster."

What if one or more is/are gay?

Showing both my lack of knowledge and my Bleeding-Heart-Liberal leanings: I assume (America-centric, as mentioned above), astronauts are military people, right? Assuming that, and projecting some bias, some firsthand knowledge and a lot of "data" from movies and tee-vee, I see people who have achieved this level of rank/competence/something as being very conservative. In a non-political sense. People who would be uncomfortable having sex with different partners who are having sex with different partners whom you each know intimately. I can probably project this one onto the women safely, but definitely the men: competitiveness. That's one of the character traits that pushed them to this level of accomplishment. Three of them perhaps not too big on co-operation in societal-sexual matters?

Religion. Might my conservative-projected astronauts be devout Southern Baptists? Catholics? Mormons? Different attitudes about birth, women and morals.

I'm given to understand that astronaut-candidates are tested for psychological stability. Might or might not that have something to do with their attitudes? What if the scenario is so far out of their mental confines--military blinders--one or more explodes mentally and turns into a raging hatchet-murderer?

Okay, I could go on, but I realize I've drifted 'way, 'way off course, but the human/personality angle is what hit me first in this situation. The genetics bit was a quick education for me, though. Thanks.

• About religion: Ideally one of them should realize the parallel between their situation and Genesis 9, which should make them more willing to "be fruitful and multiply". – Damian Yerrick Oct 28 '14 at 2:50
• Hi RMpede, welcome to the site and glad you like it! You've raised some interesting points here and you've got some great questions but but IMHO your "answer" this raises more questions than it answers. StackExchange is a very Question/Answer based format. As it stands I don't think you're answering the question. Head over to chat chat.stackexchange.com/rooms/17213 if you have any questions about what makes a "good" answer! – Liath Oct 28 '14 at 7:36
• Note that being gay doesn't prevent people from reproducing - reproducing doesn't necessarily require intercourse. In this extreme situation there is going to be a difference between who someone chooses as a romantic partner (likely to be just one person), and who they choose to have biological children with (likely as many people as possible), so regardless of sexual orientation most people are going to be having biological children with partners they are not romantically attached to. – trichoplax Oct 28 '14 at 10:50
• Although answers that raise more questions are not a good fit for this site, if you use your additional questions as subheadings and answer each one, leading to an overall conclusion on what they should do, then you could create a full and useful answer. – trichoplax Oct 28 '14 at 10:52
• Note that it is perfectly acceptable to include detail from other answers to cover certain aspects, provided you give attribution to the relevant answer. You could create a full answer by referring to another answer for the genetics side, adding a few subheadings for the points you raise yourself, and then finishing with a conclusion of the best thing to do in different situations. – trichoplax Oct 28 '14 at 10:55

There are three main challenges: Short-term survival; reproduction; and rebuilding society, agriculture and industry in the long term. The last of these is likely to be the most difficult.

# Short-term survival

The astronauts would have to find food and shelter while they recovered from months in zero gravity. This might not be too bad, assuming they could land near (or travel to) a formerly populated area. Suppose New York City has enough stored food (in canned/dried forms which can last a long time) to feed 10 million people for a day; then it can feed 6 people for 4500 years! Similarly, there would be plenty of fuel, medicine, and spare parts for the first few years.

# Reproduction

If the Earth-dwellers set up a suitable bank of sperm/embryos before the end, this would help a lot with genetic diversity. It could still be a problem if one of the surviving mothers has a nasty recessive gene; for a historical example, see the spread of hemophilia among Queen Victoria's descendants.

Frozen sperm are viable for about 50 years, so they have that long to maximize genetic diversity of the survivors. If possible, they should select sperm so that the first two generations born are (almost) all female. If each of the 3 original women (who are likely to be over 30 years of age) has 5 daughters, and each of them has 10 daughters, the second generation is a group of 150 women who all have different fathers (plus a few males, for when/if the sperm bank fails). This should provide a reasonable amount of genetic diversity.

After the "legacy" sperm is no longer viable, the survivors will start reproducing naturally. Having lots of children would still be a good idea, to ensure that disease or natural disaster doesn't reduce the population below a viable level.

# Long-term rebuilding

Things get particularly interesting about 50 years after the disaster.

The last of the original survivors are dying. The population numbers somewhere around 1000.

The good news is, they have books from the past to instruct them in agriculture, medicine, engineering, and other useful knowledge. They still have a lot of old resources to scavenge, although some have been lost as buildings collapse from neglect, and materials are damaged by weather, animals or vegetation, or simply rust away.

The bad news is, they do not have anything like the numbers to maintain the crumbling technology around them. As I argue in this answer, that needs at least 10 million people, which they won't have for several more generations. In particular, modern medicine will be gone. They will at least know about basic hygiene, but stockpiles of drugs and vaccines will have long since expired and they will not have the manufacturing base to replace them.

They will also be short of expertise. You can't learn to be a farmer from a textbook; it requires practical experience which is passed down in person. They will have to rediscover agriculture by trial and error, before the last of the stored food runs out or becomes inedible. They would at least be able to use surviving populations of domesticated food species. (Things like beans, potatoes, and a few pigs and chickens would be useful; this is a case where the doomed Earth-dwellers could help out before the end.) As long as the population remains small, hunting and gathering wild food would be an important part of their diet.

A colony of a few hundred survivors, none of whom remember the Time Before The Plague, will be living a low-technology existence in the ruins of a high-technology world. From this point, it's hard to predict how society would develop -- although a quasi-religion might well develop around the writings of the original survivors. Some well-intentioned laws and commandments laid down by the Founders might be interpreted in very strange ways a century or two later.

• Welcome to worldbuilding.SE and thanks for your great answer. I especially love your views on the low-tech society living in the ruins of the high-tech society. – overactor Oct 30 '14 at 7:31
• How can they risk going to a formerly populated area, as they don’t know how long the bodies will remain infection for. – Ian Ringrose Oct 30 '14 at 13:43
• @IanRingrose: Depends on the plague organism. If it's something like a virus which needs a living host, the area would be safe a matter of hours after the last victim died. If the plague is persistent (in long-lived spores, animal populations, or the like), then given it was virulent enough to kill off the entire human race, our astronauts are probably doomed anyway. – Royal Canadian Bandit Oct 30 '14 at 13:47
• @RoyalCanadianBandit, some virus can live a lot longer then hours in dead bodies and the 6 will have no way of knowing how long the virus can live. – Ian Ringrose Oct 30 '14 at 13:53
• @IanRingrose: Presumably this is something the Earth-dwellers could find out and tell the astronauts before they all died -- how long is the plague organism viable after the death of its host? – Royal Canadian Bandit Oct 30 '14 at 13:56

I would imagine that the biggest problem with such a small number of survivors trying to rekindle the light of civilisation is going to be inbreeding. If society saw this disaster coming and were able to prepare, as you suggested, then the sperm and egg banks of the world could be used (provided they don't defrost too quickly whilst nobody is tending to the power supplies) to widen the gene pool. The biggest hurdle in this case is likely to be viable hosts for carrying test-tube-babies to term. With sufficient advance warning and ignoring the obvious moral objections, it should theoretically possible for a non-human to act as a surrogate mother for a human embryo. The other option is that the female astronauts become serial mothers, acting as a surrogate mother for as many babies as possible during the remainder of the lives.

One more aspect not yet discussed is that for such a small gene pool to survive long term, they will have to separate. Assuming they have communications technology, they can still meet virtually, but there should be no possibility that a simple cold or localized natural disaster will reduce or eliminate the fledgling population.

This will cause problems trying to spread the remaining limited genetic material, but the even if the risk is low, the consequences are too great. Almost immediately upon learning that they are the most likely last hope, they should probably attempt reproduction in space, or immediately upon landing, and separate. Perhaps each pair would leave with the female of a given pair pregnant from a male that she's not staying with, so at least the first two births have different genetics.

Once the next generation reaches puberty, they can be traded to ensure widespread distribution of genetic material. Simple quarantine measures could be enacted to avoid spread of minor illnesses from one encampment to another.

This will ultimately lead, though, to power imbalances and conflict. But that is the nature and history of humanity, so while there will be desires to stamp that out, a generation or four down the line will likely have problems.

This, though, will actually serve to preserve humanity from a genetic standpoint, as long as it occurs generations down the line and not immediately. By ceasing trade of any kind one protects from diseases. By stealing children and integrating them with your community you can bolster your genetic pool. In either case, from the standpoint of restarting humanity, having a group or two that distance themselves from everyone else will ultimately increase human survival. Different environments, difference diseases, different diets, different mutations and so forth will cause different selective pressures.

As long as they can get past that first bottleneck necessarily caused by the small pool of DNA available, separation will improve survival.

• Personally I think the risk of spreading out and trying to survive alone is a bigger risk than some freak accident sticking together. 3 couples are far more vulnerable than 6 people! – Liath Oct 27 '14 at 15:32
• @Liath What makes them more vulnerable? What kind of challenges would they meet that they couldn't handle as two people that they could handle as six? Besides, I'm thinking short distances at first - just far enough apart that a flood, tornado, flu, malaria, etc wouldn't necessarily get all of them at once. – Adam Davis Oct 27 '14 at 23:16
• I was thinking more along the lines that if someone becomes ill or injured it's easier for 5 to care for/defend them than a single person – Liath Oct 28 '14 at 9:32

The thundering machines sputtered and stopped. Their leaders talked and talked and talked but nothing could stem the avalanche. Their world crumbled; the cities exploded. A whirlwind of looting a firestorm of fear. Men began to feed on men. -The Road Warrior

What things were done by the plague faced population? I would assume it was every man for himself. Not many people would be concerned with setting aside commodities for the (completely forgotten about) lucky VI who will probably just burn-up on reentry or drown anyway.

The VI would (and should) have absolutely no concern with maximizing humanity's chances of survival, only their own, which of course leads to the former. They need: water, food and shelter. In that order, provided they aren't going to freeze to death in the meantime, or be eaten (A serious consideration that the Russians took into account, sending their astronauts up with pistol/shotgun/rifle combination guns for if they had landed in the wilderness).

Re-population comes naturally (more so than most of us would like), especially when the drug store is closed. The first few days, months, a year; will be the hungry, hard part. (The Twinkies truck has already been ravaged, I assure you.)

Without human intervention, the Earth would be more of a mess.

Maybe I missed it, but the TV show Life After Humans seems to gloss over the initial period of neglect. Lots of stuff will start to 'go wrong' when people stop showing up for work. Factories such as oil production facilities are likely to have caught fire by now (like Morton-Thiokol's Woodbine plant explosion). Caustic chemicals left in 'temporary' containers are beginning to leak. Failed blow-out preventers? Yuck. Really my question would be:

What hasn't burnt down yet? For a limited time all the utilities would still work; gas, electric, water... which means when a tree falls on a power line, it eventually sets the entire city on fire. Most large cities would have burnt themselves to the ground, left unchecked.

Cockroaches and rats have begun to take over the world, again; setting the stage for another Dark Age style plague. The VI would soon become sick and die from the inevitable next plague or some unforeseen, unseen chemical catastrophe (like the Bhopal disaster).

If they're aboard the ISS, and must escape using the Soyuz, three of them are out of luck anyway. Also, welcome to Kazakhstan, where it's currently sunny and 7 degrees Fahrenheit.

Up to three crew members can return to Earth from the International Space Station aboard a Soyuz TMA spacecraft. The vehicle lands on the flat steppe of Kazakhstan in central Asia. The return to Earth takes less than 3.5 hours.

Lets assume humanity made it; fast-forward a thousand years. Still we would be cleaning up the mess left behind by own defunct and now ancient civilization. The phrase "to big to (let it) fail" comes to mind.

• even worse, the last people on earth might well resent the survivors in space so much that they decide to kill them, say by firing some ICBMs with nuclear warheads at their space station... – jwenting Oct 28 '14 at 14:28
• You seem to assume that the Soyuz can only land in Russia. There's no reason it couldn't land in the US with a little bit of planning. There isn't a big rail going from Russia to the ISS. – Undo Oct 28 '14 at 14:33
• @Undo, you are correct, the Soyuz does have other emergency landing sites: svengrahn.pp.se/histind/Ugol/Ugol.html#Three%20main generally between 35n and 52n latitude; still cold. – Mazura Oct 28 '14 at 20:34
• @Mazura A Soyuz docked at the ISS should be able to land anywhere along the ISS's ground track, +/- a couple degrees. See this image of the ground track - there are certainly some warmer places to land. The designated emergency landing locations are likely places that have enough area to land reliably and are places that won't get Russia bad attention. In a post-apocalyptic scenario, that's not what we're worrying about. – Undo Oct 28 '14 at 21:17

What if they don't care about propagating the race?

What if even one of the women says, "No way am I going to be impregnated even once, Buster."

What if one or more is/are gay?

Given the original premise of all 6 being the occupants of the space station, we would be dealing with 6 highly intelligent, logical people. Even a space-closet drag queen would understand the situation and do what is needed. Highly unlikely any have excessively strong beliefs - a catastrophe of this magnitude would probably wipe out any concepts of a benevolent deity, even if Pope Francis was visiting.

Two things that don't seem to appear elsewhere:

• a quick check of the current (and recent past) female astronaut corps shows that they are all rather old - the one up there now is 38, recent ones have been in their 40s. Perhaps the agencies deliberately select women past their childbearing years? Or you need to be that old to have enough experience. So you're not going to get more than one, maybe 2 children from each female. Puts a severe crimp in the math. The woman who had 27 children probably had some twins and started in her teens.

• lets assume we get a couple of early-20's girls in the mix. You can increase the genetic diversity further by breeding across generations. First-gen female A could probably remain fertile long enough for the son of B+D to reach reproductive age, original males should have no problems with second-generation females - 15 is safe enough which puts even a senior astronaut at under 60. This is the apocalypse, people - we toss ALL of society's concepts out. Need to keep careful records of who is related to whom though, and teach the descendants what to do with them.

• Each question and each answer has a "share" link below it. If you quote from another question or answer, please provide attribution by linking to it and mentioning the poster. Aside from being a requirement of the license, this is also helpful for readers so they can see where quotes have come from, especially as more answers arrive and this becomes less obvious. – trichoplax Oct 28 '14 at 11:05
• You're welcome to join us in chat if you have any questions on this (and just as welcome if you don't...). – trichoplax Oct 28 '14 at 11:06
• A post menopausal woman can still have a fetus implanted. A few of them, actually. – Jolenealaska Oct 28 '14 at 16:48
• @Jolenealaska With the original premise of the world population being 6, none of them doctors, the only thing being implanted will be a parasite. You need a working hospital and medical specialists to do that, and they are all dead. – paul Oct 30 '14 at 1:03
• @Jolenealaska 2. Jolene, meet Mike. He's going to pull some cells out of Joana here and insert them up your hooha. He's never done anything like this before but we found a book and some tools in this building that apparently used to be a hospital before literally everyone else on the planet died. He's got a PhD in Maths and another in Astrophysics so I'm sure everything will be ok. Good luck! – paul Oct 31 '14 at 14:03

Even supposing they have sperm and eggs banks, will they know how to perform IVF?

As other mentioned their knowledge is limited to aerospace science. They'll have to train someone to do that.

Since several excellent answers address the basics, I will try to fill in the gaps with less obvious problems that OP is also asking about.

• some or all of the original six may not be interested in repopulating earth. They may suffer from depression, PTSD, dissociative amnesia, as a result of the demise of the world as they knew it. The idea of being an incubator or giving birth to girls doomed to be incubators from age 15 onwards may seem unethical to some or all three of the women.
• the future generations may refuse to cooperate. There is no guarantee that every (any?) girl in the second generation will agree with the ultimate sacrifice of turning themselves into incubators. With bottled water and imperishable food a plenty, "rebel" youth may not sign up for the repopulation project.
• a tyrant may arise early on. If they take the advice given here, there will be few men around in generations 2 and 3. What if one of them is physically imposing psychopath who decides all the incubator women should serve him instead?

The question of inbreeding could be addressed by using programs used by zoos for breeding of captive endangered animals. Those track and optimize pairings to maximize genetic variability when specimens are rare. That could be kind of a kooky reference for the story.

Black sheep here but I think a good path for this scenario is to emphasize all the reasons why every conceivable attempt at this scenario is futile and in the end, which is not far away, all humans are dead.

Maybe they do believe their goal is to repopulate. Say they have a couple volunteer and quickly into this effort they realize just the basics of restoring a small colony requires more skills than they were prepared for. Nutrients, natural threats, etc. It's not like they're all forced to camp in the Amazon or anything, but a dead to the world remnant of society might prove to be more inhospitable than one may think. For one, with everyone dead what happens to the nuclear reactors left running? Fukushima is on the edge of meltdown right now. If everyone died, what does happen there? How much of the land is off limits from that one alone? Then the rest?

What about miscarriages? Medical emergencies? Basic medicine used for childbirth which could prevent anyone else from volunteering to be the next host. I mean... your crotch is going to be ripped up. Tearing is common. Shall we just guess on how to sew that thing back up?

While I am listing some negative things to dissuade the intention of the question, these pessimistic details might be what makes the story interesting to address. And ultimately the story of survival ends up being a story on failing to survive.

My personal opinion - Humanity wouldn't get even one more generation out of this. If children are born, it will take a village to raise them. This village is all but burnt down from the beginning. One flu could kill everyone. One person who can't handle the depression and isolation. Maybe people disagreed with them once and planted the seeds of rebellion, tyranny, or the belief that it is their responsibility to do "god's work" which is always insane and full of stupidity. A million things could go wrong, and whoever is left to inherit the earth will have to contend with the onslaught of mental and physical challenges most people fail to endure for a few weeks.

... someone here mentioned mormons though. Keep in mind they have enormous stockpiles of armageddon survival in unknown locations. If a plague hit, for sure some of the higher ups who know where they are may have migrated there. They are preppers, and they are not alone. It might be hard to conceivably eradicate all of human kind without some of these preppers being isolated, filtered, and much better off than the crew of this vessel. If you are adamant about all of humans except the 6 being dead, consider the mormon stockpiles probably still exist. If you get to one of those, and you can figure out how to get in, your odds are greatly improved.

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