# The last two humans are brother and sister **URL:** \ **Category:** In My Humble Opinion\ **Created:** [September 16, 2013, 1:46pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968 "2013-09-16T13:46:39Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Mr.Kobayashi](https://avatars.discourse-cdn.com/v4/letter/m/4af34b/32.png) [@Mr.Kobayashi](https://boards.straightdope.com/u/Mr.Kobayashi)\ **Post date:** [September 16, 2013, 1:46pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/1 "2013-09-16T13:46:39Z")
“Suppose atomic bombs had reduced the population of the world to one brother and one sister, should they let the human race die out? I do not know the answer, but I do not think it can be in the affirmative merely on the ground that incest is wicked.” > **[Bertrand Russell - Wikiquote](https://en.wikiquote.org/wiki/Bertrand_Russell)** Bertrand may not know, but I reckon you’ve got an opinion. When Russell wrote that it was 1954 and bombs were the big menace, but these days I doubt anyone thinks they will wipe out almost everyone. So instead, let’s say an extremely deadly virus wipes out everyone except a brother and sister who are immune. Should humanity die with them or carry on? Oh, and for the smartarses the spermbanks and frozen eggs have been without power for too long, so no joy there. However to make things less squicky artificial insemination techniques are well known these days. ---
**Author:** ![Prof.Pepperwinkle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/prof.pepperwinkle/32/13627_2.png) [@Prof.Pepperwinkle](https://boards.straightdope.com/u/Prof.Pepperwinkle)\ **Post date:** [September 16, 2013, 1:53pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/2 "2013-09-16T13:53:00Z")
Two people are not a big enough gene pool, brother-or-sister or not: humanity is doomed. I understand there are some places (Netherlands, Spain, France) where brother-sister relations are legal. So, let 'em have fun if they want for what it’s worth. It won’t change anything. ---
**Author:** ![XT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/xt/32/456_2.png) [@XT](https://boards.straightdope.com/u/XT)\ **Post date:** [September 16, 2013, 1:56pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/3 "2013-09-16T13:56:20Z")
Unless they are also geneticists with access to a lot of diverse genetic material (i.e. frozen sperm and eggs) we are doomed. So, I picked something else…they need to get busy impregnating the last woman with eggs and sperm from as many of that stock pile as they can, and then raise the children and start a breeding program asap…and hope nothing happens in the mean time. ---
**Author:** ![Bricker](https://avatars.discourse-cdn.com/v4/letter/b/977dab/32.png) [@Bricker](https://boards.straightdope.com/u/Bricker)\ **Post date:** [September 16, 2013, 2:07pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/4 "2013-09-16T14:07:34Z")
> [@XT](#): > > Unless they are also geneticists with access to a lot of diverse genetic material (i.e. frozen sperm and eggs) we are doomed. So, I picked something else…they need to get busy impregnating the last woman with eggs and sperm from as many of that stock pile as they can, and then raise the children and start a breeding program asap…and hope nothing happens in the mean time. Is that categorically true? Or could we imagine a brother and sister with exceptionally clean alleles whose children and interbred grandchildren would be reasonably healthy, and who undertake to teach their offspring the necessity of mixing it up – breed with your cousin instead of your brothers; with your second cousins instead of your first cousin, etc. Can you explain why it’s an automatic death spiral? (I don’t disagree – I just don’t understand.) ---
**Author:** ![Sofis](https://avatars.discourse-cdn.com/v4/letter/s/e5b9ba/32.png) [@Sofis](https://boards.straightdope.com/u/Sofis)\ **Post date:** [September 16, 2013, 2:13pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/5 "2013-09-16T14:13:59Z")
The 50/500 rule (which I assume is what **Prof. Pepperwinkle** and **XT** are referencing with their doomsaying) is not an absolute inviolable law of nature, it is a rule of thumb. It is in particular a rule of thumb used by conservation biologists, who typically deal with species under a lot of stress (usually by humans, in one way or another). These surviving siblings and their offspring would have little competition for resources and few natural predators, and would be in a much better shape than most species. Their odds may be bad, but that’s no reason to not have a go at it. ---
**Author:** ![Omega\_Glory](https://avatars.discourse-cdn.com/v4/letter/o/7c8e57/32.png) [@Omega\_Glory](https://boards.straightdope.com/u/Omega_Glory)\ **Post date:** [September 16, 2013, 2:19pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/6 "2013-09-16T14:19:44Z")
> [@Bricker](#): > > Is that categorically true? Or could we imagine a brother and sister with exceptionally clean alleles whose children and interbred grandchildren would be reasonably healthy, and who undertake to teach their offspring the necessity of mixing it up – breed with your cousin instead of your brothers; with your second cousins instead of your first cousin, etc. Can you explain why it’s an automatic death spiral? > > (I don’t disagree – I just don’t understand.) I don’t think mixing it up would matter much. All the cousins would genetically be siblings, since they all come from the exact same gene pool. ---
**Author:** ![XT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/xt/32/456_2.png) [@XT](https://boards.straightdope.com/u/XT)\ **Post date:** [September 16, 2013, 2:21pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/7 "2013-09-16T14:21:31Z")
> [@Bricker](#): > > Is that categorically true? Or could we imagine a brother and sister with exceptionally clean alleles whose children and interbred grandchildren would be reasonably healthy, and who undertake to teach their offspring the necessity of mixing it up – breed with your cousin instead of your brothers; with your second cousins instead of your first cousin, etc. Can you explain why it’s an automatic death spiral? > > (I don’t disagree – I just don’t understand.) Well, I’m no geneticist nor do I play one on the SD, but from [here](http://en.wikipedia.org/wiki/Minimum_viable_population): > [@](#): > > Minimum viable population > From Wikipedia, the free encyclopedia > Jump to: navigation, search > > Minimum viable population (MVP) is a lower bound on the population of a species, such that it can survive in the wild. This term is used in the fields of biology, ecology, and conservation biology. More specifically, MVP is the smallest possible size at which a biological population can exist without facing extinction from natural disasters or demographic, environmental, or genetic stochasticity.[1] The term “population” refers to the population of a species in the wild. For example, the undomesticated dromedary camel is extinct in its natural wild habitat; but there is a domestic population in captivity and an additional feral population in Australia.[clarification needed] Two groups of house cats in separate houses which are not allowed outdoors are also technically distinct populations. Typically, however, MVP is used to refer solely to a wild population, such as the red wolf. > [@](#): > > Estimating the MVP > > Minimum viable population is usually estimated as the population size necessary to ensure between 90 and 95 percent probability of survival between 100 to 1,000 years into the future. The MVP can be estimated using computer simulations for population viability analyses (PVA). PVA models populations using demographic and environmental information to project future population dynamics. The probability assigned to a PVA is arrived at after repeating the environmental simulation thousands of times. > > For example, for a theoretical simulation of a population of fifty giant pandas in which the simulated population goes completely extinct, thirty out of one hundred stochastic simulations projected one hundred years into the future are not viable. Causes of extinction in the simulation may include inbreeding depression, natural disaster, or climate change. Extinction occurring in thirty out of one hundred runs would give a survival probability of seventy percent. In the same simulation with a starting population of sixty pandas, the panda population may only become extinct in four of the hundred runs, resulting in a survival probability of 96 percent. In this case the minimum viable population that satisfies the 90 to 95 percent probability for survival is between 50 and 60 pandas. (These figures have been invented for the purpose of this example.) This is why if either the brother or sister were doctors specializing in reproduction AND had access to a bank of frozen sperm/eggs from a diverse population we might have a chance if everything went right: > [@](#): > > MVP does not take human intervention into account. Thus, it is useful for conservation managers and environmentalists; a population may be increased above the MVP using a captive breeding program, or by bringing other members of the species in from other reserves. > > There is naturally some debate on the accuracy of PVAs, since a wide variety of assumptions generally are required for future forecasting; however, the important consideration is not absolute accuracy, but promulgation of the concept that each species indeed has an MVP, which at least can be approximated for the sake of conservation biology and Biodiversity Action Plans.[2] > > There is a marked trend for insularity, surviving genetic bottlenecks and r-strategy to allow far lower MVPs than average. Conversely, taxa easily affected by inbreeding depression – having high MVPs – are often decidedly K-strategists, with low population densities while occurring over a wide range. An MVP of 500 to 1,000 has often been given as an average for terrestrial vertebrates when inbreeding or genetic variability is ignored.[3][4] When inbreeding effects are included, estimates of MVP for many species are in the thousands. Based on a meta-analysis of reported values in the literature for many species, Traill et al. reported a median MVP of 4,169 individuals.[5] From memory we hit a population bottle neck about 70k years ago where we almost went extinct. I seem to recall that at one point the human population was down to a few thousand at most. Today, with modern genetics and stocks of frozen sperm and eggs as well as the know how to use it we might have a chance with just two people (what if the woman has complications in child berth? What if one of them catches the flu or something else that proves fatal?) IF they were lucky AND they had access and know how to use the technology. If they didn’t I’d say the odds of two people by themselves breeding enough to ensure a viable human population is just about zero. Even with it I’d say it’s pretty long odds for just two people. ---
**Author:** ![JustinC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/justinc/32/8083_2.png) [@JustinC](https://boards.straightdope.com/u/JustinC)\ **Post date:** [September 16, 2013, 2:21pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/8 "2013-09-16T14:21:56Z")
They are the only survivors so must have exceptional genes. I say get on with the multiplying. ---
**Author:** ![silenus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/silenus/32/166_2.png) [@silenus](https://boards.straightdope.com/u/silenus)\ **Post date:** [September 16, 2013, 2:36pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/9 "2013-09-16T14:36:35Z")
Go for it. The residual radiation will ensure that there is enough mutation going on in the shallow end of the gene pool. ---
**Author:** ![Czarcasm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/czarcasm/32/4050_2.png) [@Czarcasm](https://boards.straightdope.com/u/Czarcasm)\ **Post date:** [September 16, 2013, 2:39pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/10 "2013-09-16T14:39:33Z")
Go for it-it’s the only sex they’re going to get. ---
**Author:** ![Loach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/loach/32/350_2.png) [@Loach](https://boards.straightdope.com/u/Loach)\ **Post date:** [September 16, 2013, 2:44pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/11 "2013-09-16T14:44:53Z")
> [@Prof.Pepperwinkle](#): > > Two people are not a big enough gene pool, brother-or-sister or not: humanity is doomed. I understand there are some places (Netherlands, Spain, France) where brother-sister relations are legal. So, let 'em have fun if they want for what it’s worth. It won’t change anything. And New Jersey and Rhode Island. IIRC those are the only two states in which there is no incest statute. As long as both parties are consenting adults of course. Can’t get married but its not illegal to have sex. In NJ at least it does bump up the age of consent from 16 to 18 if it is a blood relation. ---
**Author:** ![Yorikke](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yorikke/32/4335_2.png) [@Yorikke](https://boards.straightdope.com/u/Yorikke)\ **Post date:** [September 16, 2013, 2:45pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/12 "2013-09-16T14:45:54Z")
Some of us fuck our sisters regardless. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [September 16, 2013, 2:46pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/13 "2013-09-16T14:46:33Z")
why not. they breed and have imbecile kids, they can breed with them and them with each other making more imbeciles. sooner or later disco happens. ---
**Author:** ![Bricker](https://avatars.discourse-cdn.com/v4/letter/b/977dab/32.png) [@Bricker](https://boards.straightdope.com/u/Bricker)\ **Post date:** [September 16, 2013, 3:21pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/14 "2013-09-16T15:21:09Z")
Ok, so there seem to be a couple of different hazards. I understand the chances of extinction are high based on environmental concerns: if a hurricane hits your idyllic tropical paradise and kills the brother and sister, then the race is dead regardless of any genetic concerns. But I didn’t quite follow why the genetic concerns on their own are so certain to be fatal. So let’s assume we’re talking about physical safety for a thousand years – no natural disasters, attacks by predators, or loss of food supply. Is the race doomed solely because there’s just two people? And do we even care that they’re brother and sister at that point? ---
**Author:** ![XT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/xt/32/456_2.png) [@XT](https://boards.straightdope.com/u/XT)\ **Post date:** [September 16, 2013, 3:30pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/15 "2013-09-16T15:30:57Z")
> [@Bricker](#): > > Ok, so there seem to be a couple of different hazards. I understand the chances of extinction are high based on environmental concerns: if a hurricane hits your idyllic tropical paradise and kills the brother and sister, then the race is dead regardless of any genetic concerns. > > But I didn’t quite follow why the genetic concerns on their own are so certain to be fatal. > > So let’s assume we’re talking about physical safety for a thousand years – no natural disasters, attacks by predators, or loss of food supply. > > Is the race doomed solely because there’s just two people? And do we even care that they’re brother and sister at that point? If neither one has a resistance to some disease that crops up then their kids won’t either. Adios muchachos, to paraphrase from Aliens. Genetic diversity is what allows some humans to survive when something catastrophic like a new variant on a disease crops up. The reason it’s important in this situation is that a brother and sister are going to be less genetically diverse, and the genetic material they pass on to their offspring will also be less diverse. To be viable for the thousand years you are proposing they are going to face things like disease, and all it’s going to take is one disease that this brother and sister don’t have any immunity to at all and are particularly susceptible to. Every year someone dies from, say, the flu, while others get it and survive…and some folks, even without immunization never get it even when exposed. If a flu strain came up that this brother and sister were particularly susceptible too then so would their descendents be…and it could easily wipe out the entire population, especially since it would be pretty small and concentrated for many, many years…hell, for centuries. ---
**Author:** ![Jonathan\_Chance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jonathan_chance/32/701_2.png) [@Jonathan\_Chance](https://boards.straightdope.com/u/Jonathan_Chance)\ **Post date:** [September 16, 2013, 3:46pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/16 "2013-09-16T15:46:49Z")
Yeah, but the OP isn’t ‘can they survive’ it’s ‘should they bang’? VERY different questions. In the end, it boils down to ‘is the incest taboo strong enough that humanity should lay down and die instead of violating it?’ My answer is get banging. Anything’s better than lying down to die without trying. Hell, even with the genetic angle, extinction isn’t a lock. Assumptions: Start with 1 male and 1 female, siblings Assume each female has 12 children Assume a 50% mortality rate from whatever (including genetic defects) Assume children born are 50-50 male and female First generation 2 Second gen: 6 Third generation: 18 Fourth generation: 54 Fifth generation: 162 Sixth generation: 486 Seventh generation: 1458 Eighth generation: 4374 Ninth generation: 13122 Tenth generation: 39366 At which point warfare breaks out over who are the TRUE descendent of nuclear Adam and Eve. ---
**Author:** ![Anaamika](https://avatars.discourse-cdn.com/v4/letter/a/5f9b8f/32.png) [@Anaamika](https://boards.straightdope.com/u/Anaamika)\ **Post date:** [September 16, 2013, 3:49pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/17 "2013-09-16T15:49:42Z")
Why would laws or the creep factor of incest even play a part? I never, ever want to have children, and I guarantee you if it was the end of the world I’d be trying to get pregnant ASAP. Duties change at that point. ---
**Author:** ![drewtwo99](https://avatars.discourse-cdn.com/v4/letter/d/d07c76/32.png) [@drewtwo99](https://boards.straightdope.com/u/drewtwo99)\ **Post date:** [September 16, 2013, 3:55pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/18 "2013-09-16T15:55:40Z")
If my sister and I were the last two on earth, I’d probably convince her to try it. But no way would we actually have sex. We’d use a turkey baster or something. ---
**Author:** ![bup](https://avatars.discourse-cdn.com/v4/letter/b/6bbea6/32.png) [@bup](https://boards.straightdope.com/u/bup)\ **Post date:** [September 16, 2013, 3:57pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/19 "2013-09-16T15:57:16Z")
Guinea pigs (or hamsters) in captivity all come from a population of like 2 pigs. Incest birth-defects are over-stated because of the yuck factor. Humanity’s got a fighting chance if the woman is healthy and can produce a couple/few females (and ideally a male, otherwise Dad’s going to have to have sex with his daughters). Of course, this is easy for me to say. I don’t have a sister. ---
**Author:** ![Jonathan\_Chance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jonathan_chance/32/701_2.png) [@Jonathan\_Chance](https://boards.straightdope.com/u/Jonathan_Chance)\ **Post date:** [September 16, 2013, 3:58pm UTC](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968/20 "2013-09-16T15:58:22Z")
That’s why, Anaamika. The incest taboo is pretty far down in the subconscious for some people. Not everyone, certainly. But I bet a majority of people would wrestle with it for a while before making the approach. [Next page](https://boards.straightdope.com/t/the-last-two-humans-are-brother-and-sister/668968.md?page=2)