We turned wolves into Chihuahuas, transforming them from beasts to basically rats.

If aliens captured humans and made them into pets, what are the most extreme features humans could develop through selective breeding and domestication that just can't happen with natural evolution?

The best answer describes a possible human with features that are borderline impossible but still technically feasible without genetic engineering, just domestication—not just small humans or extra tall humans.

Think of how, for centuries, there have been people with the hobby or profession of creating new breeds of dogs, horses, cows, sheep, and whatnot.

Now imagine these aliens hold contests and competitions where they showcase their new breeds.

We have machinery and 3D printers, but we still hold contests for handmade sculptures.

In the same reasoning, those aliens hold contests for modifying species the old and more "artistic" way, even if they have the technology to genetically engineer whatever they want.

someone sugggested a time limit to make it reasonable.

the time limit is the same it took us to breed dogs,but counting only the modern era, about 2000 years

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    $\begingroup$ I would recommend the book “All Tomorrows” for a possible answer. However, it does deal with genetic engineering rather than artificial selection, so it may not be the best for this question. $\endgroup$
    – RHF
    Commented Jun 24 at 17:54
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    $\begingroup$ For a fanciful answer, read Jack Vance's The Dragon Masters. $\endgroup$ Commented Jun 25 at 2:01
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    $\begingroup$ @elemtilas Humans are animals and everything you do with animals, you can do with human. $\endgroup$
    – Jemox
    Commented Jun 25 at 6:34
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    $\begingroup$ @elemtilas we had slavery until 182 years ago and we still have slavery in some countries ....if humans are so hard to adomesticate because they rebel so much?. Why has there been slavery for as long as there have been humans? slavers aren't all mighty powerful aliens, they are just people even if you are a slave, you can kill people, maybe not powerful aliens but you can kill your slaver and free your country .... Truth is humans are just pathetic animals with no spirit. $\endgroup$ Commented Jun 25 at 10:18
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    $\begingroup$ "Weird and Worrying Questions from my Inbox" $\endgroup$ Commented Jun 26 at 9:06

7 Answers 7


That's not a Genetic Disorder, that's a feature!

Apert syndrome, Angelman syndrome, Down syndrome, Achondroplasia, Pediatric Acromegal, Albinism, Scheuermann's Kyphosis, etc... all of these phenotypes are rare in humans because they make wild humans so selectively less fit, but they all cause significant changes in a person's physical appearance and demeanor, and when it comes to dog breeding, what's cute, different, or neat often trumps what's actually good for the animal's health. Likewise, aliens could use their advanced biomedical technology to keep profoundly unhealthy breeds of human alive despite terrible health and impaired cognitive abilities.

By mixing and matching, you could get a breed of humans with pink eyes, bulbous heads that look like beluga whales, fused/extra fingers and toes, hunched backs, permanent smiles, hair all over their entire bodies, extreme shortness or tallness, the list goes on and on.

Combine this with the large number of already existing healthy phenotypes, and humans could easily be bred to be just as diverse as dogs, and like with dogs, it would come at the expense of good health.

Oh, and don't forget that these are Aliens with advanced technology

Humans are already starting to get pretty good at forms of genetic manipulation that does not even require genes in a specie's gene pool. We've hybridized strawberries with fish DNA to make them more frost resistant. We've hybridized rice with bacteria to make them produce more vitamin A. If that is the level of God humans can play on other organism's gene pools, the options available to our alien overlords would be unfathomable. They might just decide to make humans that can only survive in a specific mixture of boiling alcohol and paraffin just so that they can keep us in their lava lamps.

Even if these breeds are banned from competition, that does not mean the aliens won't make them. And over time, some might get mixed in with the mutts from which they could eventually get parts of their DNA snuck into official breeds.

  • $\begingroup$ To add to this excellent answer. If you look at breeding of farm animals, especially chickens. You can increase size by about 4x. Very large, or small humans are possible too. $\endgroup$
    – UEFI
    Commented Jun 25 at 10:52
  • $\begingroup$ How did you miss alopecia? We breed dogs and cats for that; no reason aliens wouldn't breed humans for it. Also don't forget about hypertrichosis. (Oh, you did get that one...) And, of course, various forms of dwarfism; it isn't just size that changes, but proportions. $\endgroup$
    – Matthew
    Commented Jun 25 at 21:34
  • $\begingroup$ @Matthew Yeah, I cut back the list because it was getting stupidly long as I was writing it. That's why you see a reference to Hypertrichosis in the second paragraph, but it's not in the first. $\endgroup$
    – Nosajimiki
    Commented Jun 25 at 21:45
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    $\begingroup$ Great answer. The story possibilities are endless and the stuff of nightmares. $\endgroup$ Commented Jun 25 at 21:48
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    $\begingroup$ As a bonus, this will likely create a whole bunch of new and interesting conditions for the aliens to study! $\endgroup$
    – vinzzz001
    Commented Jun 26 at 13:57

Humans are exactly the same as other mammals.

Although it's distasteful, you could breed humans exactly the same way.

This is a surprising/bizarre question because, totally evidently, the various wonderful "breeds" of humans you see before you, with enormous differences in height, skin type, facial features, body shapes etc., obviously all exist and happened in a trivial amount of time, 20 thousand years or so.

It's not really clear what you're asking... The answer is just "yes, obviously, it's weird you are asking!";

and if you're asking the time-scale of these Alien Breeders, it would be "the odd 10,000 years", a few hundred generations.

Now looking at your dog example. The smallest dogs are say 1/4 the size of smaller wolf strains, and the biggest dogs are say 2x the size of the biggest wolf strains. (Just talking "size" extremely generally.)

So let's just refer to that as a "4-2" breeding achievement.

Dogs are by FAR the thing humans are most interested in breeding, #2 is horses. Horses have much less "breeding achievement" by us, call it like "3-1.5" at best.

If you're asking the technical question, just HOW extreme could we breed horses/humans/dogs in terms of size, that is a pretty interesting biological (?) question, and I doubt anyone knows the answer.

Since dogs/humans/horses are all absolutely identical in terms of every mechanism of genetic, reproduction, and embryonic processes, I have to believe the "extreme possibilities" would be roughly the same.

You could maybe make a guess that, it's easier to breed mammals "very small" than "very large" - that is true of both dogs and horses.

(With cattle, we very much WANT them to be huge, but they are not that much bigger than the original, we have not achieved like "4x" or anything.)

In short, if you're asking "just how much bigger/smaller" we have an answer for dogs, horses, cattle and it's something like "at the extremes, probably about 0.25 through about 2.00 ..."

So that's probably the best answer.

  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Worldbuilding Meta, or in Worldbuilding Chat. Comments continuing discussion may be removed. $\endgroup$
    – Monty Wild
    Commented Jun 26 at 0:06
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    $\begingroup$ Humans are likely to be more limited in size variation than dogs, etc. because of our bipedal structure and because we already pose difficulties in giving birth. $\endgroup$ Commented Jun 26 at 7:45

Far less than could be achieved with other animals

One would naturally look at the incredible things done to dogs, cows, pigeons, etc. and think that it should be the same with humans, but this is not the case. There are two main reasons for this:

Humans breed slowly, grow up slowly, have small broods, struggle with childbirth already, and have difficult growing conditions which makes us unsuitable for effective selective breeding. If you want to breed a human for high intelligence, say, then you need to wait a minimum of thirteen or fourteen years between rounds, and probably until they reach adulthood. Especially if you want to consider any possible negative side effects, when you'll want to wait thirty or so. Plus each human is banging out a small number of progeny each time, so you're getting a limited range of options to choose from. Worse, humans are smart and social, requiring that they be bred in groups who will do their very best to breed in ways that undermine your carefully laid breeding plans. And even then, humans already have to be born in a strikingly immature state because of the limitations of squeezing our oversized noggins through the birth canal - try and breed a larger human, for example, and you're likely to rapidly hit limitations in the ability of women to successfully birth these giants.

Humans have low genetical diversity. There is less genetic difference between human populations from different continents than there is difference between Chimpanzee populations from two sides of a river. The 1000 genome project estimates that, on average, two randomly chosen humans differ in about 0.6% of all base pairs (with most of these differences being silent anyway). This lack of genetic variation means that there are many fewer differences in your starting population to use as raw material than with most other species¹.

Combined these limitations mean that the breeding programme is going to be less effective than with animals we've domesticated in the past. It would still be possible to breed for particular traits, and create some level of modification but it'd be much less dramatic than Great Danes and Chihuahuas.

¹ In case you are wondering, it makes very little difference to this if you pick humans with genetic heritages from different parts of the world. Most human variation is within rather than between populations.

  • $\begingroup$ It would just take more time. Regarding difficulty with birth: As far as I’m aware there are dog breeds which pretty much need a cesarean section for birth. $\endgroup$
    – Michael
    Commented Jun 26 at 12:46
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    $\begingroup$ The comment about the low numerical genotypic diversity seems totally irrelevant - there is clearly a large amount of phenotypic diversity among humans, regardless of the percentage of shared DNA among them - that 0.6% accounts for people over 8 feet tall, ones under 3 feet tall, people weighing hundreds of pounds or less than 5, a wide range of skin colors, etc. There are still millions upon millions of variable genetic loci that can be selected for, the notion that we don't have a sufficient number of interesting traits that could be bred for makes no sense. $\endgroup$ Commented Jun 26 at 14:15
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    $\begingroup$ Depending on the level cruelty and horror the author wants to inject, the first point can be worked around through advanced technology. Scifi like Lois McMaster Bujold's Vorkosigan Saga introduced the idea of the "uterine replicator", an artificial uterus to gestate and birth children. Why couldn't Aliens have similar and thus harvest "seeds" from their favored samples and create 'broods' en masse artificially. It would add a huge element of horror to the whole concept. $\endgroup$
    – Fodagus
    Commented Jun 26 at 17:04
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    $\begingroup$ In this sense, humans are already a very selective breed of hominids. $\endgroup$
    – alamar
    Commented Jun 26 at 21:41
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    $\begingroup$ @MindwinRememberMonica why does number of chromosomes matter? It doesn't matter if you have 20,000 genes in 2 chromosomes or 20, meiosis and sexual reproduction still work the same way with the same results. $\endgroup$
    – Philip
    Commented Jun 27 at 22:54

Humans don't breed true, at least in part because genetics of many animals (including humans) are complex and a fairly arbitrary trait is actually the product of a whole bunch of genetic effects, and you can't optimize for that trait without messing up a bunch of other things. Have a read up on pleiotropy, for example.

This is why out of the enormous number of animal and plant species on the Earth, only a tiny portion of them have been domesticated and of that tiny proportion an even smaller number can actually be bred to produce a significant number of distinct breeds. With dogs you can get things as different as Bernese and Chihuahuas and Greyhounds and so on, but cats don't exhibit nearly so much diversity.

Also consider horses, where some people tried very hard to breed the ultimate race horse and ended up with something that falls down dead expensively if you feed it slightly the wrong thing at slightly the wrong time.

There's also the issue of how long it takes to raise a new generation to adulthood to see if they actually express the traits you want, and how long it will take for their children to grow to adulthood so you can verify that the traits are neatly heritable. Lets imagine though that your aliens are sufficiently long-lived that they can see the results of their tinkering without getting bored or dying of old age.

if people are bred like dogs, what can be achieved?

Charles II of Span

(Charles II of Spain)

D'you want Habsburgs? Cos that's how you get Habsburgs.

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    $\begingroup$ "Humans don't breed true" Really? from the link you provided > "In the world of selective animal breeding, to "breed true" means that specimens of an animal breed will breed true-to-type when mated like-to-like" so why is it when you "breed" a Caucasian to a Caucasian you don't ever get what looks like a Chinese baby then? or vice versa, I think you are letting your moral distaste for the idea (and yes it is a distasteful one, for the real world) colour your words, you either don't understand the words or are not telling the truth, either way it's a bad answer as a result > [-] $\endgroup$
    – Pelinore
    Commented Jun 24 at 16:38
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    $\begingroup$ @Pelinore you do realise that “doesn’t breed true” doesn’t mean “randomizes all genes”, right? $\endgroup$ Commented Jun 24 at 17:01
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    $\begingroup$ I don't see how this proves the idea doesn't work. The Habsburgs(and other similar examples) were the results of inbreeding, not controlled breeding. That can be avoided with actual controlled breeding. As for why so few species were domesticated, it wasn't because of the way their genes work(we share mostly the same DNA as dogs), but practicality. Some are easier to domesticate than others and require fewer resources, but neither seems to be a concern here. I don't see any reason why this is impossible, it would just be more time-consuming, but that doesn't seem to be a concern here. $\endgroup$
    – Bubbles
    Commented Jun 24 at 17:57
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    $\begingroup$ Humans certainly breed true. Kids are typically both mentally and physical similar to thier parents. Things that don't breed true must be cloned to recreate positive traits. For example, if you take the seed from a Gala Apple and cross pollinate it with another Gala apple (or any apple really), 999 times out of 1000, what you get will be a far less appetizing crabapple tree. Every Gala apple in every store in the world is cloned from other Gala Apple trees as is every other variety of grocery store apple. $\endgroup$
    – Nosajimiki
    Commented Jun 24 at 22:01
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    $\begingroup$ When you compare pugs to wolves, you realize that the Habsburg were still pretty fine, compared to what can be achieved if you really go for it. $\endgroup$
    – Jemox
    Commented Jun 25 at 6:31

It depends greatly on the time span allowed. There is tremendous variety of different genes available in the human gene pool that can influence human development in a myriad of different ways.

So in the short term (ten's to hundreds of years) selective breeding could produce a wide range of shapes, sizes and features in humans. But this would be limited to the existing genetic flexibility in the human gene pool. So small, tall, fat, skinny, hairy, hairless and much more in a wide range of features that we are more or less familiar with.

In the long term (thousands to millions of years) mutations would provide a slow trickle of new possibilities allowing for all manner of extremely weird body types.

In the very long term (more than ten million years) mutations and artificial selection could morph a human into almost anything. Although there are limits to what can be achieved using biological means as in many cases there might not be a series of small steps that provide a route (no macroscopic axles, no internal combustion engines etc).

Some "short term" possibilities: very docile humans and hyper aggressive humans, humans with low intellect, humans in a range of colours.

Some "long term" orange, blue or green humans. Humans with huge arms, legs, heads or other body parts twice their normal proportions, humans with multiple sets of arms and legs.

The power of selective forces over vast stretches of time should not be underestimated, but the simple brutal expanse of these vast stretches of time should not be underestimated either. As an example a million years is enough time for a mouse sized creature to evolve into an elephant sized creature without anyone without written records noticing.

I'm not suggesting that this would happen, but using it to illustrate the expanse of time (1mm average size increase per century).

  • $\begingroup$ More specifically, it depends on the amount of generations, which is a product of available time and generation intervals. If the aliens aren't bound by human standard of ethics, they can manage to squeeze out a few more generations in the same time interval. $\endgroup$
    – vinzzz001
    Commented Jul 2 at 8:55

I'd look into modern plant "non-GMO" breeding techniques.

It is often a multi-step process.

Step 1: You identify a genetic sequence you want to express. Maybe found in a related species, maybe a protein you want to produce, or maybe something your computer models predict is useful.

Step 2: You splice it into a sample of the plant using GMO. You check that the resulting plant has the properties you want.

Step 3: You irradiate large numbers of germ line cells, like stem cells you can convert into embryos with a bit if biochemistry. You cause these cells to reproduce. Then you sequence them. Those that move closer to the genetic sequence you want in the chromosome in question you preserve.

Step 4: You breed away the other mutations and get the "closer chromosome" isolated in a viable sample. Now repeat step 3.

Step 5: You now have a viable chromosome with the changes you wanted "naturally bred". Your plant glows when exposed to UV light, or contains an anti-malarial compound, or is resistant to roundup, or has fun looking speckles on its flowers.

Much of this is limited by breeding speed. So what I'd be tempted to do, as an alien who wants to breed humans, would be to develop a dominant set of genes on chromosomes that would massively reduce maturation and reproduction times. This lets you fast-forward the slow "manual breeding" problem. They you just breed out those "speedup" chromosomes over a generation or two to get back a "true" human with "all natural" genes.

Part of our genetic code includes things like a linear sequence, from head to tail, of what limbs we get. So humans with new limbs is plausible; getting the limb attachment points to be viable is going to be difficult. You'd have to breed "second hips" before you could breed useful "second legs".

Similarly, if "C-sections are required" are allowed, you could close up humans' hips. On the other hand, you could breed a generation of humans who cannot walk and "open up" the birthing canal in the front or the back, migrate it out of the hips, then breed the hips closed again - giving a birth canal that is no longer limited by our hips, a serious block on humans evolution (it limits head size, which in turn means babies are born very premature for a non-predator species).

Completely crazy things may also be possible. A sufficiently advanced species might know of efficient sequences breed mammal style lungs into avian/dinosaur style lungs; their lungs are much more efficient. To pull this off, you might need to first give humans redundant lungs and breed the redundant lungs to be avian, then reduce the mammal lungs to vestigial, or support the transitional humans with supplementary oxygen (maybe even directly oxygenated blood for exceedingly non-functional intermediate lungs).

The ability to strongly select for intermediate "non-viable" stages is big - even bigger is the ability to support the survival of such intermediate "non-viable" stages with technology until they become viable.

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    $\begingroup$ You mention premature babies for a non-predator species. As I understand it, humans are a persistence hunting predator (obligate omnivore specifically) species. $\endgroup$
    – Iiridayn
    Commented Jun 26 at 23:40

Beyond just "what is possible", another thing to consider is that such a scenario removes certain constraints that somewhat moderates natural human development: economics. One thing a lot of genre fiction (i see it a lot in speculative scifi and progression fiction) gets wrong is "survival of the fittest". A lot of people and subcultures has this view of evolution as "Law of the Jungle," "the strong take from the weak," "only the strongest will survive". This is... not entirely accurate. "Fittest" doesn't mean strongest persay. If humans were cars, evolution would not be creating super powered muscle cars, it would be making Toyota Corollas. Why? Economics. Big and powerful needs lots of food, and it needs premium food. Look at peak performance athletes: they eat A LOT of food. Michael Phelps, at his peak, was noted in one interview to eat around 10,000 calories for breakfast. That's 5 days worth of food for a statistically average adult. True, a lot of that had to due how much training he was doing and he likely eats less these days, but to be in peak physical condition, you need a lot of food.

Out in the wilderness, this doens't pan out well. At a base line, you need more food, so you need to hunt more. The act of hunting itself consumes more food, and this rapidly spirals into a feedback loop and assumes that there's even that much food to go around. Meanwhile, the guy that can get by on 1/5 of your needs and still be at their peak performance is going to be a lot more successful at raising a family and continuing their lineage. This does translate to modern living too: a lot of people struggle just to survive as they are. If they suddenly needed to eat five times as much food... they would probably not be able to afford to live. This would be made worse if everyone, world wide, needed more food as food prices would soar and further drive people towards famine.

These are considerations that are removed by having people treated as "pets" by aliens. If the aliens can afford to feed their favorites peak diet, then said favorites could continue to develop to their genetic maximum. The aliens could better select the individuals that reach greater peaks, and thet could select for traits that wouldn't exist without the artificial support of their labs. They could create true marvels of physical ability that would dominate any baseline human in physical or mental competition.. but if you plopped down a tribe of these super humans in a primal, hunter gatherer environment, they would rapidly decline from starvation and malnutrition.

I mean... a lot of modern era humans suffer from nutrient deficiencies. It's really common for people, even in developed nations, to suffer for iron, calcium, magnesium and Vitamin B deficiencies. Our ability to develop is often constrained by what resources our environment could reasonably provide. Pets in a lab could be developed that have crazy dietary needs that would be next to impossible for natural creatures to acquire in sufficient quantities.


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