Aliens have arrived to Earth from the depths of space, and thankfully they are peaceful and only want to learn about us and interact with us.

Despite some initial trepidation, first contact goes well and the Aliens quickly learn our languages and offer an exchange of cultural and scientific knowledge. These Aliens are quite friendly and they seem quite taken with us Human Beings. Inter-species friendships soon start forming, and despite some opposition from those who do not quite trust our new friends, a peaceful political alliance is formed between the United Nations and the Aliens.

Some of us, who are particularly curious, end up in very amorous relationships with these Aliens. Like us, these Aliens have quite a high sex drive and many of them find us Humans quite attractive. Many people find the idea of sex with Aliens to be quite odd or even disgusting, but for some people: that is their kink and they love it.

It transpires that not only is mating possible between a Human and an Alien, there is also potential for procreation. Complications are abound in the successful delivery of a hybrid child, but with the combined medical and technological knowledge of both us and the Aliens we are able to make it happen.

A whole sequence of events unfolds from not only the presence of these friendly Aliens in our society but also the creation of this new hybrid race.

But what I am interested in is, speaking in terms of both appearance and genetics, how different can these Aliens really be from us, if mating and child birth is possible between the species.

I am assuming the Aliens would have to be pretty close to our DNA and fairly close to us in appearance - but I want to make them as Alien as possible whilst still allowing for inter-species breeding.

So the question is:

How different from us can we make the Aliens whilst at the same time ensuring that procreation is possible between the species?

I have left some things intentionally vague to give people some room to get creative. For instance, I am not dictating whether the children can be born in either a Human or Alien womb or both or neither - I am only stipulating that procreation is possible (although successful births can be difficult). I am also not stipulating the types of genders for the Aliens - they can have male and females sexes like us, but if you can find a way for singled-sexed or 3-sexed (or whatever) to work, go for it. The more different they can be the better.

The big green tick will go to the answer that can show the biggest difference(s) between these Aliens and us whilst at the same time demonstrating that inter-species intercourse and breeding is possible.


It has already been established by many here that the Aliens would have to be extremely similar to Humans in order for them to be able to procreate. This isn't an issue for my story and stating this doesn't really answer the question. Even if the differences are minor, please give examples of what they could be - if it turns out that the biggest difference can only be something like "the Aliens have purple eyes" then this is an acceptable answer (and preferable to being told once more about Chimpanzees).

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch - Reinstate Monica Nov 13 '20 at 20:34
  • $\begingroup$ Is it important to have the offspring genetically similar to both parents? This might be possible to some extent, even for sort of realistic Aliens with a genome very different from us, or even without DNA as we know it! I've edited my answer to account for this. $\endgroup$ – EdvinW Nov 14 '20 at 17:51

You might get away without the Aliens having close-to-human DNA, or even without DNA at all!

LSerni touched on the idea of parthenogenetic conception, but I suggest a slightly different take on it.

The easiest way, if you want to keep some biological connection to both parents, would be to let an alien be able to "impregnate" a human by planting the seed to its offspring into her womb, where it grows as a "parasite" until birth. This is not so different from what a humans normally do; a human fetus could (technically) well be thought of as a highly specialised parasite.

This would strictly speaking not require any genetic similarities between humans and aliens. It would require that a human womb is sufficiently similar to wherever alien fetuses grow though. Perhaps even more importantly, the body of the intended birth mother should somehow receives similar stimuli that she would during a normal pregnancy so as to respond as if she was indeed pregnant in the traditional sense. This might be done artificially by injections of hormones and whatever else is needed, or, if we are lucky, the alien embryo can secrete this itself and otherwise "fool" the body of the mother. I would personally go with the former as it's much more believable.

Concerning the actual birth you could go two ways.

The first is to see the alien fetuses as parasites that need to be extracted by a c-section before it matures and finds a way out itself using its claws. This could be used for some drama. It becomes less plausible that anyone would be voluntarily impregnated, but accidental pregnancies are a thing after all.

The second is to allow for actual births. I like this, as it opens up for much less dark stories. Why not making the giving birth to an alien easier than doing so for a human? Maybe they have smaller heads at birth? (regardless of the size when grown) It might just be me, but I think the idea of human/alien sex becomes much more appealing if it doesn't potentially result in the death of one of the participants!

Speaking of risks, you might also want to make failing to assist the pregnancy "only" resulting in the death of the fetus and not too lethal to the mother. This way, you can have safe sexual encounters between species. Perhaps even safer, as at least viral STDs are quite species specific!

Like you specified, pregnancies wouldn't occur naturally, "but with the combined medical and technological knowledge of both us and the Aliens we are able to make it happen."

Now, you might wish to have the human birth mother have more influence on the child than just being a vessel. She might even have significant effects on the child's genome!

Say that the baby, once born, closely latches on to it's birth giver and starts to adopt her features after birth? Chameleons can change their skin colour, and some octopuses can change their skin texture as well to mimic their surroundings (https://en.wikipedia.org/wiki/Mimic_octopus ). It wouldn't be too far fetched to have Alien babies have an instinct to mimic their parents after birth, and have the traits stick after some certain age!

It's even possible the external features of the mother that the baby adopts will ingrain itself in the baby's genes! This is actually a thing even for Earth life and even humans: https://en.wikipedia.org/wiki/Transgenerational_epigenetic_inheritance

It's well documented that the effects of, for instance, starvation will cause changes in one's organism which are then inherited and can be detected in the starved persons grandchildren. It's not just food supply that affects us though; for instance the Wikipedia page mentions a Swedish study: "The Överkalix study noted sex-specific effects; a greater body mass index (BMI) at 9 years in sons, but not daughters, of fathers who began smoking early. The paternal grandfather's food supply was only linked to the mortality RR of grandsons and not granddaughters. The paternal grandmother's food supply was only associated with the granddaughters' mortality risk ratio."

If you want to be realistic, the Aliens probably don't even use DNA as we know it to encode it's "genome", but this is no problem. If even humans can do it, letting the features the baby adopts from it's mother become encoded into the baby's genes is definitely possible!

I think this is as close as you can get: You get a way for Aliens and humans to have intercourse, which results in pregnancies and babies which, soon after birth, will show some resemblance to either parent. The child would be genetically similar to its Alien parent(s) and have a genome which is affected by its human mother, though, as many answers have pointed out, it's not really possible to let the baby directly use the mother's DNA without some pretty vivid hand waving.

EDIT2: I originally thought of the embryo impregnating the human to be genetically similar to a single alien, as in parthenogenesis, but I suppose this is not strictly necessary! Maybe the Aliens can reproduce EITHER sexually or asexually? An Alien child born by a human could thus have either a single alien parent or two alien parents. The precise mechanics of how such a three-way-conception would occur, should you like the idea, I leave up to you. Just thought I'd share the idea!

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    $\begingroup$ This is an excellent answer, and exactly the out-of-the-box thinking I was looking for. What you propose makes me think of friendly cute xenomorphs. $\endgroup$ – Jimmery Nov 15 '20 at 12:30

Procreation the easy way

The aliens are similar in their biology and biochemistry to some gasteropods, fish, frogs and assorted reptiles: the female is capable of parthenogenetic conception.

Once the mechanism gets triggered - and for that, arousal and intercourse with a human turns out to be enough - a female child will be born to the alien mother that has no genetic relationship with the human father (it is in fact a genetic clone of the mother), but is nonetheless culturally considered as the father's legitimate daughter.

Procreation (and intercourse) the natural way

You need the aliens to be, well, by definition of the same species as humans. For offspring to be born, DNA differences must be negligible. Human DNA is around 3.2 billion base pairs; the "right" differences of less than 2000 base pairs are enough to trigger miscarriage, and no offspring. Natural differences amount to about 24 megapairs (0.75%). We can handwave differences up to about 5% provided they are very, very specific (for example a human XXY, XXYY or similar genetic makeup).

But in all "important" things, these aliens have to be humans. So, very similar shape and not too different sizes. Alien humans living in lower gravity might be still interfertile and yet way bigger, lacking the drawbacks of gigantism; but not much more than that.

How far can we push differences? We need small DNA variations that yield a large difference in the phenotype. Perhaps we can twiddle the HOX genes and have a human compatible (and fertile) pair where one partner has six limbs - two legs and four arms, or two arms and four legs, and maybe eight fingers per hand too. The offspring might either be four-limbed or six-limbed.

Skin texture and colour, body hair or lack thereof, nails, eye color, size and shape can all be factored in easily. There are symbiont (and not so symbiont) viruses and epigenetic factors that can further change the appearance of a Mark I human to make them more "alien"; these are diseases in humans but might have evolved in a more controlled, even functional or useful form in "aliens". There's lots of choice, from neurofibromatosis to epidermodysplasia verruciformis (a bit of advice there: do not Google. Just take my word that you can get Klingons, Jem'Hadar, Orions, Cherons, and even Groot).

Procreation the hard way

(Liberally inspired by Diane Duane's Sarek)

The child is in truth neither the mother's nor the father's. It is a genetic construct built from traits and mechanisms inspired by their parents', but they are not inherited, they are engineered in. A team of expert alien gengineers has mapped out all appreciable human and alien features, and decided how best to compromise and bestow them on the child (which will be brought to term in an artificial womb). From the number of fingers to the details of protein digestion, the newborn is for all intents and purposes an artificial organism that simply very closely resembles, as far as possible, both parents.

This was, if memory serves, how Diane Duane decided to smooth over the too many impossibilites about Spock's birth, from Spock's blood being copper based and his mother Amanda's, obviously, being iron-based.

So: intercourse, and mutual attraction

The intercourse mechanics must be roughly compatible. But they needn't necessarily resemble either species'.

Sex between one of Cherryh's Hani males and a human female would be possible, if possibly not so satisfying for the human female. Also, fur. The reverse also would be possible. In both cases, comparatively easy.

Attraction for a lion-maned, felinoid biped poses no particular problem. Judging from Internet forum and novelty gadgets, it's already a thing. For the reverse, deferring to C. J. Cherryh's expertise, we would need strong liquor and hope - naked skin being blandly repulsive to hanis. But the eye of the beholder and that sort of things.

Looking for very different partners, sex with one of Eric Flint's Mother of Demons' squid-like gukuys would require first of all a largish gukuy - not quite a battlemother - and then the human male (since all compatible-sized gukuys would be female due to their extreme sexual dimorphism) would need to insert one or both arms inside the other's mantle. Meanwhile, the lady could reciprocate in a different manner, and the human would need to wear protection: all human biological fluids are extremely poisonous to gukuys, and vice versa. On second thought, perhaps also shoulder-length gloves would be a good idea.

In spite of all that, it would probably work. It would be straight sex for the gukuy (they make sex with their arms, as squids do) and oral - well, sort of oral - sex for the human. I cannot envision either being in the least attracted by the other, but at least they're absolutely out of each other's uncanny valley.


Procreation between aliens and humans requires a lot of handwaving.

The only examples in our biosphere of species which can procreate are those which are already closely related genetically through a common ancestor species. The idea that a lifeform which evolved in a completely different biosphere is genetically compatible with a human is just not scientifically plausible.

But there are a couple tricks you can use.

  • Panspermia. Humans and aliens didn't actually evolve independently. They are both descendants from an older spacefaring species which settled both on Earth and on the alien home planet. We would have noticed if humans weren't genetically related to anything on Earth, so humans being imported wouldn't be plausible. But perhaps the aliens are actually descendants of a hominini species from Earth and they indeed did notice long ago that their genes are completely different from everything else on their planet. Perhaps that's because 100,000 years ago some aliens abducted some Neanderthals and settled them on a different planet as part of an experiment and then forgot about them?
  • We / the aliens are so good at genetic engineering that we can make it work. The children aren't really a natural offspring of humans and aliens, but completely genetically designed. They are more like an artistic interpretation how such an offspring could look.
  • The offsprings aren't actually genetic offsprings of both parents but actually pure aliens. The alien reproduction is asexual from a genetic point of view, but they still need a partner to trigger it. That partner does not provide any genetic material, but does certain... things... to the alien which trigger their reproductive system. The alien culture expects the person who triggered a pregnancy to take part in raising the child. The alien culture considers upbringing as far more important than genetics, so they don't consider the non-genetic parent as a less important family-member than the genetic one.
  • Just hand-wave it. Star Trek, for example, has lots of examples of interspecies crossbreeding without providing any plausible explanation how that's possible. The audience is just expected to accept that as a part of how the ST universe works. Which it generally does.
  • $\begingroup$ "The idea that a lifeform which evolved in a completely different ecosystem is genetically compatible with a human is just not scientifically plausible." There are examples of different species of animals from different ecosystems breeding (e.g. the Liger), so I don't think your statement here is correct. Plus I've already stated that I assume the Aliens will be very similar to us, and yes, I am using something similar to panspermia to explain the similarities. $\endgroup$ – Jimmery Nov 13 '20 at 14:56
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    $\begingroup$ @Jimmery But tigers and lions are closely related (they are both part of the genus Panthera). $\endgroup$ – Philipp Nov 13 '20 at 15:07
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    $\begingroup$ Perhaps the confusion comes from your definiton of ecosystem being slightly different than mine. I think I will use the term "biosphere" instead. $\endgroup$ – Philipp Nov 13 '20 at 15:08
  • $\begingroup$ I've already said that I assume that the Aliens will be very similar to us in appearance and genetically. Kinda like how Tigers and Lions are very similar in appearance and genetically. $\endgroup$ – Jimmery Nov 13 '20 at 15:49
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    $\begingroup$ Star Trek did not handwave it but had a TNG episode explaining Pamspermia occured and that Humans, Klingons, Romulans (and by virtue, Vulcans and Vulcanoids), and Cardassians were descended from a common ancestor. We can also put Bajorans in this catagory. At least one episode showed that even then sometimes two different species couldn't cross breed as Worf and Jadzia had to request medical assistance in having a child because it wasn't possible naturally, despite Trill looking near identical to humans. $\endgroup$ – hszmv Nov 13 '20 at 17:56

DNA Difference of less than ca. 0.5 %

Humans and Chimpanzees share [98.8 to] 99 % of the genome. We can't procreate with Chimpanzees, so the aliens need to be closer than that.

There had been Homo sapiens neanderthalensis and homo sapiens sapiens hybrids that were fertile and procreated. In other words: to be genetically viable partners, the aliens are genetically belonging to the genus of Homo sapiens, and possibly about as distinct from us as the Neanderthal. These Two types of Homo sapiens share 99.7 %, diversity of modern humans is less.

The total difference would be about as much as between human and neanderthal, possibly twice as pronounced. The skull shape comparison between the two is this, Neanderthal on the right, modern human on the left. The general body shape will be the same, but proportions might be shifted to some degree, muscles could be more pronounced or less.

enter image description here

Generally, the level of diversity that we see in many of the very human looking StarTrek aliens is what seems to be in the possible area: Klingon Browridges (dark-skin type) and Vulcan pointy ears are most likely well inside the procreation-possible area, as might be Romulan features, Betazoid black eyes, Tavnian baldness, Trill skin patterns and many more. As an example where might be the line: While Cardassians are described as looking reptilian, they are still mammalian, and might be just on the inside of the genetic boundary. Remans might be just on the other side of that similarity edge, as they are truly different in makeup and atop that have undergone a high degree of genetic modifications.

Borrowing those ideas from StarTrek, it might be most easy to explain the genetic similarity by having a "common progenitor" (Homo heidelbergensis), which either invaded from the stars to earth or had an exodus.

  • $\begingroup$ Can you give any examples of what that 0.5% might end up looking like? Bigger noses? Different skin colour? Longer necks? Bigger feet? $\endgroup$ – Jimmery Nov 13 '20 at 16:44
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    $\begingroup$ @Jimmery Considering it: you are looking at StarTrek aliens... not so much the green-skinned aliens, but Klingons, Vulcans and Romulans. But StarTrek offers many examples of close-human aliens $\endgroup$ – Trish Nov 13 '20 at 17:20
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    $\begingroup$ @Trish: In Star Trek Lore, this was addressed in a TNG episode that stated that most sentient life in the galaxy is reasonably gentetically compatible because they all share a common ancestor in an extinct progenitor species that seeded most if not all sentient life that exists in the galaxy. That said, in DS9's season 6 finale, there's a scene where Worf and Jadzia Dax (Trill) were discussing a possible of child with the station Doctor. It was not impossible with current medical technology, but was not possible through natural means+ $\endgroup$ – hszmv Nov 13 '20 at 17:44
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    $\begingroup$ @Matthew the "low" percentages on Chimpanzee-human overlap were disproven in more recent research, they stem from bad samples as far as the current reason goes. The lowest ever credibly claimed was as low as 94%. Human and Chimpanzee cells are virtually identically in shape. $\endgroup$ – Trish Nov 14 '20 at 15:53
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    $\begingroup$ @Matthew this 2007 paper claims the 94% while this 2014 article shows again 99% for chimps and 98% for Gorilla. - Generally, it seems that the sometimes 94% includes endemic genes and if genes are active, but apparently more modern research disclaims the number again. $\endgroup$ – Trish Nov 15 '20 at 15:34

For there to be a viable alien-human child, the aliens would have to be closer to us than monkeys - so, highly unlikely. That said, you're in charge of justifying this - so, what would you need to make this viable?

  1. A biological capacity for sex between either alien males and human females or vice versa (or both, of course). This just requires the organs to be the right size.
  2. Compatible DNA - this is tricky. The DNA of the aliens would encode their different biological proccesses to us, even if they use the same system as us. So all of the aliens' vital organs would have to be similar enough to ours so that an organ that was part of each would still function.

Or, given your request for the aliens to be different as possible from us, what might work is a system where the aliens take DNA samples from sex and their body takes any useful features not present in themselves and passes it onto their children. Such a species would probably only have one sex, although they could also have a large number of sexes and en-mass mating to ensure that they get as much potentially useful DNA for their offspring as possible.


Unfortunately procreation with an alien is highly unlikely. Procreation between a chimpanzee and a human is probably not viable and we share 99% of our DNA. So the alien needs to have the same basic biochemistry (unlikely) that is: use proteins, sugars and DNA that we use and have the same labyrinthine metabolic pathways as us and share more than 99% of our DNA. In short they need to be very similar. You might get away with something like a great ape at a push. Sorry.

Note for those who haven't yet seen it I recommend looking at this link https://www.sigmaaldrich.com/technical-documents/articles/biology/interactive-metabolic-pathways-map.html This gives just a fraction of human metabolic chemistry. If you start messing around with any of it it by eliminating proteins that catalyse any of these reactions won't take long before your dead.


Some-- maybe not all-- of the aliens actually reproduce asexually, but they have genetic diversity because of a symbiotic relationship their species has with a retrovirus which implants the DNA of others into the zygote.

Your human exchanges bodily fluids with an alien and is infected with the alien retrovirus. The retrovirus incorporates a small portion of your human's DNA, and later ends up back in the same or a different alien. Maybe the virus is passed among a chain of humans or other animals before getting to the alien.

When the zygote is infected with the virus, a small portion of DNA is incorporated from whoever/whatever was infected previously.

This way you can vary the amount of DNA passed on so you don't have to have much of a match. You can also have your alien technically reproduce with whoever/whatever you want regardless of sex or species. A young man, his grandma and their dog can be involved in making the alien love child.


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