In a story I'm just beginning to write, the aliens who have invaded Earth have a ghastly secret that is about to become known: They reproduce exclusively by transforming other sapient beings into more of their own kind...

At this point, I know what you're thinking: "Isn't this just classic space zombies? Or maybe Cybermen?"

You would be right to think that at first, but there is a crucial difference here: Individuals who are transformed in this way retain their original personalities and memories; They aren't just being used as raw material to create something new, as those converted retain their selves and identities.

My question then is "How and why would this happen?" Why would this being the only way a species would maintain and increase its numbers, and by what means would it do so?

Here are some criteria answers must meet:

  • The conversion must involve radical physical bodily changes; A change of identity, possibly involving minor bodily alteration, is not enough
  • Those converted must retain all their original memories and something that could be recognised as their original personality; Original instincts and bodily drives may be replaced, but every aspect of a person's personality not inherent to their species should remain as close to the same as possible
  • This species must reproduce in such a fashion exclusively; They cannot simply convert other species into more of their own while also reproducing in the normal way; Conventional reproduction must be impossible for them
  • This means of reproduction must be inherent to the species; It cannot be that this particular culture just so happens to forbid other methods of reproduction. Note that this does not mean that the reason could not be psychological; For example, artificial beings hardcoded never to create new consciousness would be acceptable
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    $\begingroup$ Whilst the subject is interesting, this seems fairly story-based to me so I'm gonna VTC. Doubtless others will disagree, though. In the meantime, consider the parallels with religious orders, especially ones with vows of chastity/celibacy/removal of reproductive bits. For an example of the former, consider Shakers. The priesthood of Cybele might be an example of the latter. $\endgroup$ Commented Mar 26, 2022 at 12:33
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    $\begingroup$ Thanks for calling Umbrella Corp, if you seen a broken glass tube with our logo press 1, to speak directly to our staff press 2. Sorry currently all of our staffs are engaged... $\endgroup$
    – user6760
    Commented Mar 26, 2022 at 13:09
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    $\begingroup$ Ashes to Ashes 🤔 do I hear echoes of the Kobali? 😉 $\endgroup$
    – Pelinore
    Commented Mar 26, 2022 at 17:09
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    $\begingroup$ @Pelinore Probably in my top ten "Harry Kim has a girlfriend" episodes. $\endgroup$
    – Daron
    Commented Mar 26, 2022 at 22:37
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    $\begingroup$ is resistance futile, or not? $\endgroup$
    – ths
    Commented Mar 27, 2022 at 0:33

10 Answers 10



enter image description here

Drawing by kakashi on Pinterest.

The species manifests as a collection of growths on the victim. The growths change the victim's appearance. They did not have growths before. Now they do. Appearance change -- check!

The growths also modify behaviour by injecting a cocktail of new hormones into the victim's body. The behaviour includes moving to a warm humid place and finding new victims.

Since the victim retains their brain, they have the same memories and personality. Only the personality is gradually broken down as the new hormones encourage new behaviours.

For an oversimplified real world example if you were to take a shot of testosterone every morning, then you would not change your memories or personality. Nevertheless after several months you would find yourself being more irritable and aggressive. More likely to move things around the house even when told not to.

Welcome to the swarm.

enter image description here

  • $\begingroup$ From your design your species could reproduce by budding. Would both males and females exist in your species or only males? $\endgroup$ Commented Mar 30, 2022 at 2:29
  • $\begingroup$ @SabrineCrystal The growths might create buds that send out spores that implant in other victims. $\endgroup$
    – Daron
    Commented Mar 30, 2022 at 11:47
  • $\begingroup$ @SabrineCrystal If the growths are anything like Earth fungi they do not have different male and female ones. $\endgroup$
    – Daron
    Commented Mar 30, 2022 at 11:48

Upload consciousness to machine bodies.

Your aliens are aware of the benefits of genetic diversity as regards the long term fitness of a biological race. The aliens are non biological. Instead of genetic diversity, they seek intellectual diversity. A variety of minds among their kind will maximize the chance that their kind will do well, and adapt to unforeseen future circumstances.

They upload the consciousness of sentient creatures into machine bodies that they have prepared. These machine bodies vary greatly and will depend on the ultimate planned habitat for the sentient - and it is very probable one intelligence will switch bodies many times because after upload the consciousness is effectively immortal. One consciousness might actually inhabit a number of brother / sister bodies.

The original biological entity harboring this consciousness might or might not be harmed by the procedure, according to the needs of your story. I like the idea that the original is left to go about his or her business, with the duplicate spirited away to join its new synthetic race.


Can a brain in a jar be killed or have a reduced lifespan from psychological trauma alone?

And of course: https://www.hplovecraft.com/writings/texts/fiction/wid.aspx

What a fine quote from Henry Taylor! And perfect for these aliens who value minds.

Why Would Aliens Enslave Humans?

We are not enslaving you. We are helping you and ourselves in the only economics that matter... the commerce of our minds over the mindlessness void.

  • $\begingroup$ Why would they bother seeking out new conciousnesses to upload into machine bodies after they had the first few? if you can upload you can make as many copies as you like from the 'master copy', might be useful to add a few new ones once in a while to broaden your 'racial perspective' as it were but there'd be little point assimilating any entire species you come across if you don't actually need their physical bodies. $\endgroup$
    – Pelinore
    Commented Mar 26, 2022 at 18:52
  • $\begingroup$ @Pelinore I would assume they value unique identities, since the answer hinges on acquiring unique minds. Out of millions of people, you will get a true cross-section of unique beings. But the question doesn't require them to make any significant percent of the population into their own image. They could cherry-pick the best and brightest. I didn't upvote this because the answer is not sinister enough. Maybe if they only picked REALLY unique people, like the insane, and then placed them in charge of humanity... $\endgroup$
    – DWKraus
    Commented Mar 27, 2022 at 0:23

I'm shocked that the usual suspects haven't already hit this one, but we already have examples of this in human mythology - lycanthropes and vampires being the most obvious. Both of these creatures reproduce exclusively (bizarre science experiments in certain movies notwithstanding) by converting normal humans, but can I justify the rest of it?

Radical Physical Changes

Lycanthropes certainly undergo radical changes, albeit conditionally, but we can't really tell them apart from humans when they're not transformed. Vampires though? Well, that depends on the specific mythos we're looking at.

I present to you The Red Court from The Dresden Files:

Red Court Vampires

They look pretty normal, right? Well that black monster on the floor is their true form. They produce a natural skin suit to pass as human. (Well, 'natural' in as much as their body does it, but it's magic of course.)

Retaining Memory

This ones simple. While their new form comes with certain appetites and psychological effects, all the stories tell us that the newly minted vampires and werewolves retain all of their memories and at least most of their personalities. The person is converted, not replaced.


There are some tales of vampire families that reproduce the same way humans do, and a few I can think of where lycanthropy is a heritable trait, but the vast majority of the stories tell us that you get bitten or drink the blood and you change, otherwise no new vamps or weres. It's not a choice, it's a biological necessity.

But how does this relate to your aliens?

Well, clearly the stories are only ignorant superstition. Like most mythology they take a little bit of truth and wrap it in layer after layer of nonsense. The core truth in vampire stories is that your aliens exist, they don't look human unless they really work at it, and they can only reproduce by converting other entities. Any compatible animal will do, but only intelligent entities can be taught to control and conceal themselves after the change.

Also, only intelligent entities can consent to the change. Humans are ideal for this: they can be convinced to willingly become something more than human.



How: The logical way to do this is to have a symbiotic organism that is relatively simple (no intelligence on its own) that both conveys some significant advantage to its host and that itself reproduces by a primitive means like binary fission. The infected individual is compelled to infest others, and I would have them induce belief in the superiority of those infested over those not.

  • The hosts themselves may still be capable of reproducing their own species separately, ensuring a steady supply of hosts going into the future. So your symbiote won't burn themselves out when they are stuck in a limited environment.

Why: The symbiote is primitive in its reproduction, and reproduction is highly conserved. As long as the organism is successful in reproducing, there isn't an absolute requirement to reproduce any other way.

  • The host provides the adaptability to deal with new environments, so as long as the symbiote can infect a host, the host can do the adaptation for the benefit of the symbiote. Thus the need for sexual reproduction is significantly reduced.

  • If desired, a type of sexual reproduction can occur in people double-infested incidentally, but that's just if you want to maintain the diversity advantage of sexual reproduction. It isn't required.

The advantage can be anything, like telepathy with anyone else with the symbiote, or great physical endurance, regeneration, etc. but it must give an infested species an advantage. The advantage could be as simple as every host considering every other host as "one of us," taking advantage of one of the simplest tribal impulses of life to include and exclude. Combined with the impulse to spread the symbiote, it could become a near-religious obsession - raise up all sentient life in the universe to a higher, better state. Selfish and noble drives alike push the hosts to spread the symbiotes.

SO: I envision an organism that leaves the hosts as themselves, but somehow 'better'. Your "invaders" believe they are doing a great favor to your humans, and perhaps they really are. As long as you can keep making new humans, and the symbiote gives an advantage that promotes the continued success of the hosts, where is the downside?

BUT DWK, how is this converting others into their own kind? Well, from the perspective of a symbiote, every host represents one of their own kind. All hosts are seen by all other hosts as "one of us." You could even see humans embracing this as a way of becoming one with all other intelligent life, ending conflict and war, and uniting humanity. After all, you want the people to still be themselves, yet somehow now not.

  • $\begingroup$ Sounds a bit Goa'uld to me. To'kra if you're lucky :P $\endgroup$
    – Corey
    Commented Mar 27, 2022 at 8:26

In the Startrek Enterprise episode "Extinction" the crew encounters a species that was pushed to extinction by an unknown catastrophe.

The species laces the planet's biosphere with a mutagenic virus that rewrote the DNA of the crew members to convert them into a member of that species, and embedded memories into their that emerged in the form of dreams and a homing instinct.

Presumably the aim was to rebuild the race using other species as genestock.

Few details were given in the episode.


Medical advances and longevity.

In a theoretical future society, perhaps the original alien species improves medical sciences and techniques focusing on longevity. Longevity, comfort and abscence of physical pain is the focus. As this happens, the population of the planet seeks equilibrium by limiting the number of newborn. Over the decades, fewer are born, the average population remains the same. The instinct of having children is slowly but surely repressed.At the same time, the medical advances have not focused on maintaining reproductivity, since: a) over-population was considered a major threat to society for a long time b) popular goals was longevity, comfort and pain-free existence

too late to revert, the extremely aged but healthy population has come to the point loosing the ability to reproduce naturally.

While still being able to reproduce by cloning, the instinctive surge to have new and more unique individuals to be introduced into their society becomes stronger, as the individuals in the society becomes older... now essentially immune to the effect of aging.

They reach out to the stars, where they find new friends to interact with. Naturally, as a giving and open society, they share in their medical science that is focused on longevity, comfort and pain-free existence... to other civilizations. They also bring a solution to over-population, but they do not push any solution on anyone.

A consequence of the medical science they bring, is that over the years, any sapient creature subject to it, converges to the same physical characteristics... but this happens very slowly and gradually.

Modify as you please to add various degree of malignant intent, if your story so requires...


Heavy genetic engineering left the species sterile

Your scenario sounds a lot like the Kett. Perhaps you should look at the lore for inspiration.

One of the reasons provided there is that 'their genetics reached a point of stagnation, leaving the species unable to breed through natural means'. Sounds plausible enough. Couple this with Kett's genetic engineering technology being advanced beyond any other technology they might have, and... when you've got a hammer, everything looks like a nail.

One problem you might have with this explanation is that the level of sophistication of genetic engineering needed for the exaltation suggests the Kett should be able to reverse the infertility issue, should they wanted to. However, it is apparent that:

  • Their method of reproduction gives them an advantage on the battlefield, in that their opponent might be reluctant to fight what can well turn out to be their exalted brethren
  • The exaltation process has become deeply ingrained in their culture, and seems to be regarded as part of a messianic mission to shape the life in the Universe into their own image, a form of 'lifting' other species, so to speak. I'd wager to say any attempts to reverse it would be seen as a transgression against the very nature of the species

...and so, there are both practical and cultural reasons to maintain the status quo.


A particular species knows much about space travel, but only some about genetics, about the same as we do, or even less.

The species becomes mostly extinct after encountering some illness on one of their voyages, spreading across their entire network before they realize it exists. This illness increases the pace of aging, even of maturity, and the oldest of their society are already dead. The rest are left infertile, and age at a rate unheard of for their species. Although they have had the technology for cloning for a long time, it still requires a host, and none of them are in a shape fit for doing so.

However, they're ships are fast enough to make it to a nearby system before they die. Existential fear and a realization of the little time they have left make them reckless, with a seemingly newfound creativity and intellect, and they do many things to the resident population to try and make them capable of giving birth to their young. However, this isn't possible, and, while they manage to completely alter the genome of the native's children, while accidentally killing the rest of the natives they also become infertile.

They then take all of the the children, who are now aging quickly as they are, while leaving the rest to die.

This process having worked once, they then continue to use it on many other systems. They age to quickly to advance in the sciences, so this is the only way they can survive. They also do not know why they are doing this, having merely been taught to this by their parents. They lack any idea of a better way, or that what they are doing is wrong, and by the time any of them think of such a thing, it is too late because that individual will already be dead.


“Why” should be because evolution turned out that way… which might be hard to justify. How would you explain it?

“How” would prolly be harder, whether or not the why was explained, because there are so many loopholes in the exposition.

In evolutionary terms of “why”, isn’t whether the chicken or the egg came first easy, compared to how a cuckoo might snatch its victims bodies, rather than usurp their nest?

Sticking with the plan, how does converting other beings into “more of their own kind” turn out? How does retaining their original personalities and memories; their selves and identities work? In that vein, how are “original instincts and bodily drives” not inherent to their species?

In your vision, what happens when transformed individuals retain their original personalities and memories, and find themselves changed… or do they notice nothing and so never find themselves changed? Which works for you?

Do the converts need to be sapient, or will a chimpanzee or orangutang do as well as you or me?

Ignoring sapience, does this apply only to creatures with similar physiology, or is the process purely mental and apes like us are as suitable for conversion as horses or insects, octopuses or spiders? Will birds qualify, or fish? Are you saying that’s covered by the conversion involving radical physical bodily changes?

Are you calling the converted simply converts, or are they subjects or victims or what? “Convert” suggests apparent willingness, even if that’s due to deceit. “Subject” might seem neutral, except it applies less to people than to scientific specimens. “Victim” seems to me more suitable, and who am I?

Is one of those what you want, or something else?

How do you reconcile “A change of identity… is not enough” with retaining their original personalities and memories; their selves and identities? Which takes trumps: changing the identity, or retaining it?

How did “their personalities” change into “something that could be recognised as their original personality”?

Either way, how might a change of identity involve bodily alteration, and why might that be needed?

Since you seem to be hinting at a physical process, how might the reason being psychological come into this?

Whatever “artificial beings hardcoded never to create new consciousness” might mean, how could that matter here?


Ummm, don't we do this? We eat pigs, they are sapient beings, with emotions, thinking, they love each other and even humans.

And we eat them (yes including me), to fuel our bodies and reproduce, effectively converting pigs into human beings.

Other than the autotrophs; plants that convert sunlight, water and inert chemicals into more of themselves, every animal on the planet is busy converting other living things into more of themselves.

That is the endless purpose of life! Bees exist to turn pollen into more bees, birds exist to turn insects into birds, sharks exist to turn other sea life into sharks. People exist to turn all sorts of other life into more people.

Which is certainly fun and rewarding.

It is entirely understandable why aliens would want to convert people into more of their own kind. Perhaps we just have the right combination of raw materials in the right proportions to feed their children.

Or, if they are just modifying us, perhaps we are the easiest to modify: Our brains are clearly the most advanced of any other animal; if that is what you think "sapience" means. As far as I am concerned, it just means "self-aware", and there are dozens of species that are clearly self-aware.

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    $\begingroup$ "Those converted must retain all their original memories and something that could be recognised as their original personality". $\endgroup$ Commented Mar 26, 2022 at 12:28
  • $\begingroup$ @StarfishPrime This is obviously not impossible if the aliens are just modifying humans; as my last paragraph says. "Perhaps we are the easiest to modify." $\endgroup$
    – Amadeus
    Commented Mar 26, 2022 at 13:57
  • $\begingroup$ An answer that consists of the sentence "perhaps we are the easiest to modify" with several paragraphs of irrelevant fluff is not really a great answer. Perhaps it might be better as a comment on the original post? $\endgroup$ Commented Mar 26, 2022 at 14:21
  • $\begingroup$ @StarfishPrime I'm happy leaving it as it is. $\endgroup$
    – Amadeus
    Commented Mar 26, 2022 at 14:45

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