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I hope my question isn't too vague or broad. It's a follow-up to this one.

So, we have a human (genetically modified in order to support life on another place, from previous question) on an alien world - specifically a male human. Assuming that nothing on his sexual organs has been changed, is it possible or even plausible to have the alien mate with human in order to assure the alien's survival as a race? The result offspring would have none of the human's features, only the alien's.

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    $\begingroup$ long term all humans would die off $\endgroup$ – ratchet freak Oct 24 '14 at 9:40
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    $\begingroup$ Related, but not a duplicate: Would co-orbital planets with very similar convergent evolutionary paths be possible? $\endgroup$ – a CVn Oct 24 '14 at 10:52
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    $\begingroup$ This is impossible. If species A can mate with a human and produce viable non-sterile offspring, then species A is human. I'm not being flippant; this is the official definition of a species. So, if it were possible, the other species wouldn't be "alien", it would be human. $\endgroup$ – rumtscho Oct 24 '14 at 16:20
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    $\begingroup$ @rumtscho this is true, but writing good, logically consistent definitions isn't easy. How exactly does this definition fit with the very small minority of mules that are fertile? Can you really say that a horse (with 64 chromosomes) and a donkey (with 62) and a rare fertile mule (with 63) are all the same species? Rigid adherence to definitions as written regularly leads to absurd conclusions. Did you know that Stonehenge isn't officially a "henge", despite being the original "henge" from which the term derives? $\endgroup$ – DeveloperInDevelopment Oct 24 '14 at 21:17
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    $\begingroup$ @rumtscho, you are oversimplifying. First, "human" is a much older word with a richer meaning than simply "member of species homo sapiens". Second, defining "species" is actually pretty difficult. Definition authors routinely miss edge cases. Again, some mules are fertile. Are horses and donkeys the same species? What about any of these? $\endgroup$ – DeveloperInDevelopment Oct 25 '14 at 14:46
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The simple answer is "no", however that's not a very interesting answer, so lets turn it around in this case.

If the aliens had a reproductive system very different from our own then it would be possible to have the result you are looking for, I'll give a few examples below. It should be pointed out though that anything a human male can provide a reasonably advanced alien species would be able to do artificially so there is no reason they would "need" the humans unless their technology level was lower than our current present day knowledge and certainly no more advanced than us.

Catalysis

Perhaps the female's eggs are capable of developing without actually being fertilized but the development process is catalysed or fed by certain sugars present in the male semen. Human semen contains enough of the relevant sugars to trigger the process of development.

Equally maybe again the eggs do not need fertilization and the act of intercourse by itself is enough to catalyse the process.

Note that in both these cases the new child would most likely be essentially a clone of the mother, genetically nearly identical.

Joining

The Asari from Mass Effect are all female and can mate with any other species and gender. When they do so the person being mated with contributes in some not-very-well-specified way to the child although the child remains completely Asari.

Modification

The species itself could have been heavily modified to allow cross-species breeding. For example in the Culture novels most of the members of The Culture have heavily customized and enhanced bodies. This allows them to change Gender, Skin Colour and Build (including adaptation to different levels of gravity) pretty much by willing it to be so and then waiting for a few weeks while the new changes grow through. This also allows them all to be fertile with each other even when different species originally, their modified reproductive system is able to use and incorporate any incoming genetic material, even when it would not normally be compatible.

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  • $\begingroup$ This is an interesting answer! Catalysis as you describe it seems possible in principle. The the human semen would somehow influence the alien eggs purely chemically, but it would be almost guaranteed that the offspring would have nothing from the human parent - human DNA is a passive element. Joining seems to have similar problem (the influence of the human does not transmit its humanesqueness.) The Modification would probably have to be very specific to both physiologies to allow human-alien breeding. $\endgroup$ – Irigi Oct 24 '14 at 14:30
  • $\begingroup$ This is probably why the hard science fiction is not so popular - science is very restrictive to many things. But I still like it a lot. $\endgroup$ – Irigi Oct 24 '14 at 14:33
  • $\begingroup$ I was about to write a "Catalysis" answer but you beat me to the punch. To elaborate on this there could be some social or religious pressure that artificially necessitates such a coupling. (eg They self fertilize but it is forbidden to do without condition X being met and this belief outweighs the survival of the race). $\endgroup$ – Myles Oct 24 '14 at 19:12
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From the point of view of science, it seems very implausible. There are quite good reasons why lifeforms based on carbons might share the RNA/DNA-mechanism to store their genetic code (stable molecules from materials abundant in space, except for ribose). Even some of our base pairs could be used in alien species, since among many possible base pairs, cytosine, uracil, adenine, guanine and thymine are exceptionally stable with respect to harmful UV radiation and therefore a good molecules to use. But the genetic code, the translation table which tells the cell how to translate the DNA or RNA into proteins from which the bodies are built, is very random and would be almost certainly be different. In other words: DNA of both species would be mistranslated in the breeding process and no offspring could be born. Pleasant side-effect is, that we can very probably be never infected by an alien virus, since our cells will not provide the building blocks it asks for.

Note: Even with identical genetic code, sufficiently different species will not produce offspring. Any independently evolved aliens would be genetically much further than, say, alligators and horses. I pretty much doubt there could be offspring of an alligator and a horse, even if they share (almost) completely the same molecular biology.

Genetic code - short explanation:

Genetic code

From Wikibooks: An Introduction to Molecular Biology/Genetic Code

In the cell nucleus, DNA is used to store the genetic data. They are used in a following way: DNA gets transcribed into RNA and RNA is later translated according to the table of genetic code. Each three letters of the RNA code information about one amino-acid, which is used as part of the new protein. (Proteins make our bodies, but not necessarily alien bodies - then, the breeding would be completely out of question.) The table is quite universal for most of the organisms on Earth, but it evolved pretty randomly and it would take immense coincidence that they would be mutually compatible.

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  • $\begingroup$ This is interesting, but as a follow-up: what fraction of the possibility space is already represented by life on Earth? What would be the magic number of inhabited planets within which at least some collision would be expected? Given that some chromosomes can be almost eliminated without harmful effect, could just a handful of these coincidentally transcribable genes from alien DNA be active in a healthy host cell that largely ignored the unrecognised remainder? $\endgroup$ – DeveloperInDevelopment Oct 24 '14 at 22:03
  • $\begingroup$ To my understanding, if the alien DNA is interpreted according to wrong gene-code table, it is very similar to random mutations. You can insert many random mutations into a DNA and it will mostly still work as you say (since there is lot of code that is not essential or even has no function). But it will almost certainly not have similar function it served in the alien body. (For example if the alien has pointy blue teeth, if you insert that gene somewhere, it will mean "random noise" rather than anything to do with teeth at all.) $\endgroup$ – Irigi Oct 26 '14 at 13:11
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I am assuming that you are looking for a contrived reason that this would work. Scientifically it shouldn't but that is ignored in SciFi enough you don't really need a reason. I also assume that the species is completely alien (not ancestory with humans) yet phycially able to have sex due to convergent evolution or some hand waving. That being said there is one way and it relys heavily on the fact that we are discussing how to write fiction:

The mating has nothing to do with reproduction. Sex is not just about reproduction in our society. If that is the case with the aliens too it might be an important element in a story.

Say we have enough technology that we are going to nuke the living hell out of the aliens if they kill you. The females are dominant over males in the culture. The Queen decides she doesn't like you and is to have you killed. The princess however wants to **censored** and as a result takes you as her mate. The Queen doesn't kill you at the risk of upsetting her daughter. You live. So do they.

Say the species can exchange antibodies through the mating process. They are all dying of some bacteria you brought with you. You find that they can assimilate your resistance to it by intercourse and I don't mean talking. You start making your way througb the females, they work through the males, etc. I probably wouldn't read that book but it serves your purposes.

If it needs to be about reproduction: The species has eggs that can germinate using its own genetic material if the male's in incompatable. It is, however, an induced ovulator. For example, male cats have spines which scrap the insides of the queen (in this case the term for an unspayed female cat) to ovulate if she is in heat. If you cannot provide genetic material but some detail of you activities induces ovulation, you can cause the alien to get pregnant with one of her own kind. If you have a few of their females, you can mate with them until a male is born to allow actual sexual (as opposed to asexual) reproduction again.

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Meiosis

Human reproduction is a complex dance, a gamete cell splits producing two complete copies , then each of these cells splits again, with half of the DNA moving to one side, and half to the other. At this point the DNA is shuffled around a bit, producing two partial complements of DNA, the cell splits, and we get 4 eggs, or 4 sperm.

Fertilisation

When a sperm meets an egg, it's 50% complement of DNA is matched with the egg's complement, giving us a new, genetically unique individual.

Genetic machinery

Of course, this DNA is useless without the specific machinery to read it. If I gave you a flash drive containing a bunch of encrypted data, but you had no decryption software to read it with, or even a computer, or even electricity, that data would be worthless to you.

A human cell is a vast factory able to read data from the DNA and convert it to proteins which then fold in very specific ways to create enzymes. Seriously, it's awesome. Use different machinery and you'll get junk, just a mess of dead protein that doesn't do anything.

Moreover, the proteins produced are themselves molecular machines, or components thereof, they probably would not have the same effect. Imagine putting a fast washing machine motor inside a laptop and expecting it to speed up the processor. It's just not the same piece of kit, the components can't work together like that.

Even more than this, these molecular machines need to have many other equally complicated machines around them in order to work. Imagine putting a pie machine in the middle of a car assembly plant. It wouldn't do anything, there are no raw ingredients being supplied to it, there's no oven to bake the produce, no conveyor to move the pies to the packaging facility, no delivery lorries waiting outside. It just wouldn't do anything.

Alien procreation

So we have a problem. Since standard terran sexual fertilisation requires two exactly matched complementary sets of DNA, and a very specific set of molecular machinery and decoding equipment to correctly read and interpret the data, and then a very specific environment in which the molecular machines you've produced can interoperate, it would be difficult for an alien to procreate with a human in the standard way.

This doesn't mean it's impossible though.

Possible Solutions

DNA is expressed within a human, the human itself is a DNA decoding machine. Perhaps the alien can make use of the human's own expression of it's DNA.

Perhaps information is transferred in some other way unknown to us. Some holographic information is collected, allowing the alien to mimic the structure and form of the human without directly reading the DNA as with Ridley Scott's Alien, which mimics the structure and form of it's original host to gain a survival advantage.

Perhaps the alien has the ability to mimic the machinery of a human cell somehow, maybe by absorbing a complete cell, or by absorbing two complementary cells creating an embryo out of human tissue and then manipulating it somehow to produce a hybrid.

Perhaps the Alien is able to modify the development of a foetus, changing it's gene expression in clever ways.

Perhaps it taps into some other form of information exchange unknown to us.

Perhaps it's only interested in some specific feature of our cellular anatomy, such as our ability to metabolise glucose, or the general structure of our brain.

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Yes this is absolutely plausible there are organisms right here on earth whos method of sexual reproduction is the exchange of dna via touch/contact not even requiring sophisticated sexual organs now If the aliens had this method they could essentially touch you to procure genetic material and after the first genisis there may be humanoids that have false sexual organs or proto organs that could still be usefull in reproduction as their skin or outer membrane allows for dna exchange regardless. Now this could have been an evolutionary trait that was passed on for survival of the species by way of essentially becoming their predators whilst retaining some of their original primal attributes. Just a thought....

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  • $\begingroup$ This seems like quite a claim and you're not exactly explaining why it's plausible. Could you go into a bit more detail about how this would work? $\endgroup$ – overactor Oct 25 '14 at 5:27
  • $\begingroup$ Lets say that this alien species was semi-amorphous and has a loose stem cell structure that has a base program to rewrite only certain parts of itself upon contact with another organism. This would be a majorly beneficial trait as the alien could essentially add or subtract traits to enhance survive-ability. $\endgroup$ – user2218260 Oct 25 '14 at 19:24
  • $\begingroup$ Now they are alien and lets assume that DNA is not unique to this planet also lets assume that the aliens could have proteins and cellular structures yet unseen to man possibly even a mitochondrial like organisms that aid in the passing of dna fragments to specialized areas of these pseudo stem cells. This fast tracks evolutionary traits and could potentially cause this alien organism to meta-morph almost instantaneously making it possible to fend off an attack by giving it similar attack mechanisms and tools/appendages required to fight off higher ranking predators/predation. $\endgroup$ – user2218260 Oct 25 '14 at 19:25
  • $\begingroup$ Now lets fast forward to their Alpha state where they have become the dominant species on their planet or at least the most intelligent/evolved species. They now have no need for this type of reproduction as Predadtion is not an issue lets say a virus is introduced the mitochondrial like organism fast tracks it and that starts rewriting their dna causing all sorts of metamorphic failures among other ailments. $\endgroup$ – user2218260 Oct 25 '14 at 19:36
  • $\begingroup$ They have no choice but to assimilate into a quasi humanoid form this over time in some cases replaces their reproductive method entirely as it is what nearly caused their demise because of alien viral dna reprogramming that their system had no way handling $\endgroup$ – user2218260 Oct 25 '14 at 19:44

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