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Background:

Human Origins:

We originated from a space-faring civilization. A breakaway faction that hates advanced space level tech traveled here and set up shop before destroying their advanced tech. They were unprepared for the large animals on Earth before the Ice Age and they quickly lost the ability to make any reasonable tech. A few generations later and they were unadvanced. They retained their ability to solve problems though and eventually out competed the native proto-Hunan Neanderthals.

NOW:

In 3096 we have reached interstellar travel using a combination of wormhole (govt. access only) and considerably slower warp drive. We send colony ships out and quickly find that we are totally surrounded by alien colonies. We soon find out that these aliens are genetically related to us.

While some conspiracy nuts (yes, they still exist in 3096) expected this, most of humanity is shocked. The aliens are genetically different from us in many areas however as they have had to use gene manipulation to adapt to new worlds.

My main character falls in love with the High Princess of the alien empire (which is by its own nature peaceful). They decide they want to escape the obvious difficulties that would be caused by their union by fleeing known space. They want to have children.

Question:

How much genetic variation would allow offspring? The offspring do not have to be viable from the standpoint of reproduction.

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    $\begingroup$ Not a geneticist, but it seems like even a very small genetic drift would make offspring impossible. There is only a relatively small difference between human and apes... However, they do have gene manipulation, so they should be able to fix his genes to make him compatible. $\endgroup$
    – AndyD273
    Apr 6, 2015 at 16:20
  • $\begingroup$ @AndyD273 There are a few examples of what I am looking for such as mules. $\endgroup$
    – Jax
    Apr 6, 2015 at 16:24
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    $\begingroup$ Primate DNA does not seem to lend itself to successful interspecies breeding, for whatever reason. Humans are unable to produce offspring with either chimpanzees or bonobos (both very close genetically). Bovid and equid DNA, on the other hand, are very successful at hybridizing, even between species with different number of chromosomes. $\endgroup$
    – Nick2253
    Apr 6, 2015 at 16:40
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    $\begingroup$ @DustinJackson I can believe it is expensive. If only he was dating a princess or something... ;) I'm really interested seeing what someone that knows something about this subject says. It seems like it would depend more on her genetics than his, since there is no genetic drift anywhere on earth, and the earth colonists destroyed their tech really early... so if her planet is earth like then they may not have altered their genes much. I'm gonna guess it'll be a non issue. Especially if some of the alien encounters on earth were also ways to interbreed and keep genes similar over time. $\endgroup$
    – AndyD273
    Apr 6, 2015 at 18:27
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    $\begingroup$ @the_OTHER_DJMethaneMan, why would you want to change the DNA of every one of his cells anyway? Why do his brain, arms, or belly matter for reproduction? His testicles would be more than enough, the one spermatozoon used for artificial fertilization is just enough. But since they want to flee society this might not be an option depending on your decision of how much support they can expect before they leave and what kind of medical equipment they take with them. But mating is always possible and adopting any kind of young should be possible too, just saying. $\endgroup$
    – hajef
    Jun 14, 2019 at 12:16

4 Answers 4

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It mostly depends on how long ago your humans came to Earth. The process you are referring to is known as Allopatric speciation. It's the process of forming a new species by geographical isolation (in this case separate planets). The classical definition of a species (although not strictly adhered to by biologists) is the most inclusive group of organisms capable of interbreeding and creating viable offspring. So if your humans and aliens have diverged sufficiently to no longer produce viable hybrids they will be considered separate species.

So how long does allopatric speciation take? How long does it take after separating two genetically identical groups until random genetic drift causes them to no longer be able to interbreed? It turns out it can range anywhere from a few generations to many thousands. All it takes is one strong mutation (say one chromosome breaking into two to give a simple example, but much more subtle mutations would work as well) to create an insurmountable reproductive barrier between two organisms. However, the chances of such a mutation occurring are quite small. The estimates I've seen suggest at least hundreds of thousands of years and more likely millions of years between isolation and speciation.

It's well within reason that your humans (unless they showed up many millions of years ago) will not have drifted too far to interbreed with your aliens. As for the aliens and their genetic manipulation, I think that part of their society actually makes them much more likely to be capable of interbreeding. If they have multiple races on different planets it would be important for them not to drift far apart or they would become separate species. If they care at all about inhabitants of different planets being able to procreate with one another then they would need to develop a galactic standard of reproduction. They would take special care in all of their genetic manipulations to not change their ability to reproduce with the rest of their species. As a result the aliens may closely resemble the original ancestors of the humans in terms of reproduction.

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The answer lies in the genetic difference of a horse and a donkey. They can have a baby (although infertile) called a mule :)

There are some interesting articles on horse and donkey genetics on Google.

This is just an analogy but it may point you to the right direction.

My GUESS is that horse and donkey diverged from a common ancestor no more than 100,000 years ago. And if the analogy holds and your humans and aliens have a common ancestor around the same time, then you should be able to have human-alien hybrid.

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Homo Neanderthal is not an ancestor of Homo Sapiens but a cousin who lived at the same time and went instinct. If you want a species that can evolve into Homo Sapiens, go for Homo Erectus. However, there is some proofs that homo sapiens and homo neanderthal had children since we -Homo Sapiens- have some of the DNA of homo neanderthal. So you have the proof that viable hybrid human existed in the past.

Second, hybridation is a very particular science and it is very difficult to know what will happen so you are free to do what you want. You can check the Hybrid page of wikipedia.

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  • $\begingroup$ Please cite "we-Homo Sapiens- have some of the DNA of Homo Neanderthal." Last I heard this was a disproven theory. $\endgroup$
    – Jax
    Apr 7, 2015 at 12:53
  • $\begingroup$ While Neanderthals and Cro-Magnon could indeed have children they were most likely infertile, so the premise that "we have Neanderthal DNA" doesn't seem to stand.......that being said I would be interested if you could pull some official documents out in support of your answer :) $\endgroup$
    – Jax
    Apr 7, 2015 at 12:55
  • $\begingroup$ @DustinJackson Not "official" I suppose... but not made up either slate.com/articles/health_and_science/human_evolution/2012/10/… blog.23andme.com/ancestry/find-your-inner-neanderthal $\endgroup$
    – AndyD273
    Apr 8, 2015 at 11:08
  • $\begingroup$ I read it in a french science magazine Science et Vie, I dont remember the number. The article talked about the hypothesis that neanderthal didn't be killed but absorbe by homo sapiens. $\endgroup$
    – Rigop
    Apr 8, 2015 at 12:35
  • $\begingroup$ @Rigop if I remember correctly, Neanderthals and Humans could interbreed but the children would be infertile due to genetic differences. Also, modern scientific and archaeological communities do not really support that theory as a whole. $\endgroup$
    – Jax
    Apr 8, 2015 at 12:46
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Don't forget differences of environment. All of the examples of hybrids have been on planet Earth, subjected to the same gravitational pull, the same lunar phases (tides, body fluids, menstrual, etc) and the same carbon based food source.

If you have the same species dropped off on different planets to survive, I would suggest they would separate much faster than the proposed 100,000 years of earth only examples. If you have a group living in space, with no lunar/solar pull on the body, it will be considerably different from a body needing to adapt to a multi-moon or red giant sun planetary environment.

Even if you dropped off a group on Earth, and one on Mars, they would diversify faster because the body would become dictated by the two rock moons on Mars (they emit a gravitational pull of sorts) and the one moon on Earth (that also emits a gravitational pull).

Now that I think about it, if the same species was dropped off on Earth AND the moon, would they divide quickly too? I would take a guess and say more than likely.

Technology may be grand, but it can only get so far, it's no match against the forces of the universe.

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  • $\begingroup$ Interesting, but do you have any reason to believe that evolutionary changes like the ones you describe would affect the creatures’ genetics to the point that their ability to (inter)breed would be altered? $\endgroup$
    – Peregrine Rook
    Apr 15, 2019 at 14:19

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