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I am imagining the following situation:

Humanity has to leave earth due to deteriorating conditions. Most decide to leave and settle on two different worlds.

There are practically no differences between the 2 worlds, except one: World no. 1 has a surface gravity of 0.9 g, World no. 2 has 1.2 g

These 2 populations would cease to interact with each other for the next 100.000 years to come, peacefully existing without any major events that would disrupt their evolution according to these new conditions.

My question is: After 100000 years had passed, and they would for some reason meet, would a man originating from world 2 be able to reproduce with a woman born on world 1 or vice versa?

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    $\begingroup$ It's impossible to say for sure. Given 100,000 years of separation and the same conditions it seems highly likely that the two populations would be able to interbreed. But given the huge evolutionary pressure imposed by the difference in gravity, all bets are off. $\endgroup$
    – Slarty
    Commented Jul 24 at 18:56
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    $\begingroup$ I agree with @Slarty that the time frame is a problem, but I disagree that the gravity would be. You're starting with common stock. The gravity difference would rationalize changes in muscle and bone structure and cardiovascular efficiency... but actually change the DNA to become incompatible? That would be a difficult rationalization to swallow. Which answer do you want? compatible or incompatible? $\endgroup$
    – JBH
    Commented Jul 24 at 19:11
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    $\begingroup$ Nobody has any data on how evolution is impacted by gravity. (Unless the ISS microgravity experiments are looking into it?) We can guess based on how geographically separated species have evolved on earth, but it's impossible to know for sure. I think it would be fine to pick whichever scenario works best for the story you want to tell. $\endgroup$
    – craq
    Commented Jul 25 at 5:49
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    $\begingroup$ @JBH you may be right, but I remain unconvinced. Gravity changes would have many huge effects including changes to body structure, diets, predator prey balances and the environment. The real question is how quickly can speciation occur? The speed of speciation has been estimated at 1-2 million years, although in some cases it can occur very quickly and in less than 100,000 years, the greater the selection pressure and the less gene flow, the more likely you're going to get speciation. But it is hard to be definitive here. livescience.com/how-long-new-species-take-to-evolve $\endgroup$
    – Slarty
    Commented Jul 26 at 11:52
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    $\begingroup$ 100000 years is not a long time, and it's not the surface gravity that would matter. Right now, a spaceship would take about 50,000 years to get to another planet like that anyway. So, 100000 years is barely enough time to get to another planet, and immediately turn around and come home. So most, if not all of that time would be spent in space at an arbitrarily selected centrifugal acceleration (artificial gravity). $\endgroup$
    – Wyck
    Commented Jul 27 at 13:50

7 Answers 7

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Based on the latest timeline of human evolution modern humans have appeared sometime between 300 and 130 thousand years ago. Since then not so much has changed from the genetic point of view.

In your case, assuming the mutation rate between the two population is similar, it's very likely that crossbreeding will still be possible and will produce fertile progeny.

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    $\begingroup$ Plus, Neanderthals and Sapiens-Sapiens diverged some 650K to 1.2 million years ago, and were still able to interbreed. $\endgroup$
    – Escaped dental patient.
    Commented Jul 24 at 17:22
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    $\begingroup$ "Since then not so much has changed from the genetic point of view." Honestly, that's true since hominids diverged from the line that led to the panins several million years ago. I'm not disagreeing that I think we'd be interfertile, at least partially, but it's also worth noting that a little bit goes a long way in genetics. $\endgroup$
    – Jason Patterson
    Commented Jul 25 at 13:10
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  • We know for sure that 50,000 to 75,000 years is not enough.

    In the real world, the Europeans and the Australian Aborigines had had no contact for about 50,000 to 75,000 years when they met again in the 17th century. Practice showed that they were fully interfertile.

  • We know for sure that not even 250,000 years is completely enough.

    When the so-called anatomically modern humans came out of Africa and met their long-lost Neanderthal distant cousins, they were able to have children together. At that point, the two human lineages had been isolated for at least 250,000 years, maybe even more; and yet, all modern humans outside sub-Saharan Africa have some Neanderthal DNA, meaning that interbreeding was quite routine, not isolated events here and there.

    But . . . we don't have any Neanderthal maternal DNA. All the mixed-lineage children who have left genetic traces in modern populations were the result of a Neanderthal man mating with an anatomically modern woman; the two lineages were partly incompatible.

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  • $\begingroup$ Really? No cases of a modern man mating with a neanderthal woman? Now that is curious - and it would make for an interesting imaginary world. $\endgroup$
    – JBH
    Commented Jul 24 at 19:13
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    $\begingroup$ @JBH: They most certainly did attempt to mate. But we don't have any Neanderthal maternal DNA in our cells, meaning that either the mating did not produce any offspring, or it produced only boys, or somehow all the resulting girls failed to have descendants among modern people. $\endgroup$
    – AlexP
    Commented Jul 24 at 20:08
  • $\begingroup$ @AlexP likely intended this, but minor clarification, the pairing didn't produce any fertile offspring. It's entirely possible that there were sterile offspring (human mules, basically), or that they all died in childhood for some reason. $\endgroup$
    – Jason Patterson
    Commented Jul 25 at 13:13
  • $\begingroup$ @AlexP - you seem knowledgeable here, but my reading suggests there are other possibilities than Neanderthal Women / Sapiens male are incompatible. en.wikipedia.org/wiki/… $\endgroup$
    – Neil Tarrant
    Commented Jul 25 at 13:46
  • $\begingroup$ @NeilTarrant: Yes, there are many possibilities. I have even seen a very recent article which complicates the issue even further, assigning much of Neanderthal genetic diversity to even older (and multistage!) introgression of genes from anatomically modern humans. See Liming Li, Troy J. Comi, Rob F. Bierman, and Joshua M. Akey, "Recurrent gene flow between Neanderthals and modern humans over the past 200,000 years", in Science, 12 Jul 2024, Vol 385, Issue 6705, DOI 10.1126/science.adi1768. $\endgroup$
    – AlexP
    Commented Jul 25 at 14:15
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There is no reason to beleive the genetic drift itself would make crossbreeding impossible.

There is however, one snag - pregnancy.

Species that evolved to be able to handle pregnancy at 1.2 g would have different adaptations to their wombs, placentas, birthing canals and even abdominal cardiovascular systems that one evolved on a 0.9g planet. The differences might be slight, but humans already are fine tuned to the very edge of birth viability. Human babies are simply too big, too heavy, and take too long to develop, and the only reason why all births are not still, is that nature over-engineered women to the absolute max to let their bodies handle it.

It is not outside the realm of possibility, that a mother from a low G planet would be unable to give a birth to a who is a partial high G hybrid, because it would just be too big, thick headed, and its metabolic needs unsustainable. Inversely, a Heavy-worlder Mother would have no problem giving birth to a half-Lightworlder baby, only for the baby to die soon after in the 1.2 gravity, when its tiny lungs collapse, or its weaker arteries burst.

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  • $\begingroup$ This might result in the children / pregnancies only being viable if the mother was from the high-G planet, rather than full incompatibility. $\endgroup$
    – Sarah Messer
    Commented Jul 26 at 18:28
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    $\begingroup$ So a heavy-world mother on the light world would have less problems? $\endgroup$
    – Paŭlo Ebermann
    Commented Jul 26 at 22:39
  • $\begingroup$ Heavier gravity will likely push the population towards being shorter, stockier, with thicker bones and stronger muscles. Weaker gravity will permit taller, lighter boned people. It make me wonder if eventually, pelvic differences would cause difficulties in birth long before speciation occurs. But that's probably where the problem would occur - difficulties with birth rather than inability to conceive due to differences in genetics. Upvoted your answer! $\endgroup$
    – Wyck
    Commented Jul 27 at 13:41
  • $\begingroup$ @PaŭloEbermann Both a heavyworlder mother and child would have problems on the light-world, since their blood pressure metabolic processes and cardiovascular systems would be incompatible with it. For example, a heavyworlder's blood pressure on a ligth-world where gravity does not pull blood down, could very well lead to aneurism during childbirth. $\endgroup$
    – Going Durden
    Commented Jul 29 at 7:02
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"Modern" sensibilities help extend the timeline

The other answers have nicely shown that 100 000 years should not be a problem anyway, but in case you want to extend the timeframe, you can do that too. Because:

Evolution isn't given as free a hand in modern civilization as it would have in the wild.

Imagine a human with any sort of genetic disadvantage. Down syndrome, dwarfism, whatever.

An animal with a significant disability would find itself eaten by predators, starving because it is unable to catch prey, or otherwise removed from the gene pool. That's how evolution normally works.

In modern civilization - and I'm using that term very loosely, as even basic tribal societies often fit what I'll describe here - every life matters. A child is beloved by their parents, and more likely than not, they and the rest of the tribe/village/state/whatever will help the affected person live with/despite whatever disadvantages their genes gave them.

What this means is, that most likely Evolution is severely slowed down in modern civilization. Yes, attractive people will still get more mates, as will sociopaths who use this to be more financially successful than those who care about other humans. But then they will likely use contraceptives - so whether they pass their genes on more or less than other humans is still up in the air.

I don't think we have enough data to tell what that does to the evolution of the human race over long timespans - so you can make it up for your story, that's what science fiction does. Take a plausible idea for how the future will develop, and tell a story based on it.

If you want your human populations to be unable to crossbreed, you might have to invent some harsher conditions than just different gravity - maybe a strict eugenics program on one or both worlds to let their populations adjust to the gravity more quickly caused faster diversion? Maybe increased radiation caused a higher mutation rate?

If you want them able to crossbreed, no further action is needed if the timespan is just 100 000 years. And for longer timespans, just keeping basic mercy and compassion alive will probably do it.

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    $\begingroup$ I believe that there is a certain logical difficulty here, as the idea that persons with genetic disadvantages are treated with mercy and compassion (and not culled) is not the same as such persons participating in reproduction/the gene pool. I agree that parents/families/towns/states certainly help such persons continue living, but there doesn't seem to be a big push to help them have children. Just a comment on what appeared to perhaps be a false equivalence (maybe?) $\endgroup$
    – user99478
    Commented Jul 25 at 8:57
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    $\begingroup$ @user99478 they help them have children indirectly, by helping them survive long enough to do so on their own - if someone who can't hunt for themselves dies once they're weaned from their mother, they won't pass on their genes. They may still have lower chances to pass them on, but it's certainly higher than (near) zero. $\endgroup$
    – Syndic
    Commented Jul 25 at 10:25
  • $\begingroup$ Of course given a tech level capable of colonizing planets such disadvantageous genetic conditions may not exist at all... $\endgroup$
    – John
    Commented Jul 25 at 14:57
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    $\begingroup$ @John my point was that evolution as a whole would be slowed - "disadvantageous" genetics on the new worlds would be those that have a hard time living in high/low gravity. Which if evolution was given free reign might cause the populations to evolve in different directions, but if those people are given exoskeletons or whatever to help them live and thrive despite their difficulties, they might not. $\endgroup$
    – Syndic
    Commented Jul 26 at 7:48
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As already mentioned the two populations should be able to interbred without any issues. Unless there is an especially severe case of founder effect; this could cause some form of genetic incompatibility. The most likely issue would be Rh(D) incompatibility, a mother who is Rh(D) negative can be killed by a Rh(D) positive fetus during pregnancy and even more so during the birth. Rh(D) positive is genetically dominant so positive homozygous fathers impregnating Rh(D) negative mothers will result in serious complications. If, as the question suggests, the whole population left Earth and divided more or less evenly between the two worlds then founder effect is almost certainly not going to be an issue in any way but if the evacuation was actually only a small subset of the population then there could be some strange results.

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As a few others have mentioned, the timespan itself should not be a factor, but there are a couple complications which could swing it definitively either way. Several of those have been mentioned already, but I want to add biotech:

Modern humanity is at the verge of being able to genetically engineer our own children. We can't yet do so efficiently & reliably, but CRISPR, gene-sequencing, and cloning technologies have shown partial successes. It seems reasonable that we might have super-science biotech in the next century or so - and maybe the social taboos / squick factor around using it to craft children to order may have faded in another century after that.

100k years seems like plenty.

  1. Have either or both of the source populations developed such technology and the will to use it?
  2. Do their societies differ by enough that social & aesthetic pressures have shifted their genomes in different ways?
  3. Do they retain the ability to engineer children at the point when they meet again? And if so, do they want to have fertile offspring?
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@Going Durden has made a good point. Here is what would worry me. On World B it is harder to pump blood upwards, but you need to transport more energy. Any mutations that allowed blood to be pumped more efficiently, or that improved efficiency for utilizing oxygen, would be favoured and would spread throughout the population. Whether they'd be compatible with Planet A is anyone's guess.

It is also possible that the skinnies on planet A and the squatties of B would find each other ugly, in which case the question is moot. (Consider Lord of the Rings: how often did Men bonk Elves and vice versa?)

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  • $\begingroup$ I think Men would love to bonk Elves, so if you want an "each other" comparison maybe Elves and Dwarves would be more fitting? $\endgroup$
    – Syndic
    Commented Jul 30 at 5:51

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