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Would humans on disparate planets with little to no contact with one another continue to evolve along different lines?

In this universe, humans left Earth a hundred thousand years before. Or if it serves the story better 500,000 years before; and went in many different directions looking for planets they could live on. 100 separate planets were found and/or terraformed, all very far from one another, and humans have lived on them ever since.

After that long a time, would they all evolve into separate species? Would time make the difference? Would environmental differences on their planets make the changes?

What I'm looking for is a scenario in which two of those cultures meet after hundreds of thousands of years (or more if necessary) and don't realize they are just two off-branches of humanity. Maybe later they find out through genetic testing or something, but at first they should think they are "aliens" to one another, not the same species. Is this at all feasible?

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    – L.Dutch
    Commented Feb 16, 2023 at 11:57

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TL/DR: Not after a 'mere' 500,000 years. In fact we should know that the other humans came from earth, if not what type of earth animal they evolved from, even if there were half a billion years or more since we diverged from them.

Horsing around a little bit

To give a timeframe for comparison the last common ancestor of all equines (which comprise horses, donkeys, zebra etc) was about 4.1 MYA (million years ago). That's nearly ten times the length of time you suggested.

Looking at equines they look similar. You can guess if a species is equine just by looking at it even if no one told you. After all you probably were never explicitly told a zebra was an equine, you just looked at it and knew. That's because 4 MYA isn't enough time for a species to evolve into something truly foreign. Any species that is split by only 4-5 MYA would look close enough that anyone familiar with evolution would naturally presume they had a close common ancestor. Many equines can also usually interbreed to some degree, though their children are usually infertile, so humans split by 4 MYA likely could interbreed at least occasionally.

Now on the one hand you could argue that entirely different worlds would create such a hugely different environment as to encourage rapid evolutionary change. To this I'd say that's true, if it wasn't for humans technology.

As it is we won't adapt to our environment, we will adapt our environment to us. Were going to pick worlds that are 'earth like' and use technology to make the worlds more earth like. There may be some minor difference our technology can't adjust for, gravity being the biggest one we would struggle to change, which may be a driver for evolution but most of the potential drivers will be compensated for by technology. In fact I believe humans on different worlds will be far less likely to evolve into different forms then ancient equines, due to our technology.

Technology > evolution

Species living in stable environments don't change, look at alligators which have stayed roughly the same since the time of dinosaurs! If a certain form is working for your environment then there isn't much incentive to try out new adaptation and koinophilia - the instinctual avoidance of things that are different, like novel mutations, in mates - will result in the species staying mostly unchanged. You need some sort of change in your environment, some massive pressure pushing a species to need to adapt, for major changes to be likely to happen 'rapidly' - and yes 5 MYA is rapid from an evolutionary perspective. Now I definitely do not expect human's environment to stay static. However, I do expect the exact opposite, our environment will change so rapidly, due to technology, that no one mutation will be favored long enough to spread through the population before the environment and pressures change again, which will ironically result to the same effect of humans staying somewhat static evolutionarily much as alligators have.

Look at our technology 2000 years ago vs today, it's utterly different and yet 2,000 years is a blink in the eye from an evolutionary perspective. This means that technology is going to change faster then our species can adapt. As new technologies come about new evolutionary pressures will as well. For example right now car accidents are a major cause of death in first world nations. This means currently there is a fairly strong evolutionary pressure to evolve to be able to process movement at high speeds and make the the sort of rapid decision required to avoid accidents. However it will be no more then a generation or so before self driving cars likely take over and the need to be good at driving disappears. The few generation between when cars became common and when we no longer drove them were not enough for natural selection to significantly change humans to make us better at driving. The same patter will happen with new technologies, they will change so fast that evolution can't 'keep up' and since the exact pressures put on humans to survive will constantly change with our technology there will not be any one mutation favored long enough to permeate through a population. In absence of a long lasting evolutionary pressure favoring adaptation humans will default to persisting their current form.

There is no such thing as humanoid aliens

Honestly given how utterly foreign any species evolved entirely on a different would should be if we go to another planet and find anything remotely mammal like the only logical conclusion would be it likely came from earth. If your humans know that other humans traveled to other worlds and they run into any creature on another world that is carbon based with four limbs and eyes, no matter how bizarre it's appearance otherwise, the only remotely plausible explanation for such similarity to earth animals is it's from Earth origin. The only real question would be what earth animal this one originally evolved from. Of course that means DNA tests would happen almost immediately and our exact common ancestor being identified via DNA in less then a few years of meeting each other. This is true even after hundreds of millions of years when all the humans look utterly different.

How many billons of years can we cut that timeline down to?

If you want there to be any noticeable change in your human species you need to cut back on technology so more stable evolutionary pressures can encourage adaptations. If humans lost their advanced technology, possible due to disasters after landing preventing them from building the infrastructure necessary to support it, then a species reverting to swords and sandals level technology will defiantly struggle with the utterly foreign world they are living in and likely adapt to better survive it. If both of the human subspecies regressed technologically and had to rediscover their technology then their evolving to look different is far more likely since technology isn't favoring stability of form, though again not nearly enough for a mere 5 MYA to make them truly foreign.

If you take that further and presume your human species no longer remembers they originated on another world, and thinks space travel only recently became possible for species on their world, the likely hood that animals from another world came from the same common decent as them would seem much lower. I mean how likely would you buy an argument that dinosaurs built spaceships and now live on another world if I told you that since the creatures we just found on alpha centauri have eyes they must have come from something like dinosaurs?

If you make one of these two human subspecies non-sapient you would further make it seem implausible they built spaceships and flew to another world, and make the divide between the two human subspecies seem far more vast since humanity tends to define itself by it's sapience.

(Here I should mention that it seems unlikely human technology would regress this much and humans still survive, as any planet humans travel to would likely be so foreign that without a relatively high degree of technology to help us adapt we couldn't survive on the world. The idea of evolving towards loosing sapience, and such a non sapient species still surviving on a foreign world, seems particularly implausible without either billions of years of time for us to adapt to the world or this new world being far more earth like then is realistic. However, you get some poetic license here; few are going to notice and nitpick on something that trivial so I say go ahead and have technology regress if it serves your story)

So my suggestion would be both groups lost technology, one group's technology was so lost that they eventually became non-sapient. Then when your one sapient species finds another non-sapient species that looks like them; but their certain didn't come from their home world - which they think of as being the one they live on now not Earth - they would be less likely to guess they came from a the same species originating on another world.

Of course the utterly implausibility of two alien species sharing so many similarities as any of our decedents would share should still lead to experts asking allot of questions. DNA tests would still be expected to happen almost immediately, and DNA comparisons would be made. The plausibility of the two human subspecies sharing a common ancestor would still be known from fairly early on. The only real issue would be convincing people of how that happened when everyone would be making the equivalent of "dinosaurs couldn't build spaceships" argument.

You would instead get the sapient humans arguing the DNA evidence is wrong because it shows our last common ancestor as happening before humans even evolved on their home world! at that point evolutionary experts will point out that we always knew there was an unexplained lack of fossil records, or any other records, of animals earlier then whenever humans first touched down on their home planet and suggest maybe these creatures are somehow connected to that pre fossil record time period and may answer some of the huge questions as to why earth-like animals seemed to appear out of no were X million years ago. At that point things probably get political with religious leaders complaining that's heresy since everyone knows god created earth-like animals on their world back then and how dare you suggest otherwise etc etc. Eventually scientists will realize the non-sapient humans from the other world have a fossil record that dates back to, but no farther then, same time period as theirs. Then we get even more interesting questions about how two similar species mysteriously appeared on two worlds lightyears away at the exact same time. I imagine the standard arguments would boil down to 'god did it' and 'aliens did it'.

Of course this still requires a time period measured in hundreds of millions of years - tens of millions of years at minimum even if you push things- not thousands. Even then the possibility of common ancestry will come up almost immediately. The only real mystery would be how that common ancestor could exist given what the humans know of their own history.

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    $\begingroup$ Before anyone says it Yes Star Trek: Voyager did an episode with the dinosaurs built rocket ships premise I mock in this answer. It was terrible and it's understanding of evolution made me want to cry. Some of the political debate the dinosaurs got into when a common decent argument came up could be used as inspiration I suppose. However If you try to claim a computer can just predict the future evolution of a species over millions of years I will personally come to your house and beat you with a copy of "On the Origin of Species" so hard your last common ancestor will feel it! $\endgroup$
    – dsollen
    Commented Feb 13, 2023 at 23:32
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    $\begingroup$ ‘No humanoid aliens’: I would argue that you should use different terminology here given the context. There’s no reason to suppose that convergent evolution would not produce aliens with a similar overall body-plan to humans (which is what is usually meant by the term ‘humanoid’). They’re not likely to be mammalian (at least, not obviously mammalian), but there is no reason the usual traits such as bipedalism, binocular vision, and opposable digits on the forelimbs could not be found together in an alien species. $\endgroup$ Commented Feb 14, 2023 at 2:37
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    $\begingroup$ @AustinHemmelgarn I fully understand your argument, but I'm afraid I have to respectible disagree. Convergent evolution does happen, but it's usually for a single feature not an entire species body plan. It also happens because species are sharing similar environments and more importantly have the same basic evolutionary building blocks to mutate. What are the odds that animals on another world start with the four limb building blocks that is the basis of almost all animals? That eyes are the primary way to handle sensory input? ... $\endgroup$
    – dsollen
    Commented Feb 14, 2023 at 15:24
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    $\begingroup$ Since these first steps in evolution were partially down to random chance, this mutation happened and persisted instead of that one, those steps will certainly be different then ours. And without them all aliens on that world will share certain common traits we don't have, they use a six limb and primarily 'see' via sonar perhaps. The point is they will all share traits different from earth animals that drive their evolution. Plus their environment will be so different the human shape likely won't make sense, just a slight change in gravity or atmosphere renders human shape impractical. $\endgroup$
    – dsollen
    Commented Feb 14, 2023 at 15:27
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    $\begingroup$ @Perkins I agree humans will change slower then equines even ignoring their technology, but I'm not sure I agree with how you described it. Humans would be just as prone to mutation as equines, in fact since humans have slightly more genes then equines you would expect more mutations per child to occur due to pure volume of data copied. The difference is that longer lives mean fewer generations. Each child is just as prone to mutation, but you have fewer kids in a given timeframe and thus fewer chances for a child to be born with a novel mutation. $\endgroup$
    – dsollen
    Commented Feb 14, 2023 at 22:21
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After one hundred thousand years, would they all evolve into separate species? Would time make the difference? Would environmental differences on their planets make the changes?

Short answer: yes

Long answer: genetic separation and different selection pressure (read environmental condition) are what leads to speciation starting from a common ancestor. Your setting meets both the condition, so it's fairly sure that, after such a time, there will be some divergence between the inhabitants of different planets.

However, for it to be sufficient to make cross-reproduction impossible is another story. You will probably need more time to ensure mutual sterility. Or you can use it as a story element, the amazement at finding that Gilgarious from planet A was able to conceive offspring after a fun night with Squawaren from planet B.

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    $\begingroup$ @Len If your goal is for the perception to be ‘truly alien’ with them still being able to reproduce, then cultural disparity (and linguistic disparity) is pretty much mandatory, ideally with loss of historical information about those other cultures (and languages). $\endgroup$ Commented Feb 14, 2023 at 2:25
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    $\begingroup$ @Len You don't have to go too far back in history to find people, even in the sciences, who believed that the various ethnicities of humans living on Earth were in fact different species. Of course this was just a mix of ignorance and garden-variety racism. But it doesn't take all that much difference in appearance to convince people of such things. $\endgroup$ Commented Feb 14, 2023 at 16:28
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    $\begingroup$ I don't think environmental conditions will have any impact at all on reproductive selection. A civilization advanced enough to terraform planets around the galaxy are not gonna have any trouble with temperature/nutrients/predators/etc. cultural/societal values may change enough to have an impact, but it seems unlikely unless a massive change occurs. With such an advanced civilization it's more likely that one of these outposts will develop genetic engineering, which would obviously result in massive genetic deviation from each other. $\endgroup$
    – Aequitas
    Commented Feb 14, 2023 at 23:01
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    $\begingroup$ @Michael: It's complicated. Only the most committed of lumpers would argue that Panthera leo (Linnaeus, 1758) and Panthera tigris (Linnaeus,, 1758) are actually the same species (lions and tigers if you are not a biologist), but zoos regularly produce ligers and tigons. And then you get ring species where the Lesser Black-backed Gull and the Herring Gull don't interbreed, despite there being a continuous sequence of interbreeding populations connecting them! $\endgroup$ Commented Feb 15, 2023 at 9:02
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    $\begingroup$ @Michael It would be more accurate to say a species is "two groups of animals that are unlikely to successfully produce fertile offspring in their natural habitat". That means being able to breed to create infertile offspring is allowed, and being able to produce fertile offspring but unlikely to meet up and actually mate in the wild, as with most felines, also doesn't prevent one from being a species. Still even then the definition is imperfect, for instance dogs can breed with any other canine and it's quite plausible they will meet up. Species is often an arbitrary divide by scientist. $\endgroup$
    – dsollen
    Commented Feb 15, 2023 at 15:33
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As other answers have said, by natural selection? Probably not.

However that leaves un-natural selection!

If both societies gene-edit successive generations to be "fitter" for the environment, you can drastically change body shape over a couple of hundred years.

A large event could remove each civilizations knowledge of "Earth". Have one decimated by a super volcano or meteor strike, have the other do some weird religious or political cultural shift where they claim this new world as their own, and over 5k years simply stop talking about "Earth" (Where did we come from? Some say first people fell from the stars, and some were loved by the mother world, who gave them the strength to endure...)

After 100k years two superficially very different races.

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It depends on how different the environment are and how long. If the human terraform the planets to all be earth clones then the evolutionary process will be slowed.

Also remember genetic engineering as with this technology can both accelerate the divergence or stop it completely.

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    $\begingroup$ hmm, your first statement I feel isn't that useful. However, I do strongly back up your second. If OP wants two human subspecies to be so different as to be foreign to each other in such a tiny time period as 500,00 years I think intentional, and extensive, genetic engineering is the only plausible explanation for how it occurred. $\endgroup$
    – dsollen
    Commented Feb 14, 2023 at 15:51
  • $\begingroup$ I suspect you could still get some Deviations In 50000 years. Especially if your colonies are populated With over representation of a particular mutation, And that mutation was particularly useful, But still not this wouldnt be very much. $\endgroup$ Commented Feb 14, 2023 at 20:02
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The answer depends on whether their societies allow them to undergo a natural selection process or not.

Remember, that evolution depends on
1.) random mutations being advantageous to individuals (for some measure of 'advantageous': faster at hunting? longer lifetime? less cancer?) and
2.) this advantageous trait being allowed to permeate into the entire population by outcompeting the less advantageous part of the population.

It is certainly part 2.) here that would be under scrutiny from a conscious animal like the human. In our current state, society is wary of change, as we naturally become conservative when founding families, favouring stability for our children over wild social and genetic experiments.
Furthermore technology allows us to nullify any natural selection processes that would let a disadvantageous mutation die out in the animal kingdom, and amplify positive feats.

Summarizing, I think that modern humans have an effective speed of natural selection as a driver of evolution being decreased to zero. Any future, isolated society instead would chose their own speed of genetic change, either natural or artificial, depending on what they believe. They could chose to remain absolutely conservative, and disallow any systematic changes to the original genetic structure.

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    $\begingroup$ 'Society' was always wearing of change. Koinophilia exists in practically every sentient species and will affect mate choice. With the exception of domesticate species where humans now rule their mating behavior nearly every mammal out there shows signs of avoiding mating with those that are 'different'. This slows evolution but doesn't stop it. There is no reason to think that humanity would be any different then other social and intelligent mammals. $\endgroup$
    – dsollen
    Commented Feb 13, 2023 at 21:33
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    $\begingroup$ @dsollen " There is no reason " ... of course there is a reason and that's what my post is about. But of course that's mainly a story, as hard numbers on future human behaviour does not exist. As you hopefully understood, my main point was that I can hardly imagine (beneficial) mutations permeating an entire population, that is fully aware of those mutations. That's what is different in humans. $\endgroup$ Commented Feb 13, 2023 at 21:46
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    $\begingroup$ Now I think an argument about mutations permeating a population being less likely in humans is viable, but not because humans somehow have a strong version of koinophilia then any other species. I think our technology will change so rapidly as to result in changing and altering our environment faster then evolution can 'keep up'. What mutations natural selection favors will be changing so fast as our technology does that no one mutation will have time to permeate because it won't be beneficial for long enough to spread before technology changes evolutionary pressures. $\endgroup$
    – dsollen
    Commented Feb 13, 2023 at 22:00
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    $\begingroup$ If there were consistent evolutionary pressures on humanity then there is every reason to believe that mutations would spread and persist. Now I will agree that isn't guaranteed because of the rapid changes caused by technology growth. However I don't believe your premise that 'humans won't let it happen' makes any sense. If you argue for technology modifying evolutionary pressures I'll totally back you up. If you argue human will resist genes that were consistently evolutionarily favorable because we know how evolution works or some form of koinophilia as your answer suggests I disagree. $\endgroup$
    – dsollen
    Commented Feb 13, 2023 at 23:44
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    $\begingroup$ also how do you know that Ghengis Khan had more children then you or me? For all you know I'm a giant man slut with a couple dozen bastards running around. Hey just because I look like I should be a 40 year old virgin doesn't mean it's not possible! Geeks need love too! $\endgroup$
    – dsollen
    Commented Feb 13, 2023 at 23:47
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Given enough time, the answer is "yes"--Darwin's_finches. I'm not sure that 100,000 years is enough time to produce different species, though

It is unclear when the line of Neanderthals split from that of modern humans; studies have produced various intervals ranging from 315,000 to more than 800,000 years ago. The date of divergence of Neanderthals from their ancestor H. heidelbergensis is also unclear.

I have seen an argument that hunter-gathers are smarter (using an IQ test that measures spatial perception) than farmers/city dwellers, and that the latter are selected for resistance to zoonotic diseases (since our ancestors lived with livestock). Maybe different zoonotic diseases would give divergence.

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In Larry Niven's Ringworld series, he posits that humans have evolved to occupy many different evolutionary niches, as discussed in The Ringworld Engineers.

That's a fictional argument for you.

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I don't think they would.

The way evolution works, you have random genetic mutations, then if, in the given environment, that mutation proves beneficial, it 'spreads' and eventually most of the population end up having it.

The thing is, in evolutionary terms, 'beneficial' means it makes the individual either more likely to survive or more likely to procreate. Which means people without that mutation would be much more likely to die young than people without it. For humans, this kind of harsh evolutionary pressure is to a large extent negated by advanced technology (especially medicine) and social organisation. Most people will survive, even if they're 'less fit' in evolutionary terms. Now, your people have separated by colonizing different planets, so they were capable of interstellar travel, which assumes more advanced technology than we have today. So, likely, there wouldn't be much in the way of evolution of the species happening.

Now, it's possible that due to the pressures of life on different planets, these people would lose their technological society and end up with a pre-industrial civilization. But then, how would they end up meeting each other?

Most likely, starting with much more advanced technology than we have today and given 100,000 years, some of these people would become cyborgs, or would have the technology to store their consciousness in wholly mechanical bodies, or something even more sci-fi. Now if these people met (if you could even call them 'people' at this point), it's very possible they wouldn't recognise each other as human.

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