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I have read that during the Second World War, some mosquitoes got trapped in the London underground railway system. The mosquitoes never got out and eventually they became a new species by themselves.

I had a similar thought. In the next few centuries, if humans could, in theory, colonize other planets like Mars, Proxima Centauri and beyond, then the environments there are not the same as Earth. So, in the long term, humans who would be born and who would grow up on Mars, for example may become more and more suited to Martian conditions than Earths.

Now, when early humans explored and ventured into new geographical areas, they did change characteristics, but we are still one species Sapiens. But living extraterrestrial, is a whole new thing. The gravity alters, the entire atmospheric composition does. So that is going to have some significant changes on humans.

So, my question is: is it possible that in millions or even billions of years, if humans expand to space, there may arise separate species of humans? And would this new emergence of human species actually result in humans moving one step up the taxonomical ladder: becoming a genus?

EDIT: To avoid confusion and create speculations at the answers, I should specify that I am talking about a very particular case: if Sapiens are living in different planets, then is there a chance that Sapiens will become a new genus, and that Homo can be taken one step higher in the taxonomical order? There would still be Sapiens on Earth, but considering the environmental changes that could happen here too, humans then can be drastically different from humans now. So the question is: can 'Sapiens' become a genus?

Thanks to @tyersome and @jamesqf for pointing this out.

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    $\begingroup$ Welcome to Biology.SE! Thank you for taking the tour, but please also go through the help pages starting with How to Ask questions effectively on this site and edit your question accordingly. In particular, questions must be clear — how are you defining "human"? I would say the genus Homo is equivalent to "human" in which case your question is trivial. In addition, as written your question is speculative and thus will generate opinions rather than factually based answers — this makes the question off-topic as currently written. Thanks! 😊 $\endgroup$ – tyersome Jul 19 at 16:50
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    $\begingroup$ Human, or in Latin, "Homo", already IS a genus. It's just that the other species in the genus - Homo habilis, Homo erectus, Homo neanderthalensis, &c - have died out. $\endgroup$ – jamesqf Jul 19 at 17:00
  • $\begingroup$ @tyersome thanks for the note, I edited it. $\endgroup$ – PNS Jul 20 at 3:43
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    $\begingroup$ @nick012000 I don't know about the difference compared to wolves/dogs, but there is significantly more diversity within each human "race" than there is between "races", which is to say that any two black people or any two white people are generally more different than the average of all white people are from the average of all black people (and the same is true of all "races"). All that is to say that there is not much genetic difference between black people and white people $\endgroup$ – Kevin Wells Jul 20 at 16:02
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    $\begingroup$ @nick012000 Dogs and wolves are not considered different subspecies, dogs are just a domesticated sub-species. You are right about coyotes though, they are considered a separate species, but can interbreed with wolves. It's possible that they should therefore be considered sub-species, but it really just shows how complicated species distinction is $\endgroup$ – Kevin Wells Jul 21 at 15:44
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The concept you are referring to is speciation and it has been well studied in a wide variety of different natural organisms. I suppose here we are talking about the biological species concept.

The overall answer is yes it is possible, but critically depends on a few different factors. The reality of speciation in the wild is very complex, but these are some things to consider:

Genetic isolation

If two groups, such as your Martian colony and humans on earth isolate from one another, then for speciation to occur, there needs to be a significant level of genetic differentiation between them. This means substantial differences in the kinds of genetic variants found at positions along the genome.

Genetic differences are eroded by post-isolation gene flow – in your example, that might mean a spaceship flying back to Earth with Martian colonists, who then have offspring with people on Earth.

Although there is plenty of evidence that speciation with gene flow can occur, the general rule of thumb is that increased gene flow means a longer divergence time is required to fully speciate.

In nature, this is often caused by some kind of geographic barrier to gene flow, such as mountains or rivers forming, but it can also be caused by morphological differences, such as variation in sexual appendages. Of course, in our example, this barrier to gene-flow would be the large and difficult to traverse distance between the Earth and Mars.

Divergence time

You alluded to some kind of separation time, and you are right to be talking on the scale of millions of years. Speciation can occur extremely quickly; in Lake Tanganyika cichlids, it has occurred probably within the last 15,000 years. Humans have created whole new species of crops, such as maize, within the past 10,000 years. There is even evidence of some fish speciating in 3000 generations.

However speciation is often a much longer process. For example humans and chimps were thought to speciate in 4.5 millions years.

Of course, there is an interaction with several other factors. All other things being equal, less post-isolation gene flows results in a shorter time to speciation and vice versa. Stronger selective pressure between the different environments leads to a more rapid accumulation of genetic differences.

As Konrad Rudolph correctly points out, divergence time is strongly related to generation time, with all other things being equal, a shorter generation time, results in faster speciation.

Selection pressures & differing environments

I think the last main factor governing speciation is how different the Martian colonists environment was from that on Earth.

Different environments can lead to natural selection occurring in opposing directions in the two populations, leading to ecological speciation. Speciation can proceed without differing environments, where neutral drift in allele frequencies can eventually cause speciation, but this will be a long process.

Speciation will occur much more rapidly if there is a start difference in environment and selection pressures between the two groups.

So in conclusion, given enough time, genetic isolation and differential selection pressures (or some combination of the above), it is plausible that a new species of human could form. However, given the time-scales required, it seems a bit unlikely to me.

EDIT

I think it is worth pointing out what @Jaquez said in the comments. If current terrestrial humans split from an extraterrestrial source to form another species, it would be named as another species within the Homo, such as Homo extraterrestrialis, for example. The addition of a new species does not move the group Homo up to become an e.g. Tribe or Family.

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    $\begingroup$ Your answer currently implies that speciation time is the same for all species. This is very crucially not true, it depends on effective population size and generation length. The former is small in humans and the latter is very long, both of which imply that splitting into separate species will take much longer for humans than it takes for cichlids or maize, independent of selective pressure. $\endgroup$ – Konrad Rudolph Jul 19 at 21:57
  • $\begingroup$ Which part of the answer do you think implies that? It wasn’t my intention. $\endgroup$ – user438383 Jul 20 at 7:40
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    $\begingroup$ Yeah, I didn’t think it was intentional. But the paragraph under “Divergence time” reads as if the numbers from cichlids and maize could be used as a proxy for humans (or rather, they could if it weren’t for “natural” vs artificial environments — which anyway only applies for the maize example, not for cichlids). That said, the 3000 generations you cite sound about right as a (very rough) ballpark estimate. $\endgroup$ – Konrad Rudolph Jul 20 at 8:49
  • $\begingroup$ Right, I understand. In hindsight that was quite a tangential point and potentially confusing, thanks for noticing it. $\endgroup$ – user438383 Jul 20 at 9:04
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    $\begingroup$ Excellent answer, +1. However, I feel it would benefit from clarifying that speciation would not, as the OP asked, move either Homo or sapiens up the taxonomical ladder. It would result in a new species within Homo, for example Homo extraterrestrialis. $\endgroup$ – Jaquez Jul 21 at 1:26
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I do not believe it will happen. There are multiple roadblocks:

First, speciation time is measured in generations, not years. The human generation time is long, the 3000 generations mentioned in another answer for a fish translates to nearly 100,000 years. Are human populations not going to interbreed over a time span that long?

Second, the speed of speciation is a function of the evolutionary pressure. We radically alter our environment and change the whole driving force--genes which make one more or less able to survive the rigors of the environment have almost no effect anymore. To the extent that human evolution will occur it will be mostly towards things that favor reproductive success--things which attract mates, a desire for children, contraceptive unreliability. These things aren't going to be substantially different on Mars than on Earth.

Finally, we are already to the point that we can make designer babies to some degree, although we can't yet do it with the sort of reliability needed to ethically do it on humans. I find it inconceivable that we can remain a technological species (as would be required to have a population on Mars) without reaching the point that we can freely rewrite the genetic code at least during reproduction. If there is a speciation event it will come from the lab, not evolution. It's possible that some day there will be a modification developed that is a substantial upgrade but which is not compatible with existing humans--those with the modification can only breed with others that have it.

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  • $\begingroup$ Indeed. For such a speciation to occur, we had to first terraform Mars to a degree where a non-technological human population can survive, and then we would need a total societal collapse on both planets, so that space travel would become impossible for a long long time. But even then, in those 100000 years humans would either rediscover technology and space travel, or die out. $\endgroup$ – vsz Jul 21 at 4:28
  • $\begingroup$ "is it possible that in millions or even billions of years" - so definitely. Chimps and Humans diverged 5-7 million years ago. As for @vsz, that's not true. You would just need to have limited interaction. If you are imagining societies with regular flights between the planets then maybe not. But even on Earth we get 'ring species' that can interbreed at the boundaries while populations at the ends cannot. $\endgroup$ – Jason Goemaat Jul 21 at 17:48
  • $\begingroup$ @JasonGoemaat A requirement for speciation is that you have populations that do not interbreed. That means a lack of mobility. I'm questioning how you could have a population on Mars that for a vast period of time doesn't interbreed with the population of Earth. If they fall too far for spaceflight they also fall too far for survival in the Martian environment. And that doesn't even address my other points. $\endgroup$ – Loren Pechtel Jul 22 at 2:59
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As others have noted, the key concept is speciation. Imagine some H. sapiens living on Mars. How are they going to breathe? Presumably we have a nuclear reactor generating power, which in turn is used to make oxygen. What is it breaks down? Those who can tolerate low oxygen levels might do better. Here on Earth we invest a lot of energy in building bones and things to resist gravity. On Mars there may be less need to do this, and more need to be efficient with oxygen and tolerate higher levels of CO2. So some mutations that would be sub-optimal on Earth might be beneficial to the colonists. Couple this with isolation, as others have mentioned, and the two populations may drift apart, leading to the eventual emergence of H martiansis (or whatever.).

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    $\begingroup$ A driving force producing natural selection is not required for speciation. Separate two identical populations and give them identical environments and they will eventually diverge due to random changes in the genome. One of the main factors causing this is in the proteins in the coating surrounding the egg. They are like a lock that sperm have the key to. The selective pressures preventing change are simply that a male cannot reproduce if it cannot find a female with compatible proteins. Small changes aren't a problem, but the drift over time would lead to speciation with no other changes $\endgroup$ – Jason Goemaat Jul 21 at 17:47

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