# Could humans split into two species living on two separate planets?

This is inspired by The Time Machine, by H. G. Wells. In the book - in the far future - humans have split into two races, the Eloi, who live aboveground, and the Morlocks, who live underground. The two have become extremely different after living for a long time in different conditions.

Humans can obviously split at some points in time. After all, that's how evolution works. The story also takes place over 800,000 years in the future, when things are much different.

I'd like to know if this is possible in a time closer to our own - not the near future, but a bit beyond that, within about 500 years. The twist it that one group lives on Earth, while the other lives on Mars.

Given these conditions, could humanity split into two different species?

• I have this vague memory of a similar question, but I can't find it. Either way, I have an answer. Jul 29, 2015 at 14:42
• In absolute terms: of course. However it is more a question of how long would the communities would have to live separately for it to happen. Thousands of years? Millions of years?
– Joze
Jul 29, 2015 at 14:42
• @DaaaahWhoosh It's not this, is it? I think the two are still different. Jul 29, 2015 at 14:42
• @Joze 500 years in the future, assuming that colonization of Mars begins relatively quickly. Jul 29, 2015 at 14:43
• Oh, no, it's not that one. It's older than that. But I recall it being a slightly different question anyway, so it should be fine. Jul 29, 2015 at 14:45

In 500 years? Without genetic manipulation to help things along, not very likely. The aborigine of Australia were mostly cut off from the rest of humanity for 40,000 years, and while they have distinctive racial features and adapted to their environment, they are still very much human.

On top of that for a complete separate species to evolve even with gene therapy it would take much concerted effort of all involved to push that change for everyone. So unless there was a good environmental reason on a new planet to push gene changes for everyone's benefit, not likely in that time frame.

• I think the genetic isolation of the aboriginals was closer to 40,000 years. But in either case your point is abundantly made. Jul 29, 2015 at 16:43
• @Samuel I could have sworn I read 70K but what little I'm seeing now says 40K. I'll change it. Thanks Jul 29, 2015 at 16:47
• I guess with 500 years he described what he meant by "near future" and wasn't describing the time frame. Jul 30, 2015 at 7:00

An option you may consider, though it may not be what you're thinking, is that one group of humans stay human, but another group transcends their own humanity. There is a movement called Transhumanism that seeks to end many of the problems that some would say define us as human; many believe that in this century, we may cure disease, hunger, aging, and even stupidity. Not only that, but the threat of the Singularity looms, bringing with it the possibility that humans may be replaced by a new mechanical form of life. Again, many people believe this will happen in the 21st century.

The synthesis of these ideas and your question is that one day, humans may transfer their minds into mechanical bodies. That way, they can live forever, expand their minds, and just generally be better in every conceivable way. The only problem is that these 'posthumans' won't be sexually compatible with old-fashioned humans, and thus cannot be considered the same species.

As far as I can tell, this is the fastest way to split humanity into two species. That said, it has many of its own problems, such as the myriad other ways the two species would no longer be compatible, and the fact that one species is no longer what you'd call 'natural'. Other than that, though, it's a viable option, and even though many people don't believe it'll happen in the 21st century, it's very plausible that it'd happen by the 26th.

– Joze
Jul 29, 2015 at 14:59

As said, 500 years is just a blink in the eye for evolution; at least for a creature with life cycles as long as humans. It's generally too small to have any evolutionarily interesting changes.

However, to meet your answer, yes it is theoretically possible, though quite improbable. A species is defined as two creatures that can not reproduce with each other to create fertile offspring. If the two were incapable of reproducing they are a different species, even if they look identical in every other way. In theory as little as one mutation is all that is required to become a separate species.

In fact, that's the only reason humans are not the same species as the bonobo and the chimpanzee; two of our chromosomes merged together into a single larger one, causing a different number of chromosomes than the chimp or the bonobo had. If it wasn't for that one change it's possible we would still be able to interbreed with our closest genetic relatives.

Of course, the problem is having the entire population of Mars gain this gene, which is sort of impossible. Since they can only have children so fast, their genetics can only spread so quickly. One person with a mutation when they found Mars can not spread that mutation to a large percentage of the population in 500 years, unless, he, or one of his immediate descendents, started some truly extensive harem/breeding program to father all the next generation's population, or the population numbers were so absurdly low that inbreeding would almost certainly lead to death of the population.

This leads to improbable ways to technically create speciation. Both would only work if something made this mutation quite important, if not mandatory. In other words, your Martians would need to be facing extinction, or something close to it, that this one mutation can somehow fix. The most likely form of extinction would be that the Martians can't have children for some reason. Breeding issues, and this mutation, allowed them to get around whatever was preventing them from having children, by modifying the way they reproduced.

1. Intentional genetic mutation, the most likely. Facing an extinction level event on Mars, the Martians created a way to use a retrovirus, or controlled IVF, to get around the extinction level event, providing their children with a chance to survive. A side effect of this life saving treatment was to tweak their genetics in such a way to be incompatible with earth genetics; likely creating offspring that are infertile (that's still specization by definition).

2. Near-extinction. The colonists did not intentionally do anything to their genetics, or lacked a means to do so, and thus almost all of them died out. The ones that did survive managed to do so based off a random mutation that just happened to appear at the same time (note, I can't say how absurdly unlikely this is). This one person with the mutation spreads it to the much smaller martian population. Perhaps he is the only person capable of reliably fathering children due to some environmental radiation whatever and so by necessity is the father of all future children. Though I can't say how improbable this is; and ultimately a society like this would still be lowly dying to inbreeding.

In either case the two species would look and act almost identical. Their ability to reproduce being the only real distinction between the two. However, that is all it takes to technically be a new species.

Given more time this specialization could occur, but by more time we're talking tens of thousands of years at least, if science doesn't get involved to accelerate the process.

• The near extinction scenario is probably the most likely, especially if there is some event that stops space travel and the Martians (already starting from a small base population) suffer from technological failures of their infrastructure, bring ing them down to a "genetic bottleneck" where only a few individuals become the ancestors of everyone else. Humans apparently went through this, and all cheetahs are virtually cousins because of a severe bottleneck sometime in their past. I think 500 hers might still be too short a time span, however. Jul 30, 2015 at 0:37
• @Thucydides the problem with near extinction, which I had liked untill I started writing it down to think about it, is that there is so little time that there isn't enough time for the population to all gain the mutation unless their number of survivors is so low that inbreeding will kill them off. Unless you have some reason that the first carrier of the mutation somehow fathered a disproportional huge percentage of the children; and now it just sounds like someone's badly written pornographic story :P Give it a thousand or 2 thousand years it's plausible. Jul 30, 2015 at 14:18
• Great answer, though only option 1 (gene therapy applied to all colonists) seems even remotely likely. One very strange consequence would be that with this gene therapy, individuals could switch from being one species to the other (earth human to mars human). Nov 7, 2015 at 6:56

Frankly, it comes down to your definition. I know just yelling 'semantics' is impolite but the species problem is a very big issue for biologists.

Quite a lot of species are only separated geographically, some are only temporally separated. examples abound, check Wikipedia's Reproductive isolation page. Using that definition of species: Putting a viable population on mars and then stopping interplanetary travel would be enough.

If you only accept species that can't even produce offspring in a lab that are both viable and fertile, then dsollen's answer is great.

If you go so far as to accept as species groups of entities that don't reproduce biologically, then DaaaahWhoosh has the best answer.

Another problem is your measuring method for speciation. How many generations do the populations have to be separated and effectively reproducing before you call them different species? The transhumanism answer can get very tricky here, do posthumans even have generations? The bare minimum would be 3 generations: You start measuring population 1 when they are no longer capable of reproducing with population 2(first generation). You want to know if they have viable children (second generation). You want to know if these children are fertile (third generation). Of course you continually have to check if the new generations can crossbreed with population 2, the moment this happens you have to start all over.

I'd say your scenario is more realistic than the time machine.

In the time machine, one of those species eats the other one to survive. This implies that they cannot be separated or else the food chain would break. But that is a contradiction. If they were never separated, then how did they evolve separately? If they were not separated then they would share a gene pool and evolve together.

If migration between earth and Mars was stopped, then the groups would evolve separately. If there was a dark age, then they would evolve faster as natural selection becomes more savage and they need to adapt to their different environments, rather than depending of medicine and technology. If there were exposure to radiation, then evolution would be even faster, as there would be a higher rate of mutations. However, even in a 'perfect storm' evolution would probably take longer than you'd expect.

• Presumably, the one eating the other did not happen until much later. Jul 30, 2015 at 3:44
• If migration between earth and Mars was stopped... Seemingly most likely by a disaster on Earth rather than Mars. Such disaster could even be political so that all related funding was cut; it needn't be physical disaster. Early enough in Mars colonization, Martian population would already be at near-extinction levels; and it'd be difficult to fabricate sufficient facilities to start up inter-planetary travel again from that end. Survival would take precedence. Jul 30, 2015 at 7:50
• @user2338816 "Seemingly most likely by a disaster on Earth rather than Mars" - Very true. I like the way you think. Jul 31, 2015 at 2:12

The problem with answering this question, especially with the short timeframe specified, is that evolution doesn't really give us clear points where one species becomes another. It's more a case of taking a point where we have one species, and another point where we have something that evolved from it, and arbitrarily calling them species, because that's what we're measuring. There are no clear boundaries, just lots of fuzzy transitions from one species to another, because we don't really deal with the idea of "becoming" anywhere near as well as "is"

We're already becoming a bunch of different species, most of which (if not all) will probably die out before they ever get a chance to be different enough for anyone to notice.

So, probably not two species different enough that you wouldn't call one of them human within just 500 years, but even genetic drift will make humans slightly different within 500 years. (Just not different enough that we'd notice)

Evolution is like culture. Separate things, they change. Just look at Darwin's exploration. Different islands, different birds. The biggest variable here is time. 500 years won't prolly cut it, however, if one species is living on Mars, as you said, then they would probably want to adapt to Mars climate. More cold resistance could make reproduction different to a feasible degree. Lighter gravity means a lighter bone structure, and taller people. Given maybe 1000 years, they would almost certainly be different species. One would be redder, taller, more cold resistance, and more impact resistant, but only by a small amount. The other would be us. But they'd work together just fine for a long while.

• Australia's Aboriginal population was separated from everyone else for something like 40000 years and they're still not a different species, so I'm not sure that evolving into a separate species after 1000 years is as certain as you say. Evolution is a much longer process (at least on earth). Nov 11, 2015 at 17:40
• These are different planets. Nov 12, 2015 at 2:38
• Yes, and on earth 40000 years is not enough for divergence, so without some form of technological acceleration, even on mars, I don't think 500 years is going to make any noticeable difference that is outside of the usual scope of genetic drift, which means they're still going to be the same species. Nov 13, 2015 at 14:51
• We can't prove a figure that large cuz we haven't been around that long. Nov 15, 2015 at 2:46