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There are thousands of species of spiders on Earth. Ditto insects. And, prehistorically, there have been times when there have been several, or even half a dozen species of hominins in existence on Earth at the same time. But, now, there appears to be only one species of genus Homo on Earth.

What circumstances could plausibly cause Homo sapiens to fracture into multiple species again?

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    $\begingroup$ It's all down to isolation of groups and different selective pressures. Random drift helps, but you still need isolation. Can be geographic, cultural, - anything that prevents random mating. That's evolution. (And, I suppose we have to not kill each other like we may have done to other Homo's.) $\endgroup$ – DPT Oct 21 '17 at 23:31
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    $\begingroup$ This is called speciation. $\endgroup$ – imallett Oct 22 '17 at 16:21
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    $\begingroup$ How are you defining "species"? If we used the same definition we frequently use on animals, there would be many dozens (if not hundreds) of species of human. Although the technical definition of a species is what can breed with what to create fertile offspring, many, if not most species of animals are defined by different physical characteristics or appearances, rather than by determining if they're capable of procreating with other existent species. So... in that sense, humans already are fractured into many multiple species. Is "shoddy definitions of speciation" an answer to your question? $\endgroup$ – HopelessN00b Oct 23 '17 at 17:45
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    $\begingroup$ @HopelessN00b Even if one doesn't use a strict reproductive cross-fertility definition of biological species, the genetic and phenotypic difference between different members of the species Homo sapiens sapiens would still be too small to rate a different species classification for animals. For example, the degree of genetic differentiation is also far smaller than "grey zone" species distinctions. worldbuilding.stackexchange.com/questions/59252/… $\endgroup$ – ohwilleke Oct 23 '17 at 19:55
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    $\begingroup$ @HopelessN00b The Neanderthal-modern human genetic gap is 0.3% v. 0.5%-2.0% for "gray area" species. The gap between any two modern humans on Earth is much smaller than the Neanderthal-modern human gap. geneticliteracyproject.org/2014/02/04/… $\endgroup$ – ohwilleke Oct 23 '17 at 22:17
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Genetic Engineering

A computer program eventually boils down to two different characters: 1 and 0. The only thing that matters is what order they appear in. But from that alone we get amazing tools, like for instance Stack Exchange.

A human is, at conception, apart from a little bit of cellular aid function, four different characters — A, C, G and T — repeated 3 billion times. Again: that which makes us humans and not for instance apes or fish or amoeba, is the order in which these characters appears.

enter image description here

The interesting part is that it does not take much reordering to make us from humans into something else. Humans have over 99% genetic commonality with for instance chimpanzees. That is a mere 30 000 000 base pairs that needs reordering, and voilà... a whole new species.

enter image description here

You have 99% genetic commonality with these fellows

So what happens when we can do genetic engineering on ourselves? Movies like Gattaca have already explored this venue. Human beings will start fixing the most glaring errors in our genetic code, like for instance the fact that our retina is clumsily mounted backwards, our idiotic vitamin C self-sufficiency deficiency, or our completely unnecessary vulnerability to viruses and bacteria.

So how long until we have the first "patch" to our genetic code? Will everyone get it? Can everyone afford it? Will everyone want to get it?

And what happens when another company provides a better patch, that it not at all compatible with the first one?

Even worse... what happens when we find out that husband and wife cannot conceive because they were altered by two different patches?

In real life, Pandora is looking at that big box that says "Human Genetic Engineering" and goes "Hmmmm.... I wonder what's in there"...

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    $\begingroup$ It's better to use non-breaking thin space (U+202F) for digit grouping than simple space, like here: 30 000 000. Otherwise your number is word-wrapped in my browser. Even if it didn't, it still has too wide gaps between digit groups with simple space. $\endgroup$ – Ruslan Oct 22 '17 at 9:00
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    $\begingroup$ And let's not forget furries. There are worlds built around the idea of transformation to non-human forms. You're totally going to end up with communities of like-minded manimals and invertebroads once the technology to do so becomes available. Whether they become species of their own as a result is an open question, but I wouldn't be surprised if the compatibility issues mentioned above become an issue. $\endgroup$ – GreenReaper Oct 22 '17 at 15:45
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    $\begingroup$ Who wants a natural conceived baby anyway if you can get your perfectly engineered baby without that nasty pregnancy? Just for completeness: It's not only the ordering that's important but also the amount. E.g. there are only 2 ways to order a single one and a single zero but there are 4 combinations of 2 zeros or ones: 00,01,10,11. $\endgroup$ – Christoph Oct 23 '17 at 10:28
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    $\begingroup$ This is biology. The first rule in biology is that the comment "Hmm, I think you'll find it isn't quite that simple" is always true. The study of epigenetics is showing that there is more to defining an organism than just knowing its genetic sequence. $\endgroup$ – Martin Bonner supports Monica Oct 23 '17 at 14:07
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    $\begingroup$ @Alexander something something iPhone. $\endgroup$ – CodeMonkey Oct 24 '17 at 8:44
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Planetary migration.

Think of how various races of humans developed on our world, people got cut off from one another for thousands of years, mutation and natural selection did the rest. Now imagine that instead of being isolated from each other by deserts mountains and oceans they're isolated by the vastness of space. Each one part of a completely different ecosystem. It's natural to suppose that in such an environment the human race would Branch off as various branches develop various mutations. Given enough time you would have entirely new human races develop on each planet. And if they have the technology to genetically alter themselves to fit better on the new planets then the changes can be even more extreme and happen faster.

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    $\begingroup$ Each new colony will also begin with a relatively small population, a genetic bottleneck, and will likely expand rapidly. By having only a sample of human diversity they will immediately be different genetically to the original population, (likely pretty unusual too, as colonisation will take a specific mindset or type of person). The population explosion may take relatively rare traits from back on earth and make them mainstream, or take something usually conman and eliminate it by chance. Small populations and chance can change the gene pool a lot in a short time. $\endgroup$ – Troyseph Oct 23 '17 at 11:05
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    $\begingroup$ Upvoted because this is the only reasonable scenario I could think of that would interpose effective barriers to interbreeding. However, it would also take time. There would have to be a long period where interplanetary travel between the worlds in question is completely impractical. From what we know of human evolution, Neanderthals were able to interbreed with moderns with a 2-300,000 year separation. We don't find known incompatible species until you go back to about 1.5 million years of separation. That's a really long time without discovering faster-than-light travel. $\endgroup$ – T.E.D. Oct 23 '17 at 15:38
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    $\begingroup$ ...of course extreme environments have been theorized to speed the process up, and different planets are probably good places to go to find extreme environments. $\endgroup$ – T.E.D. Oct 23 '17 at 15:41
  • $\begingroup$ @T.E.D. Again that why I mentioned genetic engineering, Many Scientists have already said that it maybe easier to genetically alter humans to fit in to a new planets ecosystem then it would be to terraforming the planets to fit human needs $\endgroup$ – Bryan McClure Oct 25 '17 at 15:00
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Just be patient. We are right on the verge of elective genetic surgery as a medically available and culturally accepted option. In the next 20 to 50 years, procedures will become available which will modify your body's base code in a variety of ways. Few will be able to resist the allure of those new options.

Social and economic pressures when mixed with these new genetic opportunities, is a guaranteed recipe for the fracturing of our species into a multitude of new and reproductively exclusive species.

Dissatisfied with a single century of life, take this drug and live for five; but once you take it, you can't breed with short-lifers any more.

Having trouble in school? This simple process will double the blood supply to your brain and greatly enhance your intelligence and memory retention. But the genetic purist will consider you a gene-junkie forever more. Scratch off dating any of their daughters if you take that route.

What budding astronaut wouldn't take a treatment which greatly reduces their body's vulnerability to radiation, even if it meant that they could only breed with other astronauts from now on.

Just sit back and watch. The age of vanilla homo sapiens is just about over. In the days to come, we may all still be human, but most of us will be human+...

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    $\begingroup$ @RuiFRibeiro, I am sorry that what I wrote offended you. As my answer stands even without the paragraph which troubles you, I have made appropriate changes. $\endgroup$ – Henry Taylor Oct 22 '17 at 20:15
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    $\begingroup$ Why is that after any reasonable but politically inconvenient statement is made a SJW police (AKA RuiFRibeiro) shows up in the full force? Let me insert the rephrased deleted paragraph: genetic engineering would fix (or at least alleviate) many social ills such as racism. $\endgroup$ – Tegiri Nenashi Oct 23 '17 at 17:58
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    $\begingroup$ @TegirlNenashi, if you have edit privileges, feel free to make any changes you wish. $\endgroup$ – Henry Taylor Oct 23 '17 at 18:08
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Again? Many hominins may have been able to reproduce with each other, making them, at least, subspecies. Generally while there had been a diversity of hominins Homo sapiens have been the one species overall. This remains an open question. The theories and evidence are contested.

As modern humans spread out from Africa, they encountered other hominins such as Homo neanderthalensis and the so-called Denisovans, who may have evolved from populations of Homo erectus that had left Africa around 2 million years ago. The nature of interaction between early humans and these sister species has been a long-standing source of controversy, the question being whether humans replaced these earlier species or whether they were in fact similar enough to interbreed, in which case these earlier populations may have contributed genetic material to modern humans.

Spiders evolved 380 million years ago, humans between six to three. Spiders have been able to differentiate into multiple species by exploiting different ecological niches. Humans haven't had enough time or occupied habitats sufficiently different to evolve into new species. Reproductive isolation, adaptive pressures & an enormous amount of time are needed.

Open one-way portals to parallel Earths and you're in business. Isolated populations of humans on alternative versions of planet Earth. Vast tracts of real estate to occupy. New environments to inhabit. The whole business of hominization can go down a new road.

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    $\begingroup$ "occupied habitats sufficiently different"? Humans occupy pretty much all environments. I think the problems are: (as you said) insufficient time; long generation times; insufficient isolation. $\endgroup$ – akaioi Oct 22 '17 at 8:38
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    $\begingroup$ Although we already see lots of evidence of localised population phenotype specialisation. Compare the distribution of physical traits between an Inuit population and a Masai population. We observe physical traits that are specialised for cold-weather and low solar light levels, compared with traits for hot-weather and high solar light levels. These are both products of evolutionary pressure on isolated human populations. $\endgroup$ – DrMcCleod Oct 22 '17 at 10:03
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    $\begingroup$ @DrMcCleod Phenotypic adaptations result from selection pressures due to environmental conditions. Speciation, we know, takes much, much longer. Masai and Intuit could breed readily enough (cultural selection notwithstanding) in biological terms. $\endgroup$ – a4android Oct 22 '17 at 12:03
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    $\begingroup$ How does sending humans to parallel earths allow for the requested genetic mutation to occur? It would all be very similar environments that we have already adapted to (with the added potential of already existing humans living there). Sending them to other habitable planets with completely different environments which would/could require adaption would allow for a quicker split from the original homo sapien strain. $\endgroup$ – EveryBitHelps Oct 22 '17 at 14:05
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    $\begingroup$ @DrMcCleod We know that time periods on the order of 500,000 years may lead to clades that can reproduce fertile offspring but are subject to Haldane's law applicable to hybrids from Neanderthal admixture data, but that 150,000-250,000 years (the deepest divide among modern humans) is insufficient to do so. But, that doesn't control for selection that could speed up the process (or slow it down in its complete absence). $\endgroup$ – ohwilleke Oct 22 '17 at 20:36
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Typically, species tend to form when populations are isolated from breeding with each other. Unless the world becomes significantly less globalized, and remains so for millions of years, it is unlikely for humans to speciate through that route.

Pure gene therapy is unlikely to be enough. Today, most people see particular qualities as being "good" or "bad", and most genetic engineering would be focused on making people "better". This would likely result in decreased diversity among the human species, rather than the fragmentation you're looking for.

However, a societal reform, a change to the way we think of "individuals", along with gene therapy, could provide another route.

Imagine a world where every person is born into a particular job, where their entire worth is determined by how well they perform at that task. Families grow more suited for their task, and tend to produce children with others who have the same societal role. This could, theoretically, result in fragmentation, where each "breed" of human is built to their task - much like dog breeds have been historically. You could have bulky construction workers, autistic super-genius engineers, subterranean tunnel-workers with good night vision, hairless and adaptable space explorers, even cute and docile "human pets". Breeding humans who excel at and enjoy their assigned task may prove to be easier and more efficient than creating robots for the same purpose.

While it is unlikely that such a society could last long enough for true speciation to occur (even dogs, which have been bred for thousands of years, and have tremendous diversity in form, are still the same species), genetic engineering could accelerate the process. Given enough time, human society could wind up as an ecosystem, with each human species providing something of value to the whole.

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    $\begingroup$ The thing is that this has basically been tried - genetics studies show India's caste system has been in place with strict endogamy within Jati for about 2000 years, and this is still nowhere near leading to either the differentiation or the incompatibilities needed for separate species. Indeed, even if dogs quite extreme breed differences can and do result in perfectly healthy puppies. Labradoodles anyone? $\endgroup$ – ohwilleke Oct 22 '17 at 14:52
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    $\begingroup$ @ohwilleke Well it has only been 2000 years, far too short a time period to expect any species-defining mutations to arise $\endgroup$ – somebody Oct 23 '17 at 4:25
  • $\begingroup$ @somebody But, you would need an exceedingly stagnant society for jobs to be that stable over that time period. One would have to go back far indeed in human history to reach such a time period and that was a time period with little or no specialization by job period. Every society that is complex enough to have specialized jobs is not stable enough in the nature of those complex jobs to persist long enough for speciation. $\endgroup$ – ohwilleke Oct 23 '17 at 4:37
  • $\begingroup$ @ohwilleke True, which is where genetic engineering comes into place (see Krypton) $\endgroup$ – somebody Oct 23 '17 at 4:46
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I'll start by quoting Darwin:

A second great fact which strikes us in our general review is, that barriers of any kind, or obstacles to free migration, are related in a close and important manner to the differences between the productions of various regions.

On the Origin of Species, Chap. XI

You need barriers that limit to the minimum the breeding of two (or more) populations for enough time to allow mutations to arise that make it almost impossible to interbreed.

For current human conditions (low evolutionary pressure), it would take a lot of time… but on other conditions, it could be less time. Some factors that can explain an abrupt divergence:

  • Founder effect: "the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population".

  • Convergent evolution: "the independent evolution of similar features in species of different lineages". It may happen that two populations or even species found homologous structures but with different coding.

  • Ecological niche: "the fit of a species living under specific environmental conditions".

  • Evolutionary pressure: " a quantitative description of the amount of change occurring in processes investigated by evolutionary biology".

With that, you can imagine, as an example, a cataclysm that isolates a small populations (then we have founder effect, but enough big to have a minimum diversity). The cataclysm (or the isolation circumstances) put a high evolutionary pressure, that prioritizes some characteristics that makes better adaptation to some ecological niche.

Even if the other (or others) population has similar circumstances, the chance favors different evolutionary ways, giving convergence at the most. But also there are chances that evolution produces two different approaches to the same problem.


I'll add some examples:

  • Humans begin to colonize Mars. There are some thousands of humans on Mars, they can produce food and some medicine, but still depends on Earth. Then, a cataclysm on Earth (maybe just a political crisis) made impossible to move things and people from one planet to the other. That would made some pressure on Mars (low resource disponibility), it would be a niche (there is no gene exchange), the high radiation (low atmosphere) would increase mutations, etc. It would be verisimilar that in some hundred years Human specie began to fracture.
  • The economical class fracture intensifies. Rich people lives in the upper layers of cities, and they began to do some selective breeding and ask "pedigree" in job interviews. Poor people are exposed to low medical care, high pollution and no planned breading. In some decades the social fracture can be so bigger, than even if they can interbreed, they choose to not. Along centuries, the genetic drift could be so that they are become two different species.
  • Climate change and melting of Arctic ice diminish the agricultural suitable and habitable surface. At the same time, mountains become islands (which isolates populations), some began to live on open sea and can give arise to sea adaptations (see Waterworld), other people live in the top of the trees of some forest (while the bottom is under water), etc.
  • Human genetic engineering is totally allowed. In some countries are State controlled, in others are controlled by private corporations. Even if they can predict the result of the new genes in the body, they can't predict interaction between other new genes (mainly because they do not know which that new genes would be, as they are design by other corporations and countries).
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Reduced Time Between Generations

Humans don't reach sexual maturity until after puberty, which means that there is at least ~15 years between generations. This means it can take a very long time for any trait to develop.

There is some evidence that puberty is occurring earlier. If the trend continues or accelerates, that could make evolutionary changes that would lead to speciation quicker.

Genetic engineering would also fall under this category, as large genetic changes could be implemented and tested for viability quickly.

Increased Mutation Rate

Either an increase in the rate of genetic mutations or a decrease in the ability to prevent mutations could increase the mutation rate.

For example, some condition (e.g., a disease/virus) that decreased human DNA repair enzymes could reduce mutation prevention and lead to a greater mutation rate.

Similarly, some event that increases global radiation could increase the frequency and severity of genetic mutations. (nuclear weapon, damaged atmosphere, change to sun resulting in more UV, etc.)

Selective Breeding / Selection

The same process that humans have used to selectively breed plants and animals to emphasize certain traits could be applied to people. This could either by encouraging/ forcing certain people to mate or by preventing them from doing do. See eugenics.

Isolation

Some sort of physical or virtual separation of groups would be necessary. Physical separation would likely have to involve some restriction in travel via air or water - perhaps a nuclear war, severe climate change (e.g. ice age), or massive tectonic event.

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  • $\begingroup$ I like the mutation rate observation, particularly in light of increasing evidence that there are biological systems that control their own mutation rates. $\endgroup$ – ohwilleke Oct 23 '17 at 22:00
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Social/regional segregation. Split a group of humans into two separate groups. Let's say one lives in a rural area in the arctic, the other on a big plain around the equator where there can be 100s of meters, maybe even kilometers between houses. The first group will eventually evolve to get better eyes, and eyes on the sides of their faces because those with better eyes can spot polar bears more easily and have less chance to be eaten. In the second group, more athletic people can travel more easily and more quickly between houses as a child, which makes them get easier social contact, so they become more successful and have a higher chance of reproducing. After a long enough time group 1 will become hyperaware humans with very good senses and see high with contrast, and the other will be very strong, athletic people who can run very fast.

For the second scenario, let's get 1 country full of normal people. Now, a group of tall people thinks they are superior. All the politicians are tall, and they make a law so anyone above 1,85 is considered tall, and anyone below 1,85 short. Tall people get more rights, short people less. Now if a tall person marries a short person they get the same rights as that short person. All the tall people will only marry tall people, and short will marry short people. After a while, you get very tall people, and the rule is changed. Anyone below 1,90 is short. After a while, anyone below 2m is short. Now anyone between 1,90 and 2m isn't allowed to get children. After a few thousand years, human race is split between superior giants and small weak dwarfs.

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