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Setting: Earth, 5 million years in the future (see note 1). Continents are still recognizable but sensibly altered by tectonic movements. Climate is drastically changed. Civilization does not exist anymore, having already risen and fallen many times (see note 2), but humans are still around. Humans have evolved into a plethora of species that, although partially capable of interbreeding, are on the verge of becoming different kinds of humanities. Among them, however, one is morphologically identical to present-day humans: how is that possible?

I know it’s improbable that a species may exist as a contiguous entity for many millions of years, so WHAT SPECIAL CONDITIONS ARE TO BE MET TO MAKE HOMO SAPIENS A LIVING FOSSIL SPECIES?

Note 1: I'm willing to reduce this timespan to at least 2.5 million years if I need to.

Note 2: In this setting human civilization on Earth rose to space travel levels and then fell to pre-industrial or even prehistoric levels at least a few times, so while no Homo species on Earth is technologically advanced there may be, somewhere, somehow, space humans which have retained varying degrees of techonological capability. They may have a part in the explanation, but I would appreciate a justification as “natural” as possible.

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    $\begingroup$ If I can find the time, I will provide one possibility later today. In the mean time, google "The Event of 774 AD". $\endgroup$ – Vince 49 Dec 30 '17 at 19:08
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    $\begingroup$ I'm not quite sure how natural an answer could be. Evolution is not something so easily avoided over such large timescales. Genetic diversity and random genetic modulation are almost a given over thousands of generations. My only suggestion is that these humans devolved to seem like present-day humans, though even that has some pretty giant gaps, considering the two species are to be identical. $\endgroup$ – B.fox Dec 30 '17 at 19:09
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    $\begingroup$ Visually, there was not much to distinguish between homo sapiens and homo habilis. You'd have to measure the volume of the brain case to make a definite identification between the two. That's 2.5mm years evolution $\endgroup$ – pojo-guy Dec 30 '17 at 20:41
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    $\begingroup$ someone could have figured out biological immortality during one of the high points and those humans could just be few lucky surviving individuals. $\endgroup$ – John Dec 31 '17 at 15:48
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    $\begingroup$ Possible duplicate of Humanity after 100 million years? $\endgroup$ – Mithoron Dec 31 '17 at 23:51

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Stephen Baxter wrote a story The Children of Time published in Asimov's Science Fiction magazine in 2005 (I saw it in the Asimov's Science Fiction Magazine: 30th Anniversary Anthology) that poses exactly that question over approximately 400 million years, and answers it in an interesting way.

He says:

It had never been necessary for humans to evolve significantly, for they always adjusted their environment so they didn't have to - and in the process stifled evolutionary innovation.

He says this type of lack of evolution should always be a consequence of the evolution of intelligence, because intelligence is always able to figure out ways to prevent the need for further physical evolution.

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    $\begingroup$ except human have noticeably changed through competition with each other for mates. interspecies competition has always been a major driving force of evolution. $\endgroup$ – John Dec 31 '17 at 15:50
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    $\begingroup$ Humans have changed. For example, humans have adapted to drinking milk in their adulthood; indeed, that gene is recent enough that it has neither spread to all of humanity, nor gotten extinct. Other evolutionary adaptations are more visible: There is a reason why the typical skin colour is the darker, the closer you are to the equator. Of course you may question whether that is a significant adaptation; but AFAIK in the same time frame, most other species didn't change more dramatically either. $\endgroup$ – celtschk Dec 31 '17 at 17:48
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    $\begingroup$ @MosheKatz how about the fact that only ~300 years ago the average height was only 167 cm? That's a pretty significant physical change imo. $\endgroup$ – theonlygusti Dec 31 '17 at 19:04
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    $\begingroup$ @theonlygusti That isn't a genetic change, that is due to average people getting enough to eat. Previously, only a few were well fed enough to be 6 feet tall; now, anyone with the genetic potential to reach six feed is fed well enough to do so. $\endgroup$ – kingledion Dec 31 '17 at 20:25
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    $\begingroup$ Baxter's argument is not a very deep thought. He ignores that humans are a result of enormous evolutionary pressure (humans went nearly extinct more than once). If intelligence loosens that pressure, and hence ends selection for certain traits, these traits will go away. I have hey fever, and I'm myopic; I wouldn't have survived long in the stone age. The current genetic drift of humans is almost entirely determined by fashionable looks and behavioral traits. $\endgroup$ – Peter - Reinstate Monica Jan 1 '18 at 15:00
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Genetic fluctuation in populations has been studied a lot. Two things can alter the genetic composition of a population: selective pressure and random drift.

We will assume no selective pressure on the genome which is a big assumption. I could imagine a very large population with heterogeneous selection pressures and lots of mixing of individuals. A selective pressure cannot keep up pressure because individuals move around so much and are likely to produce offspring with individuals from very different regions.

Random variations can happen. The larger the population, the less randomness (think of selection bias with a small sample) will be able to move the genome. This image shows a population with a 50/50 mix of alleles a and b. In the first generation of offspring, what is the mix of alleles?

from https://www.nature.com/scitable/topicpage/genetic-drift-and-effective-population-size-772523 genetic drift and population size

Larger populations are less easily skewed by chance. Like two cellmates who are equally good at poker: a series of 10 games might show win loss for one of them to be 7:3 but over 10,000 games it will converge on 5000:5000 if the two are equally good.

Other aspects of the population which might artificially minimize it and cause random fluctuation to be more important. From same above linked source.

An "ideal" population has the following characteristics, and most deviations will decrease the effective population size:

  • There are equal numbers of males and females, all of whom are able to > reproduce.
  • All individuals are equally likely to produce offspring, and the number of offspring that each produces varies no more than expected by chance.
  • Mating is random.
  • The number of breeding individuals is constant from one generation to > the next.
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  • $\begingroup$ "Mating is random.": you'll need to suppress free will to support that condition, i.e. all humans became slaves. $\endgroup$ – Cœur Jan 2 '18 at 7:18
  • $\begingroup$ @Cœur I'd agree it's not rolling dice random, but it's probably under a lot less of your control than people want to believe $\endgroup$ – Xen2050 Jan 2 '18 at 7:56
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    $\begingroup$ @Xen2050 To make mating random, it would require that improved traits, immunities and unique traits are just as popular as degraded traits, disease predispositions and normality. And to prevent a natural bias, it would require birth control so that unsuccessful people get as many kids as successful people. It's ain't easy to stop evolution of an intelligent being. $\endgroup$ – Cœur Jan 2 '18 at 8:16
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Humans live in the Matrix

Imagine that the Machines have taken over and keep humans as pets. After 5 million years, the humans rebel and overthrow the machines, re-taking their planet.

In the 5 million year interval, the only genetic selection on humans is artificially introduced by the Machines. For whatever reason, the Machines like us just the way we were when we invented them.

Therefore, 5 million years later, humans are genetically very similar to the present.

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    $\begingroup$ no need to complicate it with rebellion. The world described IS the simulation. Nothing changes in 5 million years of simulation because that's not supported by the simulation. $\endgroup$ – Devstr Dec 30 '17 at 21:23
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Since you mentioned space travel, don't forget time dilation!

In our near future, space travelers set off at substantial fractions of light speed to various destinations, then returned to earth. Due to time dilation, the travelers' societies have experienced the passage of only a few centuries or millennia, while millions of years have passed for the stay-at-home branch of humanity. If one of these expeditions has returned recently (say, within the last couple centuries), then you can have a human subspecies which is not only morphologically, but also genetically more-or-less identical to modern humans.

Add in cryosleep pods or a similar hibernation/stasis technology, and someone who is alive today could conceivably be returning home to earth 5 million years from now.

While high-speed deep-space journeys of this sort would imply a high level of technology to build the ships and mount the expeditions, the returning ships would not particularly affect the technological level of the world they returned to, as the industrial and technological base to reproduce them no longer exists. At most, the returning explorers would provide additional scientific knowledge, but it would be so localized and of so little practical value that it could easily be lost within a generation or two. (And they may not even provide that, depending on how automated their ships are vs. how much of the maintenance is provided manually.)

Also note that I said "various destinations" above. Different trips of different durations opens the possibility for periodic injections of small modern-human populations into your world, which would help to limit the divergence of the stay-at-homes from humanity as we know it today.

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    $\begingroup$ The old original Planet Of The Apes scenario. Good one. A slight variation could be a "seed ship" designed to colonize some far away planet (probably controlled by AI with a large payload of human genetic material and rapid cloning technology) which had to return to Earth rather than complete it's journey. It managed to survive the malfunction, but ended up far in the future compared to it's point of origin. One thing about the space travel-time dilation possibility is the movement of galaxies over the course of millions of years. Not sure if this would throw off navigation too much. $\endgroup$ – JBiggs Jan 2 '18 at 18:08
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The big question is what do you mean by the "same". Humanity is quite diverse as it is. Do you want to preserve all races and ethnicities in present day proportions? Do you want them unmixed? What is the criteria?

Well you have to work around evolution. I can think of several angles to attack it in addition to what JBiggs mentioned.

  • prevent environment selection pressure. For example, wise ancestors perfected climate control and fixed it at ideal settings for 21 century humans. Automatic systems survived more or less for 5 million years (unlikely)
  • artificially select for stability of genome. After another rise of nazism there was another world war where genetic warfare was introduced. Artificial viruses or bacteria killed everyone too far from the "uber" race ideal genome. They became human symbionts and kept genome clear by killing fetuses with wrong mutations.
  • fix genome on the go. Mad narcissistic scientist devises CRISPR-like mechanism that slowly turns everyone into her clones.
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A couple things could make this plausible:

1, conditions where these humans live can be such that the current form of homo sapiens is ideal for life there, which reduces the natural pressure to adapt. Why would this be the case? Probably because of an environment at least partly man-made. Clearly in some earlier time, mankind built environments that were ideally comfortable and optimized for them, and their descendants now live in these (probably self-maintaining) habitations which are perfectly suited to their needs. One likely reason for this to go on so long is space colonization, which would force humans to live in completely synthetic environments or "terraformed" environments or giant dome areas which were specifically tailored and engineered for their comfort.

2, There could be a "baseline" genetic reference which keeps being reintroduced into the gene pool. This could also be the product of an earlier, more advanced phase of humanity. Shortly after perfecting genetic manipulation, humans may have preserved specific genetic "libraries" of human genes with what were considered desirable (probably profitable aka: "designer") traits. Sophisticated, automated systems can replicate this genetic stock which is still available for use, and probably includes some cool new or optional "features" which people might still find desirable, but to use the old stock, they must be close enough to "baseline".

2.5, Alternately, these "baseline" genes may be introduced from clones which are based on old-style genetic stock. There may have been a legal loophole which allowed clone "slavery" or servitude if the clone was created from scratch and grown entirely in a lab. Perhaps a "second class citizen" type system more like the highly educated Roman slaves who ran the empire's administrative apparatus. Over time, for legal/humanitarian/political reasons, the stock from which these clones could be produced was frozen to one basic "pool" and production continued for some considerable time. Inevitably, the descendants of the clones would mix with the non-clones, pulling the entire human gene pool back toward "baseline". Also, if there were cultural effects where non-clones were considered "superior" for some period of time, and there was a long term, low birthrate, the higher class strata may have faced a milder version of the inbreeding problem faced by European nobility during the Renaissance. This would make it necessary to integrate clones into the gene pool as well. After that kind of a long-duration imperial structure collapsed, humanity might be very close to baseline. Even farther out, perhaps the cloning facilities were kept active for religious reasons in some post-imperial religion where creating a clone based on "baseline" (ancient) genes was thought to bring back in some spiritual way the virtues of ancient humans from before the time of imperial decadence.

Basically, most of this would all boil down to some very sophisticated self-replicating and repairing machines which had been developed by humans at the height of their development, which continue to function.

Edit: since I answered, yet another plausible scenario came to mind which might even make a more interesting story. What if humanity (that is, 20th century humanity as we know it) has actually gone completely extinct and then been reintroduced? This could be either through a quasi-conservation program similar to current efforts to clone a woolly mammoth, or through some high tech "worst case scenario" fail-safe mechanism that was reactivated and began cloning modern humans again. Even if this event occurred far in the past in your future timeline, it would still put modern humanity far into the future.

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    $\begingroup$ What if every time the human species gets advanced enough to perform genetic manipulation they use it to stop genetic drift. This would be a small percentage each time, so other species would evolve $\endgroup$ – bendl Dec 31 '17 at 21:18
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There are answers I've seen here suggesting that this lack of change could be because advanced technology removes the pressures that would result in evolutionary change. Well, OP said that high technology on Earth doesn't currently exist in their world, and wasn't consistently present, as civilization rose and fell multiple times. Also, there's stuff such as genetic drift even in the absence of natural selection, and the only way to intentionally avoid it would, again, be advanced genetic engineering. The space humans with advanced technology have also been mentioned, but the OP said they would like a "natural" justification. It's also quite difficult to have technology, even advanced technology, operating without maintainance over literally millions of years. And why would space humans tinker with Earth humans over that time for so long, without detection (note that even if they have advanced cloaking technology, someone from their civilization could still decide to reveal what's going on, such as for ethical reasons)?

A solution I suggest is that it was chance, to some degree. Among the many descendants of humanity, it just so happened that one kind were similar to their ancestors. This may seem completely unbelievable, but it may not be. Some creatures, such as some shark species, have been around for millions of years with few changes. Humans have only been around for a few hundred thousand, but perhaps even without advanced technology, they could be adapted enough that some wouldn't change much. We aren't sure. Also, even if technology doesn't fully survive, parts of it perhaps could be used somehow in ways that would make some environments suitable and reduce evolutionary change (metal structures make building simple metal tools easier, for one). And perhaps the mutations, (which happen randomly, after all) , that would result in bigger changes didn't appear much in one population. Finally, the more change is allowed, the more plausible the desired result becomes. For instance, if "morphologically identical" means "outside looks the same, but internal organs, microscopic structures, and so on can be different," then that would be far more likely than "no genetic changes what so ever. "

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Although I favor the artificial genetic reservoir idea myself (Ergo Proxy springs to mind), there is also the Planet of the Apes idea wherein time and/or space travellers return to earth and end up re-colonizing. I think there is more fun to be had by playing with conventional expectations of "human" -- for example, a "tall man" today is different from a tall man of just two hundred years ago -- so what sort of expectations would mean "human" to an observer?

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  • $\begingroup$ The returning generation ship is good. Maybe their destination turned out to be horrible so they came back. $\endgroup$ – Willk Dec 31 '17 at 6:55
  • $\begingroup$ I was thinking more along the lines of "Gee, I wonder if earth ever survived that crisis gramps ran away from." But it is an open situation -- we leave earth for what was to us years/generations, and look what happened! $\endgroup$ – Lurker Larry Dec 31 '17 at 14:05
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Evolutionary change happens when unsuccessful models die before breeding and don't pass on genes that don't work to the next generation. This may occur by being eaten or having inferior gathering capability compared to a rival. Therefore the most successful models survive to breeding age and pass on the genes that have worked so far. Over multiple generations this filters to the best set of genes for a species at that time.

In modern society nothing kills off unsuccessful genes. Successful genes and unsuccessful genes reproduce at the same level. Provided technology hasn't progressed to a level where they can fix the genes this will cause a stagnation of evolutionary forces. There may be local changes due to diet and exercise etc. but the base model is still human as we know it.

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  • $\begingroup$ No they really don't breed at the same level, survive is not the same thing as breed. $\endgroup$ – John Jan 1 '18 at 6:17
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Combine three things:

1) Genetic engineering removes old age as a cause of death. Full regeneration is enabled.

2) Medical technology can deal with most everything else that isn't covered by #1. Death is basically reduced to suicide (which will be rare as the mental health and medical reasons will be gone--suicides will be those who have grown tired of life) and major trauma beyond what can be put back together in time. (Or John Varley's Ophiuchi Hotline universe--memory recording/playback. Normal procedure is to get periodic recordings of your brain, if you die the recording is played back into a clone.)

3) Once this point has been reached genetic engineering becomes anathema--perhaps due to some event such as in the Trek universe where it almost lead to a final war.

Put this together and you don't have that many generations between now and then--and evolution is a combination of generations, pressure (the only major pressure now is for a desire for children) and gene pool size (very large, as we are becoming very globalized).

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  • $\begingroup$ An interesting idea about "full regeneration." What if it's like Wolverine and enough damage to the brain is a memory wipe? Also what if they sometimes "reproduce" by being cut in half or smaller parts and the parts somehow manage to regenerate individually. One would have to relearn everything, but they'd be genetic twins. $\endgroup$ – Zan Lynx Jan 2 '18 at 1:02
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    $\begingroup$ @ZanLynx I don't mean superhero-type regeneration, more like starfish regeneration. Note that we start with partial regeneration but it's lost before birth. (In-womb surgery young enough leaves no scar.) $\endgroup$ – Loren Pechtel Jan 2 '18 at 4:37
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You don't understand evolution. Evolution is essentially the persistence of genes that help an individual live long enough to pass them on to the next generation.

There have been (and are) many species that have been genetically stable for quite some time. Search oldest animal species and you'll see that of the top 10, the YOUNGEST is 120 MILLION years old (source).

And our survival mechanism is directly counter to evolutions usual means of change - we change our environment to suit us, making physical adaptations irrelevant.

Given your scenario, the question isn't why we would be unchanged in 5 million years, but why there are branches of our species that have evolved so far from the core. Rise and fall of civilizations don't really matter, unless they have gone back to before the stone age.

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  • $\begingroup$ I do not agree that it applies to intelligent beings like humans as we have the unique ability to do breeding of species: so we can and we do genetic selections. $\endgroup$ – Cœur Jan 3 '18 at 1:40
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Define "same"

Currently humanity has vast physical diversity, there are many different forms that you look at and automatically think "human" even though adults can have a 50% disparity in height, 400% disparity in mass and totally different surface colouration.

  • If you keep all the humans in the same place, much of that disparity will be lost, humans will no longer be different, which also means they will no longer be the "same" as they are now.

  • If you allow the humanity to spread among the stars then that diversity may well increase as the different groups spread off to different worlds and face fundamentally different forces.

However there's no reason why a strain of humans who remained on Earth or similar planets wouldn't remain fundamentally the same as a particular group of humans are now. Though from our own perspective they may appear to fit a "generically foreign" (tv)trope by appearance. You could potentially manage to maintain some regional variation in appearance but it's likely that time, and our increasing freedom of movement, will blur this to be minimal.

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You would have to argue that the current morphology is a stable one. Evolution would have to reach a local maximum such that movement away from the current setup pushes us back towards where we are now.

This seems unlikely from our current viewpoint but is possible.

Note that this works best if science is also somewhat similar to what we have now. Or it would be hard to explain why we didn't adapt to the changing circumstances. The technology level doesn't have to be identical, but it needs to support the current morphology.

Look for places where we seem to be evolving and try to come up with technological or societal changes that would push us back. For example, our increasingly sedentary lifestyle would seem to lead to changes. So come up with a reason for us to be less sedentary. Perhaps environmentalism leads to less machinery and more human-powered activity. Or maybe it's a health thing.

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Evolution working the way it does, there is no remotely believable way that it just went on holiday for a few million years. Humans have changed dramatically over the past one million years. Even if you assume that evolution slows down due to less selection pressure thanks to technology, the future humans would definitely not be identical to present-day humans.

If nature doesn't give you an answer, technology must. The only plausible way to create a total stop to evolution if it was intentionally engineered. One of the first high-tech civilizations in your history developed genetic engineering, and for whatever reason encoded an evolution stop into the human DNA in a widespread manner. Why they did and how it came that all non-arrested-development DNA died off is lost to history (or maybe not, if you want to make it a plot point).

This could also be a side-effect of immortality research. Improving the regeneration capabilities of DNA could cause mutations to not happen anymore, slowing evolution down to the possibilities already present in the DNA (i.e. recombinations of existing traits, but no changes in traits anymore).

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Natural evolution is driven by environment and food supply. Creatures evolve to use a food supply that other creatures aren't using, or live in an area that other creatures don't inhabit due to environment variations.

Looking at a few 'living fossils', creatures that don't change, the vast majority are aquatic: gar, shark, lamprey, chambered nautilus, coelacanth, etc... where the environment tends to be more stable due to the insulating effect of liquid water.

Humans are stabilizing both environment and food supply, certainly in the economically advanced nations, so that may be removed as a cause of evolutionary change.

However, humans seek to improve themselves by evolution in their choice of a mate. We are innately attracted to a potential mate by qualities that would improve the species: taller, stronger, smarter, etc... ask yourself: why do you find that person attractive? It isn't the result of a logical thought process, you just like them. That's a desire that is preprogrammed into most humans. And it shows... the height of the average human in the 1500's was around five feet. It's getting closer to six feet today... in only 600 years, we've added about eight inches to our height.

So, in order for homo sapiens to become a static species, they would have to advance to the point where they had full control over environmental factors, yet have their ambition, curiosity, and even choice of a mate restrained, so they didn't seek to improve themselves. That would run contrary to how almost all life on earth lives, from the lowest to the highest forms. Almost all life makes an active effort to improve itself.

Could make for an interesting story, about how some humans try to stop evolutionary processes to make the species static, and end up being defeated in that effort by a bunch of teenagers who just want to get cozy with that hot guy or gal.

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