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A relatively common idea in Sci-Fi and especially speculative evolution is the idea of humans or some other intelligent species ending up losing their sapience(often done for the sake of horror).

While the most common explanation(forced genetic modification) makes sense, I've always been skeptical of the idea of it happening naturally giving how much of an edge it's given us, especially in regards to the idea of it being from some massive cataclysm.

So what are some potential explanations for why a sapient species would end up losing its intelligence?

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    $\begingroup$ "giving how much of an edge its given us" Toss you in the middle of humanity's native environment of the African wilderness and see how well you do. If you do the same with most other creatures they will survive just fine. It's a form of survivorship bias. $\endgroup$
    – DKNguyen
    Commented Jun 8, 2023 at 21:38
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    $\begingroup$ Idiocracy and The Marching Morons give some clues. $\endgroup$
    – RonJohn
    Commented Jun 9, 2023 at 7:13
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    $\begingroup$ Kit kot will do it. $\endgroup$ Commented Jun 9, 2023 at 11:33
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    $\begingroup$ Cable "news"..? $\endgroup$ Commented Jun 9, 2023 at 13:55
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    $\begingroup$ For scale, it's estimated that Kasparov burned about 3000 Calories/day playing chess. He apparently had to drop out of a few tournaments because he wasn't able to keep his weight up. So that's 1000 Calories/day over the average man's base rate, just from brain usage. $\endgroup$
    – Perkins
    Commented Jun 9, 2023 at 22:05

16 Answers 16

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Losing Our Natural Intelligence: Idiocracy

The movie Idiocracy so famously answered this question, that the movie's Title has become a bit of a trope. Idiocracy predicts 3 evolutionary pressures that work in tandem to reduce intelligence in an advanced society.

1. Technology replaces the need for intelligence.

As technology becomes more advanced and automated, the intelligence required to survive is no longer important. Being smart enough to find ways to irrigate your fields with water gives you a much better chance of survival in a neolithic society than the guy who does not. But, in the modern world, you can be pretty much anywhere between mentally disabled and a genius and still only have a negligible chance of actually starving to death. So stupidity is not selected against like it used to be.

"One of the most consistently replicated findings in the social sciences has been the negative relationship of socioeconomic status (SES) with mental illness". This means that in places where socioeconomic status impacts your odds of survival, that it can be used as a good predictor of if you are likely to pass on psychological deficiencies.

So, in countries with less access to technology, where starvation is still a major source of natural selection, being mentally deficient increases ones odds of starving to death by a greater margin than places that have the technology and resources to care for thier poor. For example, when you compare the food security rate of Iran with the United States, you see that the effect of being poor increases your odds of going hungry by very similar ratios, but in the United States, the food insecurity rate is only 12.5% whereas in Iran, it is 37.8%. This means that your socio-economic status is about 3 times as likely to cause you to starve in Iran as in the United States.

So even if people with mental disabilities in poorer countries are being born at the same rate as mentally healthy people, they die off more quickly removing them from the gene pool encouraging the evolution of greater intelligence. This factor alone does not encourage people to evolve to be dumber, but it does take away the evolutionary presure that causes us to evolve to be smarter.

2. The intelligent are often targeted in politics.

Anti-intellectualism is a common part of modern politics for a number of reasons. People in power often target the intelligent middle class because smart, well educated people are much harder to control, so they use the media to push ideas that smart people are dangerous or otherwise socially contemptable. Different kinds of 20th century governments attacked thier own intellectuals in different ways. In Communist Russia, the "Great Purge" included the slaughter of writers, artists, and anyone else perceived as having the mental faculties to be a threat to social equality. In the Fascist states of Central Europe, philosophies arose that condemned any knowledge that was not "practical" making it easy for leaders to suppress any line of thinking that moved against the state, and the people doing that thinking along with it. Then you have the Populist movements more common in the far Western nations that emphasized a uniformity among all people which lead to the out-casting of the overly smart.

While anti-intellectualism does not in every case lead to the intelligent members of society being killed, it does make them less sexually fit. Up until very recently, being labeled a "Nerd" was a good way to loose a lot of your reproductive rights; so, smart people had far fewer children. While many of the Anti-intellectualism trends of the 20th century have been reversed in Western Civilization, in many parts of Asia, being too smart is still a good way to have society turn against you. If these trends ebb and flow over enough generations, you will see a strong selection towards lesser intelligence.

3. Intelligence itself selects against reproduction.

In the past, smart people had a lot of children because it was the duty of a child to support thier parents in old age. You did not plan for retirement by investing in 401ks and Social Security, you planned for retirement by having as many children as you could afford to support when you were young so that when you get too old to support yourself, they would do it for you. However, now that there is no longer a practical benefit to have kids, smart people are choosing to have fewer children than they did before. It used to be common for people of lower socio-economic status to have 3-5 children and people of higher status to have 6-9 children, but now that there is no security in having children, we've seen the lower echelons of society continue to average about 3-5 children, but in many particularly educated parts of the world, the wealthy are averaging less than 2. This means that the drop off in reproductive rates among socialized nations is not uniform, but that it is specifically causing a drop off in the reproductive rates of its smarter members.

Since smart people are generally not meeting a the basic rate of replacement, then even if they are intelligent enough to survive to reproductive age 100% of the time, their population is dwindling and being replaced by people with people less intelligent than themselves.

Losing Our Nurtured Intelligence: Dark Age Collapse

Intelligence can best be described as a combination of Nature and Nurture. For example, IQ tests performed on in the Sub-Saharan region where there is minimal access to education and technology show an average IQ of 68-71. By civilized standards this makes the average Sub-Saharan person qualify as legally retarded. Thier brains are not smaller, they are just as genetically capable of intelligence as civilized humans, they simply fail to develop that intelligence due to lack of exposure. Even though they are just as good at solving the problems that they face on a daily basis as we are, they face much fewer complex problems that require logic, math, literacy, and/or science to overcome.

I've always been skeptical of the idea of it happening naturally giving how much of an edge it's given us, especially in regards to the idea of it being from some massive cataclysm.

Going from a state of High civilization to Low civilization has certainly happened several times in human history as the result of massive cataclysms, and has had profound, measurable impacts on how innovative the peoples of those regions were. The Bronze Age collapse for example was a time period between 1200-1150BC where a series of wars and famines destroyed most major cities in the Eastern Mediterranean region causing population declines in some areas as high as 90%. The sophistication of art, writing, and technology that was common of the late bronze age largely disappeared from this region for 100s of years as these regions reverted to a more-or-less stone-age civilizations before new technology, socio-economic structures, and classical philosophy took root. Something similar happened after the fall of Rome too. Rome brought a level of technology similar to the Early Industrial Revolution to most of Europe. They had indoor plumbing, water powered factories, complex logistics networks, a formal school system, etc. but after Rome was destroyed in the Goth War of 535-562AD (No not 476AD) most of that disappeared and triggered the Early Medieval Dark Age.

These cataclysms cause a lose in general intelligence because they destroyed the underlying framework that educates, encourages, and enables the development of intelligence. But, the effects of a cataclysms is only temporary. Within a few hundred years, people tend to rebuild those missing frameworks, and it is usually followed by an intellectual golden age like the Greek Classical Era or the European Renaissance.

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    $\begingroup$ As sad as it is, this is actually a very good answer. And not only is it good, it is pretty much proving itself more and more daily. Look at everything from our politics, entitlement programs, and average education level of the generations replacing us because the world demands no better anymore. The rise of torrents of information sources, most, largely inaccurate, at a the touch of an apathetic finger... map.barbarabush.org/overview/#intro so it is not as much "how can it happen?" as much as a model for "how is it happening?" $\endgroup$
    – Sabre
    Commented Jun 9, 2023 at 16:06
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    $\begingroup$ Surprised I had to scroll down this far to find this answer. $\endgroup$
    – Evorlor
    Commented Jun 9, 2023 at 17:01
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    $\begingroup$ Obligatory xkcd xkcd.com/603 - nice theory except everything it says is wrong, false, the opposite of true. $\endgroup$
    – James K
    Commented Jun 10, 2023 at 18:52
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    $\begingroup$ That was the worst XKCD I've ever seen. Just an ad hominem rant that didn't even attempt to confront the fact that the more education people have, the less children they have, and the recent reversal of the Flynn Effect. $\endgroup$
    – user86462
    Commented Jun 12, 2023 at 9:54
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    $\begingroup$ @Nosajimiki I'm genuinely curious, what reproductive trends? If it is just the increase in mental illness diagnoses, then that is almost certainly correlation caused by improved diagnostic practices and social acceptance. Just a few decades ago, not talking about the "retarded" kid was the social norm. $\endgroup$ Commented Jun 12, 2023 at 18:05
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Environmental Pressure

As other answers have stated, the environment dictates evolution. Take, as an example, H. G. Wells' The Time Machine. For millennia, the Eloi have had their needs met without any effort on their part. After a certain point, the "edge" intelligence gives a species that never has to work for survival will likely become vestigial and die off.

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    $\begingroup$ Endless peace, comfort, and welfare. $\endgroup$
    – user86462
    Commented Jun 8, 2023 at 23:28
  • $\begingroup$ @AncientGiantPottedPlant "Endless peace" - it may very much depend on technological level. In modern society, people are frequently drafted into war regardless of intelligence (except arguably for people who lie their way out of the draft, but whether that's a beneficial trait for humanity is highly doubtful), and war happens at far too big of a scale for intelligence to be hugely beneficial to an individual soldier. And across societies, any society that gets the upper hand at one point can keep other societies from advancing through force. It might work for primitive settlements though. $\endgroup$
    – NotThatGuy
    Commented Jun 9, 2023 at 8:37
  • $\begingroup$ @AncientGiantPottedPlant "Welfare" - assuming everyone is given equal means and opportunity to succeed, regardless of where they're born (most definitely not the case in modern society). "Comfort" - discomfort doesn't stop people from having children, unless you push them to depression or suicide, but that's... In any case, much of modern "comfort" factors affect people across entire genders, races, etc., that has little to no correlation with intelligence. You'd need to implement these measures in a very specific way, in a very specific society, to negatively affect intelligence selection. $\endgroup$
    – NotThatGuy
    Commented Jun 9, 2023 at 8:38
  • $\begingroup$ @NotThatGuy Whoosh, have you read The Time Traveller? $\endgroup$
    – user86462
    Commented Jun 9, 2023 at 13:36
  • $\begingroup$ Evolutionarily speaking, it's more about benefit to the individual than to the species. If people didn't need intelligence for survival, there would still be an evolutionary benefit if being intelligent helped you to outwit rival humans and breed more. (Though it's also possible for more intelligence to lead to fewer children...) $\endgroup$ Commented Jun 9, 2023 at 15:53
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Intelligence is costly: it takes time and resources to build such brain, and the environment has to be above certain standards, otherwise the result can be intelligent but emotionally dysfunctional mind.

The social environment can also impose restrictions: eg. inquisitive and questioning mind -- typically associated with higher intelligence -- can be classified as an unwanted trait. Intelligent creature can be seen as a threat to the status quo. Powers that be might not like to be challenged.

The cons are always weighted against the pros. It does not always pay off to be intelligent. If the cons are too prevalent, viability of the whole population can be enhanced if everyone is less intelligent.

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    $\begingroup$ "otherwise the result can be intelligent but emotionally dysfunctional mind" - Damn, I didn't come here to see people talking about me. ;-( $\endgroup$
    – Red Banana
    Commented Jun 11, 2023 at 22:29
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    $\begingroup$ @RedBanana Don't worry, emotional dysfunction can be healed, stupidity cannot. $\endgroup$ Commented Jun 11, 2023 at 22:32
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Two environmental factors, perhaps working in tandem:

  1. Brains are expensive in themselves, costing energy, taking time to build, etc. If they are excessive, so that a stupider being lives better and has more children with that energy and time, the being will evolve stupider.
  2. Brains may lead beings to do stupid things out of curiosity. In a hazardous environment, this may be a danger in itself. A stupid, incurious being will live to have children.
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    $\begingroup$ There was an example of the tides going out in...Sri Lanka I think? I forget where but elephants lived there. When the tides went out people headed to the beach out of curiosity but animals like elephants moved away. The tsunami ended up killing many more people than animals like elephants as a result. It wasn't known why the elephants moved away but it was theorized they were simply moving away from the source of infrasound. However other animals also moved away from the incoming tsunami. Just people did not because they were curious about an unusual event. $\endgroup$
    – DKNguyen
    Commented Jun 9, 2023 at 2:25
  • $\begingroup$ @DKNguyen I had heard a similar story of "ancient wisdom". A native tribe knew it as a warning signal, and so moved inland. So, intelligence for the win, since the dummies drowned. $\endgroup$
    – user458
    Commented Aug 2, 2023 at 16:02
  • $\begingroup$ brains are expensive but they vastly improve your ability to find calories, from fish weirs, to cooking roots, to building snares. $\endgroup$
    – John
    Commented Aug 3, 2023 at 23:52
  • $\begingroup$ @John Some species increase brain power, some species decrease it. Clearly evolution can go both ways. $\endgroup$
    – Mary
    Commented Aug 4, 2023 at 0:02
  • $\begingroup$ Sure but not for the reason you give, more intelligent does not equal better at getting food, but human level intelligence does, humans are way better at finding food than other primates, its hard to beat the beneficial effects of cooking and tools. cooking alone doubles your calorie intake. $\endgroup$
    – John
    Commented Aug 4, 2023 at 0:10
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Evolution takes orders of magnitude longer to kick in than how far movies are willing to look forward, therefore they need to have a bigger impact events than what evolution may provide. That being said, within hundreds of thousands of years any of the below could happen.

Constant factors impairing brain

There are multiple factors that could cause the big brains of ours (I use humans as an example but should apply to most carbon-based sapience) to be using up too much energy for the gains they provide:

  1. Low oxygen. In a low oxygen environment the brain is one of the first organs to catastrophically fail. The humans surviving lower oxygen % would be the ones who have lower oxygen needs by having lower weight, brain activity etc.
  2. Low air pressure. Low air pressure reduces humans' ability to absorb oxygen. This ties back to 1.
  3. Constant exposure to psychoactive substances (e.g. alcohol). If the brain is impaired it does not provide that much of an evolutionary advantage.

Factors impacting birth

Anywhere where a narrower hip is an advantage above brains the hips may get narrower leading to higher birth mortality of the bigger headed infants.

Factors impacting lifespan

Currently humans take about 12 years to be fertile and they take much of this time to develop mental capabilities (complex language, mathematics, drawing etc.) which is a fair tradeoff for a 50+ year expected lifespan. If external factors (famine, lack of water, pandemics, radiation etc.) were to reduce the expected lifetime drastically then there would be less time to be dedicated to education. This in turn would probably lead to being smart being less of an advantage.

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A lot of what you consider "sapience" is really just nurture, not nature. If you look at primitive tribes, they are quite a bit different from what you likely take as a "normal" human, but even they have substantial culture. You should really look at wild children to see what the "physiological foundation of sapience" looks like.

The basis for development of intelligence, culture and society is steeped in mystery. You can't have a "hard science" answer to it. However, we can speculate that human brain capacity is just barely over the threshold for what's needed to be civilized. Clearly, even a little capacity for culture snowballs quickly and amplifies what faculties the brain has. Therefore, it's unlikely that truly advanced sapience would have evolved in humans. As soon as basic sapience came about, culture would begin to dominate fitness, and the evolutionary pressure on innate sapience would disappear.

If you buy this line, you can conceptualize the sapience of modern man as an extremely fragile crystal vase (=the brain being just barely smart enough) cushioned by a thousand mattresses (=millenia of cultural, societal and civilizational heritage). While the mattresses are there, the vase will secure and nothing could explain people suddenly regressing, it would be incredible. But take away the mattresses, and you're a hair away from disaster. That means, enduring global societal collapse - not hard to imagine, early civilization prospered because of freely available resources (coal and metals literally just lying around in fields). Those resources are now exhausted and require significant expertise and investment to extract, retracing the steps of our ancestors would be much harder now.

The assumption that sapience gives you an edge is not that robust. After all, if it was so powerful how come none of the other animals bothered? Humans did it in a few million years, and yet dinosaurs just never did after 100s of millions? It seems more likely that sapience is almost always not worth it selectively, but humans arose from a rare coincidence: Intelligence happened to deviate into the less fit sapience just as the environment was a perfect storm to kick off and nurture culture. The implication is that when you repeat the "birth of sapience" experiment, most of the time the environment will not be a perfect storm, and highly intelligent species will occasionally see the sapience mutation arise but it will never "take". In fact, these repeated experiments have already been done: Parrots, dolphins, dogs, cats, pigs, squids, octopi...

In sum:

  • Sapience will not devolve while culture continues, because culture itself is a strong selective pressure maintaining sapience
  • If culture is erased by catastrophe, sapience is left with only a weak positive selective pressure, or even neutral or negative
  • The environment of early man might have been uniquely suited to promote sapience and culture, natural environments at other times are not necessarily the same (obvious reasons are resources which are now depleted, but there may be other subtle ones)
  • Without culture, sapience can easily devolve
  • In absence of culture, if non-sapience is beneficial, it can become fixed over time to reduce the risk of sapience arising in the future
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A metaphysically immense screwup

As you (and various answers) have mentioned, the classic answer is some sort of genetic shift that causes congenital brain damage (or, at least, lack of intelligence), a la Planet of the Apes. However, I don't fully buy this. As a former assisted living aide, I've spent much of my life around brain-damaged and/or mentally retarded (in the technical sense of the term) people over the years, and I've never seen either produce anything approaching non-sapience. You're either still sapient but impaired, or it makes you a vegetable; there's no middle ground. Perhaps there is some condition capable of doing this, but if so I've never heard of it. Instead, it seems to me that there must be something else which imparts sapience, some sort of soul, "image of God", or imago Dei as the theologians call it.

Now, traditionally, this is something imparted to humans (and, in a sci-fi context, other sapient species) by God/gods/the Flying Spaghetti Monster. Anything with it will be sapient, no matter how stupid; anything without it will be non-sapient, no matter how smart. With this in mind, the answer to your question is obvious: somebody goofed (to use an immensely technical term /s), and in so doing disrupted, destroyed, dispelled, or otherwise *-ed their species' imago Dei.

Perhaps the species experimented just a little too much with transhumanist bioengineering, creating things so different from the species' base form that they could no longer be considered the same species. Perhaps they committed such heinous crimes against pasta-kind that the Flying Spaghetti Monster revoked their sapience privileges. Who knows; perhaps an eldritch entity from the 42nd dimension sneezed in their general direction. The point is, they had souls, and now they don't.

This route can lead in a lot of very interesting directions. For example, in John Scalzi's Old Man's War series there's a species called the Obin.

Now, whereas pretty much every other species in the known universe uses something along the lines of "the people" as their name, the Obin are different. A literal (and quite accurate) translation of their name is "the lacking". This is because, out of all the species they know of, they are the only ones that aren't sapient. While they think, create technology, work technology, and do other intelligent stuff, they have absolutely no sense of self. They're all just really smart animals.

It turns out, this is because they are a herd species uplifted by another species, the Consu, which are well on the "sufficiently advanced" side of Clarke's Third Law. More importantly, the Consu think that the imago Dei is a curse, so they explicitly chose not to import soul when uplifting them.

The Obin really don't like being soulless, so they go to great lengths to try to acquire them.

Edit: I realize that most people probably wouldn't view this as science-based (although many theologians would beg to differ). I'm just putting this out there as food for thought.

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    $\begingroup$ Your argument isn't very credible either. You might as well claim snakes don't make sense because you've worked with quadruple amputees and never seen them start slithering around. Sudden catastrophic injury is not the same as gradual change over many generations. $\endgroup$ Commented Jun 9, 2023 at 9:32
  • $\begingroup$ Isn't what you describe with the Obin more like "sapient, but not sentient", rather than "sentient, but not sapient"? Apparently, "sapient" usually means exactly "think, create technology, work technology, and do other intelligent stuff" (having a complex language being one noticeable part of the "other intelligent stuff") - in other words, the things that humans do that other animals can't, or that humans do orders of magnitude better than other animals. $\endgroup$
    – A. B.
    Commented Jun 9, 2023 at 18:50
  • $\begingroup$ @A.B. Good point. I'm always getting the two mixed up. I'll go through and edit my answer to fix this when I have the time. $\endgroup$ Commented Jun 9, 2023 at 22:50
  • $\begingroup$ I don't want to denigrate your experience with the mentally damaged, but I think people's quick willingness to believe their dog loves them indicates that people, you included, are predisposed to believe the person in front of them has sensible thoughts. In other words, I don't think we can trust your perception, or anyone else's. $\endgroup$
    – user458
    Commented Aug 2, 2023 at 16:13
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They develop artificial intelligence and robotics that completely frees them from any need to support themselves, so sapience becomes unnecessary.

Then genetic drift slowly degrades their intelligence whilst the AI figures our smarter people are less happy and subtly encourages the population away from intellectual pursuits and towards physical prowess, boosting the mating potential of the stupid.

Enough time passes and you have a stupid species permanently maintained by machines it has no hope of comprehending.

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  • $\begingroup$ I would also add to this: As society becomes more complex and mature, thinking becomes a liability to all but the elites (as in 1984). What was once advantageous is now selected against, perhaps outside of a small clique of elites who do not interbreed with commoners. In your case, the AI takes the role of the elites. $\endgroup$ Commented Jun 9, 2023 at 9:53
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Environmental conditions that favour brawn over brain in addition to social and cultural regression (e.g children not being fully/correctly taught the language of their parents) could produce an evolutionary pathway that results in reduced intelligence. This answer is purely speculative though, I do not have the knowledge to make confident statements on the matter.

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Two options that stem from the same idea.

A very infectious bacterial or viral airborne parasite that infects the population which doesn't kill but instead attacks the frontal lobe reducing the average IQ by 70 to 90%.

The second option is again an infectious pathogen but now it infects our reproductive system causing all newborns to have multiple mutations. These mutations are often fatal and or affect the brain reducing the population's IQ.

Achieving non-sapience wouldn't be instantaneous but it would be a byproduct of being affected by either of these conditions for multiple generations (10+). As each generation comes and goes the push to naturally select intelligent beings would be reduced, because in the first there is an external source outright limiting it and in the second there would be a greater push for any being that could actually survive to sexual maturity.

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Sapience Must Become Maladaptive...

Contrary to popular opinion, species don't just lose something because "they don't need it anymore." A species that is fed and happy isn't going to have much evolutionary pressure because, as a fed and happy species, they are successful. To lose sapience, it has to be maladaptive.

...But Not Too Maladaptive

It would be pretty easy to just say that there's a virus that hits sapience somehow (attacks that part of the brain?), but let's be honest, anything hitting a species that fast is probably going to wipe out sapience by wiping out the species completely.

So how can you slowly kill off sapience in a species?

Introduce a Predator

Make a predator that hunts your species. It'll eat any member of this species, but it has a special organ that "hears" sapient thought. To this predator, intelligent, self-aware members of your species might as well be screaming. Dumb members are whispering. And those without sapient thought? No "noise" at all. They hunt all members of your species, but the most sapient are easy prey.

This need not be a fast solution. Imagine that these predators might kill 5-10% of each generation.

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    – DKNguyen
    Commented Jun 12, 2023 at 22:23
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Welcome to a brave new world!

Huxley had explored this in his book, Brave New World. Workers - or Epsilons - were 'cloned' by making sure that the embryos would split, and to do so, the worker embryos were subjected to different nigh-deadly doses of stuff, among them radiation and alcohol, which also had the "desirable" side effect of making the workers dumb and docile. Or let me quote you how Epsilons are made, from chapter 1:

He pointed. On a very slowly moving band a rack-full of test-tubes was entering a large metal box, another, rack-full was emerging. Machinery faintly purred. It took eight minutes for the tubes to go through, he told them. Eight minutes of hard X-rays being about as much as an egg can stand. A few died; of the rest, the least susceptible divided into two; most put out four buds; some eight; all were returned to the incubators, where the buds began to develop; then, after two days, were suddenly chilled, chilled and checked. Two, four, eight, the buds in their turn budded; and having budded were dosed almost to death with alcohol; consequently burgeoned again and having budded–bud out of bud out of bud–were thereafter–further arrest being generally fatal–left to develop in peace. By which time the original egg was in a fair way to becoming anything from eight to ninety-six embryos– a prodigious improvement, you will agree, on nature. Identical twins–but not in piddling twos and threes as in the old viviparous days, when an egg would sometimes accidentally divide; actually by dozens, by scores at a time.

...

"But in Epsilons," said Mr. Foster very justly, "we don't need human intelligence."

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There is a great story by Larry Niven and Steven Barnes called "the Locusts"

The locust is a grasshopper that, when facing overpopulation, grows larger and more gregarious and nomadic. They begin traveling in swarms until the overpopulation issue is resolved, then revert to their smaller, more docile state.

Spoiler alert The story suggests that human intelligence shares a similar transient quality. Depopulate the earth by migration to other planets, and they revert to an earlier condition.

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Most of the answers thus far have focused on "intelligence" as being a spectrum or gauge, which is the most common definition. On that scale, we may consider sapience, as H. Beam Piper once wrote, as "a mental boiling point".

Most answers then consider how a species that has achieved sapience would end up reducing their average intelligence, but remaining sapient, staying above that boiling point identifiable by traits like a means of symbolic communication (speech/writing). While those answers are good at answering that reading of your question, there's another way to interpret that question, by considering "intelligence" as "the quantity of mental capability required for sentient thought" and thus a rough synonym of "sapience". Read that way, the question is, how can a species that has developed a sentient mental state lose that ability to think at a sentient level?

We have not, in the real world, ever seen this happen, at least not to our knowledge, because to our knowledge we are the only sapient species on this planet, and there have never been more than a handful of distinguishable (but at some level interrelatable) sapient primate species. However, our knowledge of the taxonomy of life on Earth is woefully incomplete; less than 1% of all species to have ever lived on the Earth are extant and thus available for behavioral study, and the fossil record is a notoriously poor source of clues as to a species' mentation (and that's of the species that leave a good fossil record).

So it's absolutely possible that some sauroid species in what's now the Yucatan Peninsula achieved some recognizable form of sapient thought, less than a million years before the nearby Chicxulub Impact wiped them off the map completely, fossil record and all. If the species that ascended to sapience were located further away from the impact site, it might have survived longer and/or their remains might be better preserved, so you'd think we'd have found a couple good fossil skeletal examples. Again, we think we know of less than 1% of all species ever to have lived on this planet, so the unknown space in which such a creature could have existed is fairly large.

Now, that leads us to a pretty good possibility. Sapience tends to require fairly high macronutrient intake for a given body size; that brain needs a lot of blood glucose to function properly. Mass extinction events like the Chicxulub Impact cause a pretty thorough collapse of the food web, and the species with the highest caloric needs often come out on the losing end pretty quickly. Climate change, asteroid impact, extinction of a key species (honeybees are a good candidate) are all really good ways for h. sapiens to find ourselves at an evolutionary dead-end.

Now, that's kind of cheating, because losing our ability to think because we're dead probably wasn't what you had in mind. However, if you consider our genetic relatives in the primate family as kindred enough, much as we consider modern-day reptiles and birds to be the closest descendants of the dinosaurs (however actually distant), it may well be that some marmoset in the tropical latitudes, on a land mass big enough to stay above the oceans' surface, may end up being the primate family's scion, that some future sapient octopus-derived species will describe as "the closest living relative to these far larger and more intelligent bipedal primates, whose fossil record is endemic on every land mass".

There is an important and closely-related point to make here; The definition of homo sapiens (and, we think, any other life form we identify as sapient) is defined as a species in large part based on that trait. If the human genetic line were, by any chain of events, to "devolve" to a sub-sapient mental state, in whole or part, any remaining sapient observer may well no longer consider those examples to be homo sapiens and therefore no longer "human". So there's somewhat of a tautology here; we're human because we're sentient, and if our genetic descendants lose that trait, they'd no longer be human.


Anyway, bringing this to an actual answer, it's kind of hard to envision an Earth populated with a genetic descendant of modern humans are no longer sentient, where that didn't happen due to an initial cataclysm that, ironically, makes life easier than before for those who survive the event. The cataclysm would have to be minimally damaging to the non-human ecosystem (otherwise the food web collapses, which definitely makes life harder and thus forces the remaining humans to have to think more), so asteroid impact is right out, and nuclear war is iffy, but possible if the theories that nuclear winter wouldn't happen are true. Pure climate change is going to make life harder for most as well.

The only such cataclysm I can think of would be pandemic, something that makes COVID look like hay fever. Contagion's fictional MEV-1, but even more contagious and deadlier (IIRC, MEV-1 had a mortality rate of 20-30% with an R0 of 4; something closer to 90% mortality with the same contagion rate would about do it), would literally decimate the world population, but for less advanced civilizations, isolated from the developed world and its deadly disease, life would actually get better in the next few hundred years after the virus had burned through the world's population. The severe cut in CO2 production and the dramatic release of pressure on game, fish and forest would, within just a few decades, produce an abundance of living biomass helping to sequester atmospheric CO2, not only reversing climate change but making the caloric needs of your tribe that much easier. If you don't have to think as much about where your next meal comes from, or coordinate as much with fellow humans to get it, you could, conceivably and eventually, arrive at a new steady-state where humans don't have to use their impressive brainpower anymore, and over many generations those abilities could atrophy, seeing the species regress to ice-age or earlier intelligence levels.

The biggest counterexample refuting such a theory is the Sentinelese people. The inhabitants of North Sentinel Island in the Indian Ocean are fiercely territorial, attacking any outsider who so much as sets foot on the island, such that there is no record of a successful, friendly communication between the Sentinelese and anyone else. There really is no talking to some people. In near-absolute isolation from the rest of the world, the Sentinelese are basically frozen in a neolithic state of development, one of very few known societies to be completely unpolluted by the last, oh, maybe 8,000 years or so of human mental, societal and technological progress. Yet despite all this, the Sentinelese are definitely sentient based on everything we have been able to observe at a distance. They have a spoken language, if not a written one, they know of fire and how to produce it, and they make force-altering machines, such as knives and bows and arrows.

While there are some glimmers of these traits among more intelligent animal species, there's no true comparison to be had except to ourselves; these are human beings, however primitive, and they're sapient. Thousands of years of human history passing them by has not changed that, and it's very unlikely thousands more will see much difference. They know what they need to know to survive, at least within their own isolated environment, and whatever folklore they pass down that keeps them so violently wary of outsiders has served them well, as most of the rest of the Andamanese people that populate the Andaman and Nicobar Islands have fared considerably worse against encroachment by non-indigenous cultures (technically part of the Indonesian archipelago, the Andaman and Nicobar Islands are an Indian possession and also close to Myanmar and Thailand).

So at the end of the day, having no more complicated a possession than a bow and arrow doesn't mean you have no inherent capability for sentient thought; quite the contrary, this level of technology is well inside our definition of sapience, and this knocks a big breach in the idea that the human race could ever "lose" our sapience. Merely not progressing isn't regressing.

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  • $\begingroup$ Just FYI birds are not the closest separate group to the dinosaurs, they ARE dinosaurs, the direct descendants of one group of dinosaurs in particular. Birds are dinosaurs in the same way you are a great ape. $\endgroup$
    – John
    Commented Aug 4, 2023 at 0:24
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you rule our forced genetic tampering what about, accidental genetic tampering?

  1. Colony worlds will be especially prone to this, since the starting population is small. They decide to tamper with the colonist to make them better adapted to the local planet but oops it turns out the tampering has no adverse effect on adults but made all your kids stupid.

  2. Then you have voluntary tampering, designer babies people making babies who are more attractive, no few are likely to intentionally make their kids dumber but what if it is not a direct effect, the effect skips the first generation or Gene X is determined to be responsible for brain cancer type Y so people get it "fixed" in their kids, but it also turns out to be necessary for normal brain development. Or more realistically gene babies type A breed with gene babies type B and the two alteration don't play well together in the brain. You would need to combine this with something that kills off a lot of the population otherwise it will get fixed, maybe a plague that gene babies are immune too.

  3. Another option is war, country X creates a gene altering virus to make all (choose your minority here) stupid or their kids stupid and low a behold people willing to commit atrocities don't have the best minds or safety protocols and it affects everyone. Or they make it to kill and instead it just shuts down the tiny number of genes needed to make us smarter than chimps.

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Peak of Evolutionary Intelligence has surpassed and reboot via Devolution

Intelligence is an evolutionary trait that may take millions of years and several thousands of generations to develop. Your species is sapient and intelligent which makes me think that they are very likely to hit the peak of the collective species intelligence very soon. It's almost impossible that the species would continue to evolve eternally. If they are careful enough not to bring up their own doom, Nature may be likely to bring them the catastrophe.

Note that the biggest power a sapient and intelligent species has is its own sentience which if lost would render the species to perish. Non-sapient species might not be at that great disadvantage because as we develop more and more sentience, our ability to survive become more dependent on our sentience. For example, humans have created artificial means of curing diseases and prolonging life. If all our technology were to vanish suddenly, we wouldn't quickly return to way we used to survive some hundred years ago or so. There will be required something much more tremendous for that. So once the peak of their evolutionary intelligence is surpassed, they would start to decline their intelligence and probably sentience too. This would not happen overnight. It might take several thousand years. But it would be inevitable since it is largely natural.

If you want to take it further, you may say that after a continual decline of intelligence, there would be a reboot and a new cycle could start such that another sapient and intelligent species would arise, would reach a peak and again declining. This all could be natural and inevitable regardless of their efforts to undo it. The peaks of the successive cycles could be more or less drastic.

It is kind of similar to the fall of the Galactic Empire in Foundation trilogy by Isaac Asimov in which the Empire falls apart no matter how much the Emperor resists.

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  • $\begingroup$ collective species intelligence is not something that undergoes evolutionary selection, group selection is always overridden by gene line selection. $\endgroup$
    – John
    Commented Aug 4, 2023 at 0:27

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