What Natural Forces could Make Intelligence/Technology a Disadvantage or at Least Inferior to Something Else?

Question

Premise

I'm creating a pre-history world to attempt to demonstrate an alternative sequence of events. I would like natural forces to somehow make natural selection view intelligence/technology as inferior. At current, it all seems too predictable; the most technologically advanced tribe always wins.

If the earth gets hot, burrow underground. If the earth gets cold, weave some fur. If you can't fend off scavengers, drag the kill up a tree. Intelligence has an answer for almost everything and it gets to the point where it is almost boring. It seems the only thing that can challenge intelligence is being more intelligent. However, I would like to challenge this notion.

Time after time in natural history, the primitive species yield to the more cognitively developed competitor species. Additionally, the few primitive species that remain in strong numbers today tend to be rather smart for their genus. There are a few exceptions to this generalization, but let me list some examples to land my point:

• Thylacosmilus (marsupial sabertooth, late Pliocene) presumed to have gone extinct from conflict with sabertooths (who had more developed brains)
• Neanderthals, though their brains were bigger in terms of sheer volume, their technology was thought to be inferior and went extinct.
• One of the oldest species on earth, crocodiles, have primitive reptilian brains, but in general terms are some of the smartest lizards on the planet.

In my pre-history world, I would like to see what it would take to undo this trend. It seems to be a bit of a tall order, at times I can't think of anything other than smiting Prometheus down with a strike of lightning before he gets a chance to share his discovery. Hopefully the community here will be able to help in this regard...

Question: What could nature throw into the mix to make intelligence or technology an evolutionary disadvantage? Such that brute strength, brute endurance, brute whatever would be more effective than thinking through natures challenges.

Further Clarification

• Era: About 2 million years ago (Pleistocene)
• Natural Forces: By this I mean geologic things like climate, weather and tectonics, but also species/ecosystem forces like competition, reproduction rate, ect
• Intelligence Threshold: Presumably a dominant species would have to be at least intelligent enough to perceive its surroundings. The species that results from your natural forces answer doesn't have to be brain-dead, but intelligence would not be its defining characteristic.
• Realistic Threshold: I want to keep it fairly realistic, but freak occurrences in nature to a moderate degree will be acceptable.

Answer 1

Some wonderful answers here that really got me pondering. I have a few ideas.

Evolution is a robust thing. Specific traits, though, are fragile. Just because intelligence is an advantage in many cases doesn't mean it's always worth it, as Amadeus expertly points out in his answer. There are also distinct risks that come from being smart. Playing on that idea, I think the key here is to arrange several forces in such a way that intelligence just isn't worth the price in comparison to something else. A few small tweaks or accidents at the wrong time in human evolution and smart monkeys would have become a dead end (as nearly happened to us at least a couple times in our various bottleneck events).

So, I think there are several ingredients that you can mix and match as necessary to get rid of those pesky humans.

• A competitor that's as smart as humans, but without the physical characteristics necessary to support technology. Imagine a jaguar that's as smart as a person. Would it make tools? Nope, sorry, no thumbs. But it'd probably be darn good at hunting humans and many other animals (see The Ghost and the Darkness). Early on, tools were largely ways to make up for a lack of physical strength; we don't have claws to take down a mammoth, but we do have spears. In those early stages, I find it easy to imagine that a smart predator with physical strength could outperform (or simply eat) humans. But really, the trick here is not that this creature would hunt humans to extinction (though maybe it could), but more that it increases the cost of getting food. If you can't gather berries without 10 people with you, you can't cover as much ground. If every morning of your hunting expedition you wake up with one fewer hunter, you decrease the likelihood of taking down that mammoth. If you can't follow the herds because leaving your easily defensible cave will mean your children get eaten, you might not have enough food that winter. Once the humans are dead, the jaguar can stay smart or not; it doesn't have the dexterity to develop technology.

• Climate. Make a world with climate fluctuations that are characterized by both sharp, large amplitudes and fluctuation periods just outside of human memory. Have a severe drought century immediately followed by a lush, rainy century. During the rainy century, human population sky rockets; then, one year, all the crops fail (whether gathered or farmed directly). War breaks out and the strongest/bravest are selected for. These people scrape by until the next rainy century when the population skyrockets again, just after the people who remember what happened last time are dead... rinse and repeat for tens/hundreds of thousands of years and the evolutionary pressure is likely skewed towards individual physical power over intelligence and numbers. (Something like this happened to the Anasazi in the Southwest US – check out Collapse by Jared Diamond for a fascinating analysis of that case and many other societal collapses which may be relevant to your question.)

• In social creatures intelligence often comes cultural sophistication, but that's not always a good thing. During the recent Ebola epidemic in Africa, one of the challenges people fighting the disease encountered was ritual mourning and burial in the communities most affected. Infected bodies were washed by hand, which spread the disease to mourners. If this had happened in a pre scientific society without knowledge of microorganisms, this kind of disease could be devastating to intelligent creatures and not to ones that don't have such cultural practices. Another example is the Viking Greenlanders: they were used to farming in a different climate, and their cultural norms kept them from adapting to their environment (to the point of their own starvation).

• Make certain technologies dangerous for whatever reason. There are many great examples of this, but for two currently relevant examples, see nukes and man-made climate change. Past ones include, for instance, clear cutting forests for farms. If you make that practice even more dangerous for whatever reason, people could be doing it all over and suddenly be screwed.

• Make the problem too hard for anything but several tech leaps. What if some problem requires either metal OR physical speed? Or the only food available needs to be either specially preserved OR eaten by the pack immediately (at risk of possible death)? What if vocalization is a disadvantage, but writing hasn't been invented? Lots of ways to exert evolutionary pressure here.

• Change the behavior of prey. There are two main things humans do better than any other animal: think, and run long distances. Predators since time immemorial have known that when you chase something, it runs away. For much of evolution, eating meant running faster than the prey. Humans, though, discovered that if they just kept on running, the prey would tire (especially old, weak prey). Then you could kill it. Some scientists think that, among other changes, this is actually helped cause us to become more intelligent. If, for some reason, prey didn't behave in this way, you might stop intelligence from developing in the first place. Off the top of my head, I can imagine that if male buffalo aggressively defending the pack when threatened rather than retreating, that could pressure human evolution towards strength and speed rather than cunning and long distances.

These are just some examples of how environmental factors and competition could disadvantage early humans in particular, and any intelligent animal in general. Just because something can be reasoned out doesn't mean it will be in time, and intelligent social behaviors have their own trade offs. Playing around with combinations of these things can probably get you what you need.

Answer 2

What could nature throw into the mix to make intelligence or technology an evolutionary disadvantage?

Starvation, and/or an easy life. Brains are extremely expensive; they consume 20% of your calories for about 2% of your weight. [1] [2]

They are no good if they consume calories and do not pull their weight (or ten times their weight!) in added opportunity, that is exactly what happens if there are very few threats. If calories are also very precious, wasting them on idle contemplation will be selected against.

Our brains are necessary to navigate our world, mostly of our own making, but of course we don't choose to be born into it, we arrive and have to cope, and those of us that cannot, due to brain deficits, typically have a very low reproductive rate. In short our big brains are the result of a feedback loop.

Other species don't have that: you don't see Elephants and Dolphins making 100 year plans, and chimpanzee / gorilla society is based largely on brute force. (Bonobo culture uses sex as a medium of exchange; also not high intelligence or planning).

We are the oddity: For 99.9999% of species, brawn IS more important than brains. To select against brains, make them not worth the biological expense.

Answer 3

Let's address making technology somehow not be an advantage.

Now, technology can be something as simple as a stick made sticky with saliva so it will pick up more bugs when you put it in the hole.

And you want a world in which not thinking of that is more advantageous than thinking of it.

Because that's a tool. And tools, no matter how primitive, as long as they are modified somehow to fit the purpose, are technology.

You're asking for a world in which any tool we can think of doesn't work as well as smashing the termite pile--and you want it to be true for every situation.

I'm stymied as to how to build a world in which all tools are useless. The only thing I can think of is that the rules of physics change every few minutes or something. That's all I got.

Now, as to intelligence being a disadvantage--the only thing I can think of is: make the cost outweigh the benefit. Amadeus spoke of caloric cost--but there are other costs you don't see.

For instance, take the jaw strength of a human vs. any ape or monkey relative to size.

We lack a muscle across the tops of our heads that has to do with jaw strength, but lack of that has allowed us to spend longer developing our brains.

If, for instance, survival was predicated on being able to crack things with our jaw or bite strength---then the cost for intelligence might be too high.

Except.

Except that any drawback that you can think of evolutionarily, a big brain can mostly defeat. You can't crack it with your jaw? Take it away and build a tool to do so--or watch how the birds drop the shelled animals they catch on the rocks until they break and do the same.

Basically, what I would look into is ALL the trade-offs for intelligence, and find a way to make at least a few of those untenable.

Intelligence is unusual (depending on how you define it, which is not really clear here). That means that specific conditions have to be in place to hit the sweet spot between having enough bounty to develop a brain and having enough evolutionary pressure to make using that brain worthwhile.

Answer 4

I can think of a fast-changing environment with quick mutations.

For example, the climate is getting cold. Less intelligent species are trying to adapt naturally. More intelligent species are wearing furs and maybe even fabrics. As time goes by, first species adapt better to a new, even harsher environment, while the second species progress can not compensate for the environmental changes and they eventually go extinct.

But I still think that this mechanism is, at the very least, debatable. Intelligence would eventually find a way to improve in a "permissible" way.

Answer 5

your asking for a scenario where intelligence is a disadvantage

maybe you have heard of a movie called idiocracy. If so, this answer steals the core idea behind it.

The idea is that intelligent members of a species reproduce less often than their less intelligent counterparts. So technology could be developed but never in a widespread fashion, because the smart one is not reproducing.

Most things have a few basic requirements to be invented. creativity (who knew a stick and sting hold tension!?), spacial awareness (a stick cannot go into this tunnel well?), and efficiency (why push a stick into the earth when I can screw it in?)

creativity is an element of intelligence, lets leave this alone. Spacial Awareness is a combination of eyesight and intelligence, a candidate. Efficiency is a physics issue, lets not mess with the fabric of the world.

Out of the above, Spacial awareness is probably our easiest affected. This is a good thing as eyesight is a complex thing that took a long time to develop, now we need to discourage it. Thankfully we have some preexisting modals, blindness. This typically happens in Nocturnal and Subterranean animals, the commonality is a lack of natural light, that gives a few obvious solutions.

solution 1 Dim your world a bit across the spectrum, this promotes a lack of food that will increase the cost of brain power. it will also delay technological advances. between the two, less intelligent animals should mate fast enough to offset intelligence growth through evolution. It will also lead to advances of other senses, like smell.

solution 2 make the surface nearly uninhabitable. either through solar radiation, harsh climates or something else. if the majority of your animals are evolving without light for sight, it will never happen and will cause the same impediment for evolution as solution 1.

Answer 6

Make the world hostile, unpredictable and hard to change

If it is hard to survive, evolution will likely steer towards r strategy - just produce a lot of offspring. Now if the danger is largely unpredictable and almost impossible to avoid (let's say frequent volcanic activity or other natural disasters, or lots of diseases), intelligence can only increase your likelihood of survival a tiny bit and will not be worth the energy.

Note that this is not much of a stretch from the current state of the world. While we as humans value intelligence, only a small fraction of the Earth's biomass consists of species that we could consider highly intelligent/technological. Yet all of the contemporary species represent an unbroken chain of 3 billion years of evolution. Basically all microbes live in a world that's (from their perspective) hostile and unpredictable and their survival depends on a few simple heuristics (e.g., following a concentration gradient of nutrients) and producing a lot of offspring. At the same time microbes can easily survive catastrophes that would wipeout all multicellular life.

On a slightly related note, during my studies of AI, I did some simple experiments on when a specific form of intelligence is worth the computational burden (increased time to think). When you make the world unpredictable, simple reflexes that are faster to compute lead to more efficient behavior.

Select for small size

This is kind of related to the previous point: bacteria are so succesful because they are small. And you simply cannot cram that much intelligence into a small space. A similar case is with earthworms or krill that are very succesful thanks (at least in part) to their small size. Even if they would benefit from increased intelligence, they are less likely to evolve it due to their size.

One way to select for small size is to have small isolated islands of nutrients. The organisms need to be small enough so that a self-sustaining population (~50) can survive on a single island with only rare mixing with others.

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In general, I think it is impossible to select against any intelligence (even bacteria and plants have certain amount of intelligence), but it is relatively easy to make anything beyond reflexes and sensory processing just not worth the cost.

Answer 7

A lot of interesting ideas are around in this thread.

I would like to throw something else into the ring: the evolution part.

Intelligent life is offspring of not quite as intelligent life. So it all started from "a group of equally stupid ancestors" (yes i am simplifying).

One feature of intelligence, sadly, is the ability to jump to conclusions and to come to false positives.

False positives are actually very handy when trying to identify a fairly well concealed large predator: while "there's a tiger! defend!" does not harm you when it's wrong, and the opposite ("i'm not sure this is a tiger, let's investigate further") turns you into a smart yet tasty meal, they can also be very problematic in other situations. There are many interesting experiments that show how jumping to conclusion and all those biases that result in favouring false positives can make someone a victim of all kinds of frauds, it should be conceivable to invent a scenario where a not very bright (only slightly brighter than their fellows) group of proto-people are being doomed by jumping to a conclusion.
The route might go into the direction of creatively misunderstanding some coincidences or correlations (something you often see in not too bright and not too well educated people), and as a result having an unhealthy tendency to end up in dangerous settings.

If you really wanted to you might even have some early overly eager priest having not only one virgin sacrificed for better hunting, but all the smart females (because they don't want to mate with him, or something), or you could have those semi-smarts migrating somewhere (for very good reasons), where hunting is better, and life generally is much more pleasant, but lacking an understanding of viable populations and thus dooming themselves in their new-found paradise to extinction or at least degeneration due to inbreeding, because no other humanoids migrated there.

Answer 8

I'm not sure if this is the answer you're looking for, but ask yourself why there was no "intelligent life" for hundreds of millions of years before we came along. There was no discernible trend toward intelligence. Really, any inevitability comes from hindsight.

Our "big advantage" is also a huge drawback. We are learning animals. Ever see a newborn horse? They can usually stand 30 minutes after birth. Birds fledge in a few weeks. Humans take years. There are a number of reasons for this but one is that humans (I think) have less hardwired knowledge and more learning ability. They have to learn practically everything.

This means that intelligence doesn't work very well unless the creature concerned is highly social. You've probably seen nature programs where the mother animal loses lots of weight while taking care of newborns. Imagine if that went on, not for days, weeks or a few months but for years. What animal can afford to make that sort of investment in its young? Most often, the winning strategy is to be self-sufficient as soon as possible.

I think the "intelligent human" strategy wouldn't work if food was scarce and widely scattered. We think early humans followed herds and used persistence hunting. Without a herd to prey on, maybe selection would favor smaller, less expensive brains and shorter periods of helpless infancy.

If there were a bunch of much stronger predators (like the dinosaurs who were conveniently wiped out by a comet) then maybe it would make more sense for humans to be scattered rather than tribal. Arboreal rather than savanna-based. Strong, quick monkeys rather than tool-using thinkers. Maybe.

Answer 9

Intelligence ultimately leads to self destruction.

There is a fine old SF anthology downstairs. One of the stories (I will find it and link it up tomorrow) features future humans exploring a hive of these aliens which are like social insects. They are extremely ancient. Various commensals and hive parasites are the degenerate descendants of intelligent species which had offered them competition over the millions of years.

When the hive perceives a threat from intelligence, it produces a temporary intelligent appendage. In the story the appendage explains why to the humans: intelligence is a formidable attribute but inevitably intelligence leads a species to self destruction. The real use for intelligence is not gathering prey or fending off the elements, but outcompeting conspecifics. An intelligence arms race inevitably ensues. Individuals and groups of individuals become too clever for their own good and either destroy themselves or destroy their capability for intelligence.

I suspect this resonated with the Cold War era audience for this late 1960s story.

Answer 10

There are two sources for knowledge: Instinct and learning.

Instinct is determined by genetics and so has the advantage that it's there from before the creature is even born. The disadvantage is that changing instincts takes many generations. Many of the ills of our modern society can be traced back to the fact that our social instincts still assume that we are members of a tribe of 30-50 individuals at a stone-age tech level, fighting with nearby tribes for resources.

Learning has the advantage that it doesn't require hundreds or thousands of generations to alter, but the disadvantage that the organism has to experience things, calculate the inputs and outputs, and store the knowledge for future use. Since the new knowledge is not hard-coded into the brain's circuitry, retrieving and using it takes longer than running on pre-programmed instincts.

So, to de-prioritize intelligence, fill the world with a constant barrage of threats such that either the creature's instincts are correct, or it dies before it has a chance to learn much. The threats need to be stable enough that the slow pace of genetic evolution can keep up with them though. Intelligence is the tool for adapting to a world that changes faster than your genome. If the world doesn't change faster than your genome, intelligence is a waste.

For purely natural forces, you'd be looking at something like a constant barrage of random lightning bolts at fairly high saturation, along with frequent earthquakes so that caves wouldn't be safe, and whirlwinds that will suck up anything hiding in the water and splatter it across the landscape. Creatures in this world will have to devote most of their processing power to sensing electrostatic buildup, listening for the infrasonics of a crust plate starting to slip, and feeling for the shift in the winds that presages a tornado. The advantage to natural forces is that they are likely to remain a constant for a long time, so adaptation speed will not have to be faster than evolution speed.

Another classic method would be monsters. Big predators in the air, land, and water, and even ones that burrow through the ground, so there's never any opportunity to stop and think. Get the threat level high enough that most creatures sleep half their brain at a time like dolphins do so that they can be ever-vigilant. The problem, of course, is that this leads to an evolutionary arms-race wherein whatever species can adapt to the tactics of the other fastest wins... Which makes intelligence and learning a winning strategy for any species that can get it. There are two scenarios I can think of that would prioritize lower intelligence and faster reflexes.

One: assume all photosynthesising life is dead. The sun went out, the herbivores ate all of the dying vegetation, and then the carnivores and scavengers ate all the herbivores, and now all that's left is a pile of carnivores desperately trying to hunt other carnivores without getting themselves killed as the world slowly spirals down into icy doom.

Two: This one I find to be more interesting. Some species has already won the arms race and developed rapid learning and high intelligence... This species is not your protagonist. They are small in number, large in tech and viciousness, and systematically wipe out any species that appears to be gaining enough intelligence to be a threat to them. They live in small colonies and 90% of the planet never sees anything but their aerial drones. But any species they notice using weapons or learning to control fire gets exterminated as thoroughly as they can manage, with extreme prejudice and absolutely no concern about collateral damage. ("Burn the forest off half a continent to get rid of those pesky apes? Sure! Why not? We're not living there anyway. Don't forget to bomb it hard enough to collapse any tunnels they may have dug, we wouldn't want to have to come back and do this again in a hundred years.")

Answer 11

Two words, "nerd sniping," all you need is something odd enough to make it interesting, better if it's not a clear cause of death. I'm told you can catch some kinds of monkey with a ball through a hole just bigger than it is. They can't get their hand out holding it and don't want to let go.

A secondary problem is brains. They require lots of food, careful temperature control, and are a huge cause of death during childbirth (either the mothers or the childs). There were several near human races with bigger brains than ours, they didn't make it. Brains are a huge deficit, if they don't pay their way really well the competition will destroy their owners.

Answer 12

Evolution is a slow progress. Sure a species can eventually evolve to survive in a new environment but that's a process that's going to take hundreds or thousands of generations. Intelligence on the other hand allows for changes to spread through a population within a single generation.

By the time of divergence (2 million years ago) early hominids were already using stone tools. We have evidence of cooked food from 1.9 million years ago. They were already reaping the benefits of intelligence from at this time. Any change that would challenge early humans would also challenge the rest of the species in the environment. Humans are better equipped than other species to adapt to these changes. The won't need to evolve to thrive, they just need to learn.

Answer 13

Having an intelligence capable brain has an high methabolic cost: the brain needs to be supplied with energy to operate. As long as the brain usage brings some advantage, it is worth doing this.

Make therefore intelligence useless by removing all the situations where it is an advantage: create a stable and easy environment, one where the only effort to survive is to lay below a tree and wait for a fruit to drop in your mouth.

In such an environment an energy-hungry brain is a huge disandvantage and evolution will work by eliminating or reducing it, like it happens to fishes in underground caves which lose their eyes with evolution.

Answer 14

There is something that already exists, that if you do not have it, having a lot of intelligence and technology can be tremendous disadvantage. It is wisdom. Possessing a lot of intelligence and/or technology without the wisdom to appropriately apply these things is quite destructive. With intelligence and technology comes new discoveries, new discoveries can be dangerous, especially if misused. Many times, with new discoveries, we only partially understand the situation, and there are hidden dangers that we not realize the danger that they pose until further investigation has been done. We generally posses fair amounts of both in our current culture/society, and thus are quite attuned to discovering and eliminating the ill effects. If you need more motivation, take a look at the early history of X-Rays, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3520298/.

Answer 15

Two words: prions and scarcity Prions attacks only brains. Let just imagine a prion that thrive on grey matter and leave more or less intact the white matter. Dumb animals that kept a more primitive brain such as Koala (on top of being small its brain has less grey matter also) would survive, not others.

Scarcity of food (wikipedia)

The koala's small brain size may be an adaptation to the energy restrictions imposed by its diet, which is insufficient to sustain a larger brain.

Prions

pathology of prion diseases During the incubation period and active clinical phase of prion diseases, PrPTSE is probably present (and can transmit the disease) in all tissues and body fluids, especially brain, nerves, and skeletal muscle. Pathology develops only in the brain..../... Spongiform encephalopathy Advanced cases show neuronal loss, gliosis, and brain atrophy.../... The changes are confined to the gray matter (the primary pathology involves the neuronal body). In some prion diseases, PrPTSE precipitates as amyloid plaques.

Answer 16

I believe the answer would be equal egoism and pride/arrogance in all species.

What I mean with this is that, in real life, those are some of the worst attributes of human nature. We have that need of conquering, of feeling powerful somehow, of searching for revenge. We kill animals for fun, instead of just for a need. And we kill each other for all of the above.

Homo homini lupus (man is wolf to man), which means that we're actually our worst enemies.

Imagine those attributes in all kind of species. Strong ones like elephants and hippopotamus, or predators like lions and tigers (even a T-Rex in prehistory), wanting to dominate or "be owners" of all the land they can, or just killing a whole other specie for fun.

In that scenario, intelligence might be a little advantage, but strenght and speed would be dominant.

Answer 17

Perhaps a huge but steady growth in cold temperatures over a large number of years?

Imagine a situation where because of tech, let's say animal furs, leathers, fires and maybe even house insulating architecture or something, less was said to the DNA to adapt to the environment, and as such humans grew with a simulated temperature.

However, the temperatures wouln't stop dropping over a large span of years, resulting in wild animals adapting slowly over time, meanwhile humans not adapting as much.

Although I couldn't possibly know any plausible numbers, I can try to give an illustrative example of what I'm saying.

Imagine if it got -0.15ºC/-32ºF colder each year over 1.000 years, where animals felt 100% of that each year and evolved with the change - even if some didn't - while humans felt 40% of that and so didn't evolve as much over time. Finally, when the cold 'stacked up' enought, animals endured it, while humans lived all that time "protected" from the small temperature drops. At those pre-historic times they wouldn't have the tech to help them against so much cold (I'm talking -150ºC/302ºF total).

Second thoughts: Maybe not a steady growth but a scalating one, where the lowering of temperatures got bigger and bigger. (Resulting in a 'sudden' shock to humans and less so to wild life).

Mind not the actual numbers, -150ºC is too brute for any living organism... but the idea is a steady growth in cold, where animals DNA changed enought and the inteligent species didn't because of their protective tech.

Answer 18

Why not (As a thought experiment) put yourself in the position of a plant and ask yourself ... why has this plant survived in the evolutionary battle for survival and other plants have not? (assuming there is such a thing as plant genus going extinct)...??? Plants lack intelligence, yet they do evolve over time ...

Ultimately it would seem to me that the organism that is most adapted to its environment would by default win out over organisms that are not well adapted to their environment... thats the only way I can picture evolution happening progressively without intelligence.

As a side note, it was my understanding that Neanderthal's went extinct not necessarily because they lacked technology, but because they failed to spread out on the planet and populate new areas ... they stuck together making them "fish in a barrel" as it were ... yet, one could argue that only a lesser intelligent species would behave in such ways ...

Answer 19

Simple environments

Intelligence isn't the perfect solution to all problems that you're making it out to be. If it was, all animals would continuously evolve to be more intelligent, which they don't.

Most evolutionary adaptations are trade-offs, and intelligence is no exception. In fact, the cost for intelligence is quite high: the brain is an extremely energy-hungry organ, and bigger brains require exponentially greater amounts of energy to keep running. Energy, being nature's "currency", is a big limiting factor on any adaptation. The human brain uses a whopping 20% of the body's total energy consumption. That's energy that could have been used for building strength, speed, armor, big teeth, fast running, reproduction, survival without food, or any one of the countless other "purchases" that could help an organism to survive.

Intelligence also tends to greatly slow down the reproductive cycle. Intelligence doesn't play nicely with complex, hard-coded instincts - so usually the solution is to toss out the basic instincts and let the animal figure things out as they grow. This means that intelligent animals will tend to be less capable from birth than less intelligent ones and might require the parent to invest energy in teaching them how to survive. And the energy cost (both from the brain itself and the time spent teaching) means that parents have to invest a lot more energy in raising children, which typically means having smaller numbers of children. Both of these things are detrimental for the survival of the species.

For intelligence to be worth the cost, it has to convey a pretty significant advantage. For most animals, that advantage is adaptability. Without intelligence, most organisms can only adapt to new situations through evolution; intelligent animals can adapt within a single generation. While this seems like a worthwhile advantage, it is only really useful in a changing environment that has a lot of new resources and dangers popping up on a regular basis. And there's your key.

Most organisms are specialists, meaning they do a small number of things really, really well. Cats rip open prey, polar bears can live in the cold, crabs can smash through shells, tortoises can deflect sharp objects, ants can grow food or raise livestock, beavers cut down trees to build shelters. Humans, and other intelligent animals, tend to be generalists - nature's plug-in devices. We can make knives to rip open prey, fur coats to live in the cold, hammers to smash shells, armor to deflect sharp objects, and we can make farms and houses for food and shelter... but we need to "pay" for this ability by eating more food, and we aren't born with the innate knowledge of how to do any these things. We can live almost anywhere and exploit almost any resource, but we usually can't do them as well as the creatures who evolved to do so.

So what's a circumstance that would make intelligence a disadvantage? A simple, unchanging environment, with a single, basic resource, and few or no predators. Such an environment will favor animals that are born with the physical and instinctual means of exploiting the resource and evading the predators; intelligence will be a pointless waste of energy, reducing reproductive fitness while conveying no real advantage.

Answer 20

No hands.

Intelligence without being able to manipulate the environment does not help much.

Or make the world have few elements. Say a small island without trees. All food is obtained by catching food by hand on the sea shore. (No trees = no boats)

In both situations above intelligence is expensive but does not help much.

Answer 21

Intelligence on a world like ours is definitely paying of. But as someone stated above, intelligence is very costly as of energy/resource/evolution. To be able to pay that cost, you need high surface of land, that has a strong environmental complexity and can provide abundant and diverse food sources. If your world has smaller land masses, and for some reason they don't have abundant diverse food sources, then specialized creatures would have the upper hand in evolution. So maybe make it a world that has mostly inhabited islands, with the continents in the global areas where deserts would form. You should increase the global sea level (in the early history of the planet it was bombarded by more ice meteors and comets).

Answer 22

Here's a few of my ideas:

Make other pressures orders of magnitude more important, or make time the enemy

The Brain is a huge, massively complex organ, which, as others have pointed out takes a lot of maintaining. The more complex the system, the more points of failure it has. For example:

Increased Solar Radiation

Deplete the magnetosphere or ozone of your planet. Not enough to kill everything, but to the point where selecting for large brains is likely to come with a larger downside in terms of reduced radiation resistance.

Increased toxicity

Lead is a good example, as brains are especially susceptible - Maybe your planet's soil and water is heavy in lead content

With the above as examples, they also have the added benefit of killing over time: a large, complex brain requires a minimum size of organism, which requires time to grow. I.e. you could essentially design the planet such that there is a 'maximum lifespan' of any even semi complex organism - even with the benefit of evolutionary protection.

Everything races to produce offspring in a few days or weeks before dying and leaving a glowing, lead filled corpse.

Reduced Oxygen Content

Replace some of the oxygen in the air, with, say, Helium. It's non toxic in and of itself, but if making the most of every molecule of oxygen is a significant pressure, feeding the oxygen hungry brain may be a step too far.

Answer 23

Do it like Warhammer 40k which is so grimdark that intelligence can be a huge liability. Being curious about the wrong things usually causes trouble or brings one to the attention of the Inquisition (usually both), and many Inquisitors of the Xanthite faction walk on a razor-sharp ridge when acquiring and using arcane knowledge.

In that setting, big brains also usually imply psionic powers, which carry the inherent risk of being om nom'ed by warp entities, thereby threatening entire planets.

Another risk is being intelligent enough to amass huge power, but not intelligent or powerful enough to look through the plans of one's enemies, or failing to vanquish them. Horus for example, though being one of the brightest minds of the Imperium, was corrupted. Even the Emperor himself apparently underestimated the risk of the Ruinous Powers being able to corrupt His sons and turning them against him.

Answer 24

Intelligence is, at it's fundament, the ability to 'say' yes, no and maybe, to impose a non-binary state on an apparently binary environment as a means to 1: Decision making with insufficient data and 2: The ability to act and allow for 'luck' even when the data suggests failure.

It works for us and other animals because we always have insufficient data from a (hah, because of a) point of view.

What if nature evolved a complex binary organism that had the same drive to reduce threat as mammals? That is to say, everything outside of itself was a threat. It would not spend time pondering the stars, or wondering why a cube thrown into water would make a circular ripple, but be a plague upon all things, a force of nature as it were.

Or in other words, Intelligence is always a threat unto itself. Complacency, optimism, speculation and all those other vices.