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It seems like two potential motivations behind human ancestors' evolution of intelligence were their complex social groups and behaviors, and hunting behavior requiring more strategic thinking in carnivorous. How could human-like intelligence evolve in a species that is herbivorous (or omnivorous leaning mostly to herbivorous)? How could human-like intelligence evolve in a solitary species, and would their social behaviors change during that process of evolution?

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    $\begingroup$ This one falls foul of the "answer would need to be book-length" check, I think. Also, we don't have a good idea of how intelligence evolves in any species, or even a particularly good idea of what intelligence is. $\endgroup$ Jan 4, 2022 at 15:19
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    $\begingroup$ This might be very tricky, because the only type of solitary creature I'm aware of that is regarded as highly intelligent to the point of potential sapience are octopuses, and their diet is centered around anything but greens. The creature doesn't need to worry about its food running away, it doesn't need any major brains for living in a group and if it's big/strong enough it might not even need to be smart to avoid predators as adults (see some sauropods). Remember that our brains are VERY costly to maintain, so you'd need some strong pressures towards intelligence for it to evolve. $\endgroup$ Jan 4, 2022 at 15:45
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    $\begingroup$ The question is a bit unclear - are they meant to stay non-social as they evolve intelligence, or is this about taking a non-social species and evolving social and intelligent behaviour? "human-like intelligence" is, fundamentally, a very social thing. $\endgroup$
    – user22917
    Jan 4, 2022 at 18:05
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    $\begingroup$ In retrospect I probably should have said human-LEVEL rather than human-like, but they don't necessarily have to remain non-social; I was more just thinking of what the descendants of a species evolving from very solitary animals could be like in terms of social behavior. $\endgroup$
    – inkwell87
    Jan 4, 2022 at 20:15
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    $\begingroup$ @inkwell87 They would develop a highly individualistic culture, basically how the rest of the world sees Americans (might be difficult for Americans who haven't been abroad to understand this), and they will all be libertarians. I'm only half-joking. Tigers are a good example where cubs leave very early on and once in the wild, family bonds are no longer recognized and they will attack each other defending their territory. Americans are a little like that in they move out at 18 (few other cultures do this) and define family unit to not include the grandparents. $\endgroup$ Jan 4, 2022 at 21:21

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Smart Enough to not be Eaten.

The SmartyPantsSaurus evolved on a planet with humans. Humans are very intelligent and can coordinate to hunt things much bigger and stronger than them, like Bison and Wooly Mammoths.

The SmartyPantsSaurus is big and strong because it eats grass, which grows in abundance. Unlike the Wooly Mammoth it has an extra trick up its sleeve -- to outwit the hunters it evolved to be as smart as them.

Human-like intelligence in a large body is a good tradeoff, because to make something smart as a human you just have to add an extra 2lbs of brain matter, rather than say doubling the existing brain size. So for example a guinea pig with human smarts cannot eat enough to power its brain. But a blue whale just has to eat 10% more. Or eat the same amount but have 89% the bodymass.

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    $\begingroup$ actually brain size scales, so they don't need 2lbs more brain tissue, they need 2 times the ratio of the animals mass to human body mass. $\endgroup$
    – John
    Jan 4, 2022 at 23:40
  • $\begingroup$ @John That doesn't make sense to me. That would mean a human has two times as much brain mass as itself. $\endgroup$
    – Daron
    Jan 5, 2022 at 12:39
  • $\begingroup$ check your math the ration for humans is 1, so they need 2 lbs. of brain mass to do the same thing as 2lbs of human brain mass. $\endgroup$
    – John
    Jan 5, 2022 at 20:33
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Agriculture

It's a stretch, but solitary herbivores could develop progressively more complex grazing strategies that eventually evolve into something resembling farming. They develop a human-like intelligence because they are pressured to understand complex concepts like irrigation, crop rotation, planning harvests to avoid famine, etc.

The trick here is that social transmission of knowledge (i.e. culture) is a huge part of human intelligence. Even if they evolve human-level intelligence, if they remain solitary then their intelligence would never be human-like, and an intelligence that evolved specifically to solve tricky puzzles would be deeply alien to us. One way to develop social behaviour would be that as they grow smarter, their grazing grounds can be larger - eventually, they overlap. Then the animals start to learn cooperation, initially without communication or contact. They see what the other guy is doing, and figure out what they can do with it for mutual benefit. They would evolve to behave very predictively and communicate very little. They would, compared to humans, be better at reverse engineering (how did they get their wheat to grow in rows like this?) and worse at learning by watching and imitation.

It should go without saying that there is not evidence that this has happened, so there is probably a good reason why it doesn't work like that. But it's narratively workable.

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  • $\begingroup$ The problem is the OP didn't define "human-like" intelligence. Does that include scientific achievements? Does non-social mean they never form a civilization? Because that would be impossible. Such a creature would at best every few thousand years birth their own Archimedes, except he doesn't socialize so his knowledge/tech isn't shared. His work might be inherited by his offspring for a couple generations then lost because they were unable to understand/replicate his work. It would inevitably remain very primitive stuck at early agriculture tech busy fending off everyday predators. $\endgroup$
    – user93359
    Jan 4, 2022 at 20:21
  • $\begingroup$ agriculture does not necessarily aid intelligence, ants practice agriculture. $\endgroup$
    – John
    Jan 4, 2022 at 23:50
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How smart are your plants ?

Suppose you have a mix of toxic and non-toxic plants with all kinds of fancy colors

Antisocial, non-communicating herbivores could develop talent for analytical intelligence, that is connecting rationally what is received with the senses. For instance, being able to interpret plant colors as toxic or non-toxic is regarded "intelligent" behaviour for animals. If the differences would be very subtle, or color patterns would be involved (intelligent plant life ?) or origami leaves, or complex nut shells, the herbivore would have an incentive to develop the the brain for it. These skills could even enable the creature to pass a human IQ test with similar riddles, involving color, patterns or folding.

Your animal (?) getting used to observe plants very closely, could gain more insights about plant life, become aware of the seasonal rhythm of plants and develop tool production, smart agriculture etc

but..

Suppose you always have summer and the grass is always green

There is abundance, everything is safe to eat. Your isolated herbivores will condition themselves to graze, which does not require any intelligence. You just graze and look at the grass. When solitary, such a creature could derive some intelligence from becoming a prey animal (see Daron), but it has little stimuli in life. The brains do not need to train for analytical intelligence.

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Really short answer: Orangutans. They are solitary but social, mature slowely and spend years with their mothers and siblings learning to use tools and build nests. But they do not really live in groups and they eat mostly fruit and leaves. Yet they are among the smartest of apes.

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Probably not

An herbivore could possibly for intelligence. Given ideal conditions and plenty of time to develop, an herbivore could overcome the advantages humans had in being omnivore. As they develop better food gathering practices, they can develop higher level of intelligence. However, this may be difficult with climate change. Animals focusing on narrow food options (strictly herbivores or carnivores) tend to suffer more than opportunistic eaters (such as omnivores) when weather conditions change.

Non-social creatures, not so much. Social creature share the burden of survival. They can share resources, provide protection to one another, give warning of danger and more importantly, share knowledge and experience. Herds of elephants rely on their elders to find the local watering holes, best places to eat and locations of danger. A solo lion male has to reinvent the wheel on its own. Without a social structure, knowledge is not shared and passed on to the next generation and thus nothing would be improved upon and advancements tend to get lost. Plus mating is more based on luck of the draw and has less to do with selecting the best in a large group of potential mates.

To sum it all up, vast majority of species roaming around the world we consider intelligent are omnivores (with few exceptions, elephants are herbivores and dolphins are carnivores) and almost all (I could be wrong, but I think its actually all) are social.

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Dumb predators, accidental intelligence.

Humans are not natural predators. We have no claws, we have no fangs, we have very little strength compared to other predators our size, we don't have super eyesight like an eagle, we don't have super noses like a dog, we don't have any venom like a snake or spider, we don't have super hearing like many predators. We are pathetic.

Although social interaction may have created some evolutionary pressure for intelligence, we developed hunting strategies after we became intelligent, they did not drive our intelligence. We had to be intelligent first to invent weapons, a naked animalistic human without any weapons is going to get killed even trying to tackle a wolf. And kill it, how? Trying to choke it while it rips my throat out?

Even with evolutionary pressures, intelligence is still just rare but crucial accidental mutations. There is a good plausible reason there are no other technological species, including amongst the thousands of other social species: The mutations required for intelligence are so unlikely that they occur once in a billion species.

There is no compelling reason those same mutations could not have developed in a herbivore, making them more clever than their predators.

The one mutation that seems to drive intelligence, that humans have and no other animal seems to have, including other apes, is unlimited foresight and hindsight.

This has nothing to do with hands or opposable thumbs, and not much to do with being social (so are hundreds of other species including most forms of ape), or with being hunters (many social species are foragers, not hunters).

What we can do that other animals cannot do is relate causes and effects across large stretches of time. Most animals can't manage days, we can follow cause and effect over centuries. Not only in the sense of past events causing current events (hindsight), but in the sense of successfully planning for the future (foresight).

The first farmers were, in fact, hunter gatherers, and they noticed that where they had spat or crapped seeds of melons and plants they had eaten, the same plants grew again, a year later. We have evidence that hunter gatherers on year long migration circuits intentionally carried and planted seeds along their circuit to feed themselves in the next year. (They also planted inedible flowers far from their natural origin; nobody knows why; maybe they just liked the way they looked.)

They were farmers without farms. Herders without pens, they just led goats with them and let them feed, perhaps for milk or meat when the time came.

My point is, that accidental mutation that allows unlimited cause and effect could occur in any brained animal, including a non-social herbivore.

So imagine what they would do with that intelligence. Protect themselves from predators, to start. Perhaps even intentionally trap and kill predators, luring them into pit traps.

Perhaps they become limited social animals, cooperating for intelligent purposes, like farming and storing food for the winter. Even non-social herbivores have to mate and raise young, there would be evolutionary pressure to use their intelligence to ensure both happen successfully.

Perhaps they master fire.

We did not start out as either carnivores or intelligent. We suck at both, naked in the wild without intelligence.

Just like your herbivores.

My advice is to stop thinking of Intelligence as something that is the result of evolutionary pressures; it is not.

Think of it as a rare combination of mutations that happens entirely by chance to some animal, any animal, and then evolutionary pressures act on it and shape it.

So first pick your animal that wins this lottery (as we did), and then project the rational future of that animal once it has more intelligence than any other animal.

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    $\begingroup$ The common ancestor of the African great apes, including humans, was an African great ape. It was quite intelligent (all apes are intelligent) and it lived in social groups (all African great apes live in social groups). They never had to face a predator alone. There is strength in numbers. Moreover, humans absolutely are predators, and were predators from day one. We are not "pathetic", we are adapted for endurance. Endurance is our evolutionary advantage as pure animals; yes, a cheetah can easily outrun a human over 100 meters, but the human can easily outrun the cheetah over a kilometer. $\endgroup$
    – AlexP
    Jan 4, 2022 at 19:35
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    $\begingroup$ Correction: we have fangs. They're just not very long. $\endgroup$
    – Jedediah
    Jan 4, 2022 at 19:59
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    $\begingroup$ "stop thinking of Intelligence as something that is the result of evolutionary pressures; it is not." is an objectively wrong statement. Nearly every single trait that goes on to be an integral part of a species is related to 2 things: the mutations that eventually cause that trait to exist and the natural selection that deems it helpful or at least not a hindrance to the thing's survival. Being intelligent isn't just a random 100% beneficial thing for every creature, it takes a lot of energy to maintain, and if the extra brains can't "pull their weight" properly, they become a hindrance. $\endgroup$ Jan 4, 2022 at 20:00
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    $\begingroup$ "Why aren't all the apes pursuit predators": for the same reason that not all descendants of the first bony fishes crawled onto land. Some fishes adapted to live on land. They were not perfectly adapted from the beginning; just like immediately after the split humans were not perfectly adapted to run and walk long distances. All apes can walk on two legs, and they do so if needed; our ancestors had fewer trees in their environment, and the environment favored walking upright. Endurance, long attention span and intelligence co-evolved over some two million years. $\endgroup$
    – AlexP
    Jan 4, 2022 at 23:07
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    $\begingroup$ chimps hunt, so it is likely out common ancestor also hunted, so hunting predates high intelligence in hominids. chimps even use spears in hunting. if anything caused intelligence in humans it was the advent of cooking which doubled our caloric intake. pursuit predations could have happened before or after rapid inflation of the brain. All the anatomical prerequisites predate it by a wide margin. intelligence is very obviously a result of evolutionary selection, we a step wise advancement in hominid evolution. $\endgroup$
    – John
    Jan 4, 2022 at 23:47

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