[acchow]

"Why humans evolved intelligence but orangutans did not".

There's a different way to think about this that is closer to how evolution actually works and will make the answer clear.

Our common ancestor (common to orangutans and humans) did evolve intelligence (concurrently with harnessing fire, clothing etc.). Not all of them, but some of them. And they broke off from the group. We now call them humans.

[jjk166]

Intelligence was evolved millions of years after the most recent common ancestor. Harnessing fire, clothing, etc. came later still. The lineage that would ultimately give rise to humans split from the chimp/bonobo lineage as the human ancestors adapted to savanna life, likely due to aridification brought on by the formation of the Himalayas.

It's possible that selective pressure towards intelligence was greater for the human lineage than for the others. It's also possible that the evolution of intelligence was equally likely across the different lineages and humans just happened to be the one where the mutation happened. Regardless, once human ancestors filled the niche, it would have been difficult for another lineage to get in on the game.

[_AzMoo]

Is there a specific definition for intelligence?

[jjk166]

What definition are you using to say chimps don't have human level intelligence?

By any useful definition, the intelligence of human ancestors very closely resembled that of chimps for about 4 million years after the human and chimp lineages diverged. While it's impossible to say for certain, that's around the time that endocranial volumes started growing consistently beyond the range seen in chimps. That is also around the time of the first evidence of stone tool making.

[dotancohen]

Like life, many sources define it differently.

[dmbche]

Is there a specific definition of definition?

[Arwill]

Substitute orangutans for Australopithecus. That is (one of) the branches that did evolve more intelligence, but didn't survive. I suppose there were lots of such branches, that either merged back into humanity (like the Neanderthals), or died out.

[jjk166]

Australopithecus is essentially on the human branch, and likely was still several million years before the development of advanced intelligence. Our common ancestor with Australopithecus was not any more intelligent than a typical Australopithecus, as far as we can tell.

As far as we can tell, no branch developed significantly increased intelligence after splitting off from our own lineage. That's not to say it definitely didn't happen or that our lineage was always the smartest, just that there isn't any evidence demonstrating a qualitative difference which has survived to the present. But it's weird that no such evidence exists.

Conversely different primate groups did independently evolve similar levels of intelligence, like Capuchin monkeys (which are new world primates) developed their intelligence after splitting off from the old world primates some 40 million years ago. Baboons and Macaques likewise each evolved intelligence independent of the great apes. Likewise similar levels (if different specializations) of intelligence have evolved independently outside the primates, such as cetaceans, elephants, and corvids. For cephalopods, which likewise are highly intelligent, their common ancestor with us didn't even have a brain.

[karmakaze]

I always believed that it was the group that had first mastered fire. Cooking food fundamentally changed human energy budgets. And keeping a fire meant that the group would congregate and form a larger social group, which would then lead to greater communication.

This of course changes the question as to why only/mainly homo erectus developed the capability.

[thrdbndndn]

I think their question is not about why humans evolved intelligence, but why one and only one single species did.

[HelloNurse]

It's the other way round: we are a species because we are the ones that evolved intelligence, which was certainly an enormous difference between intelligent humans and physically identical unintelligent apes.

[whimsicalism]

> why humans evolved intelligence, but why one and only one single species did

Well, that's false. But we killed off/interbred with all of the peer/near-peer species.

[Aachen]

Wouldn't the first ones always wonder that?

[mathgeek]

This led me on a tangent that eventually took me to this line on Wikipedia:

“Humans have visited the Moon and sent human-made spacecraft to other celestial bodies, becoming the first known species to do so.”

How would we know if we’re the first known anything? You don’t know what you don’t know, as they say.

[Aachen]

Ah, I meant on our planet. Of course, we wouldn't wonder if it's evident that there is aliens (other intelligent life from another object) at a similar time as when we evolve this level of intelligence. This would be far before we can make technological means to rule out intelligent life on nearby celestial objects, or someone would probably have come up with the question why we don't talk with (the equivalent of) apes and dolphins and such. The other intelligent life would have to be either among us, or visible from where we live (mountains, ocean surface, idk) with whatever we have for senses (like eyesight good enough to observe structures in orbit that are clearly not natural, as an example)

[WalterBright]

Intelligence must surely be a cluster of evolved changes, let's say A-Z. Each of those letters must have appeared, and been advantageous on their own (or they wouldn't have persisted).

So why didn't chimps get some of them?

For example, chimps have hands, but do not exhibit anywhere near the dexterity and agility of human hands.

[asdff]

Think less in terms of "this must be inherently better than that" and more in terms of the thermodynamics at play. Dexterous hands probably have some cost. Maybe they aren't as durable as a chimps hands. Maybe they take more calories to run. Maybe they need more brain power dedicated toward the hands and respective energy requirements. I'm not really sure what they may be, but there are usually tradeoffs between any A vs B in an organism.

Now if these costs are indeed less than the fitness advantage of a chimp having more dexterous hands, and that is in biological fitness as in reproductive success not the colloquial 'fitness' as in going to the gym, and that mutation for dexterous hands is present among the breeding population, you will expect to see offspring with that mutation, having higher fitness, to increase in frequency in the population.

There are a lot of potential edge cases to consider as well. Maybe the dexterous hands allele is very close to a very bad allele in chimps, such that through recombination it is likely that these two alleles are inherited together (called linkage). You'd see both these alleles purged from the population over time through purifying selection.

There is the population history aspect to consider. Maybe you don't need dexterous hands if your population is still living in the jungle among plentiful calories like the chimpanzee. Maybe it is more relevant to comparatively more feeble humans that were pushed out of that jungle by physically stronger ape populations into more nutrient poor environments, where suddenly the increased fitness from the advantages dexterous hands might bring now pay for their energy costs.

[andrewflnr]

They did get some of them. Functionally, chimps are pretty smart compared to almost anything but a human. Only if you define intelligence specifically as the gap between humans and chimps (or whatever other reference) can you say chimps didn't get any of the pieces. We can ask why humans have more of the pieces, but that's basically the same question as why any species diverges. So, some inscrutable combination of chance, path dependence, etc

[TheOtherHobbes]

Underrated point wrt intelligence is the extent to which it depends on fine motor control. Whether you're building tools, writing, or speaking a complex language, you need fine motor control to make that possible.

So it's not just brainpower, it's likely a combination of potential brainpower - which many species have/had - and fine motor control, which set up feedback loop that translated a mind/body synergy into practical evolutionary benefits.

[dyauspitr]

It’s more like A appeared and there was a split off. Then B appeared and another split off from the A group and so on until you get to modern day Z.

[WalterBright]

So why didn't other lines evolve X and Q?

[dyauspitr]

Pure chance, you might as well be asking why don’t monkeys have wings.

[acchow]

> So why didn't chimps get some of them?

The chimps that did get them we now call humans.

There were no chimps back then. We had a shared common ancestor, and subgroups gradually emerged and gradually became different enough that they stopped interbreeding (or were physically separated).

[SamBam]

It sounds like you're saying that the common ancestor of humans and orangutans harnessed fire and made clothing. I don't think that's correct.