Posit an evolutionary history without primates. Would another order of animals have evolved a sapiens-like intelligence? Which one?
I realize that the opposable thumb is given a lot of credit for our development, but does that mean it couldn’t possibly–or even speculatively–have happened without them?
Dolphins? Dogs? Birds? All are unusually intelligent. Any chance they’d ever become fully, consciously intelligent like humans? or have they bumped up against the thumb ceiling?
Elephants? They’re smart, and they can grab things by either coiling around with their whole trunk or plucking with the opposable bit on the tip.
It was far from inevitable that a species of primate would develop intelligence. In fact as far as I’m aware of, no one has a really convincing theory of what would guarantee the development of intelligence in any particular circumstances. It’s not inconceivable that but for some lucky accident the primates would never have produced anything smarter than a chimpanzee.
Uh, I never suggested it was inevitable, if you read my OP. I’m just asking for speculative theory on what other order of animal might have taken a similar route.
Insects? They bread fast so that allows them to evolve and adapt quickly.
Cetaceans are pretty smart already. Some birds, like ravens, seem to have a primate level of reasoning. Elephants have already been mentioned. Of course, rodents are so adaptable that maybe something would arise from that line.
I’d think that the creatures would probably need both some sort of manipulative ability ( to give it something to exercise intelligence on ), and perhaps even more important to be a social species. Internal competition against opponents that are always about as smart as you is more likely to drive a constant intelligence increase than competition against some other species that once you reach a certain level you’ll always be smarter ( what I’ve heard called the Machiavelli theory of the development of human intelligence ).
Elephants do seem like a good candidate. Ravens too, but they have the disadvantage of being birds which gives developing a big, heavy brain an extra penalty.
But their brains are just too tiny, and their bodies unlikely to be able to support anything bigger.
In the absence of primates, I suspect strongly that the Dermopterans would have diversified to fill many of the Primate niches. It’s probably anyone’s guess whether they might have made it to anything like human, or even ape, level.
Among the Carnivora, probably the best candidates would be among the Procyonids. (I’m skeptical whether cats could evolve to sentience – I think they’re too biologically specialized, although they’re intelligent and adaptable enough.)
So far as dogs go, their recent evolutionary history has been pretty considerably affected by us, so far as that goes. I’d guess that any domesticated or farmed animal might well be somewhat different (though likely not sapient) if we hadn’t been there at all.
The fact that it apparently took so long for human-type intelligence to arise after the evolution of land-dwelling vertibrates suggests to me at least that it is a fairly rare and unusual set of characteristics that lead in that direction. My uneducated suspicion is that animals of the right size - neither too small to contain a brain of sufficien size nor too large to easily manipulate tools and other objects - tend to specialize in some respect that make human level intelligence unlikely.
I think being a social animal is crucial; it seems to me that all the smartest species are social. That’s just my own observation though, not sure it has any basis in reality.
Another “ceiling” factor for birds, I think, is that they have flight: that pretty much trumps almost any dangerous situation. They have less incentive, it seems to me, to adapt the kind of intelligence and strategical thinking that we developed as a non-flying species. Again, just my own thought experiment. But whereas we “perfected” intelligence as a survival strategy, their main strategy seems to me to be escape. Again, no hard rule being proposed here; just sensing a lack of evolutionary pressure from that angle.
Birds do have cetaceans, canines, and elephants beat in the manual dexterity arena, though; many birds essentially have opposable thumbs. The draw back is that for the most part they can only use one “hand” at a time.
Without question, it would be the rodents. Rats are intelligent, social, adaptable, good with their hands - all the things that goes us off to a good start.
Some rat-like thing would take our place in the smartipants niche.
On the contrary, intelligence in pretty much every clade increases over time. And that includes both mean maximal intelligence. So it seems that it is is inevitable that an organism of human level intelligence would evolve and that the circumstances are neither are nor unusual.
It does however take time. The intelligence increase seems to be driven primarily by competition, so it’s a kind of arms race with each organism being driven by the increasing intelligence of its competitors and predators. That’s a slow process, but it also seems to be very certain. The path of intelligence is always upwards.
Some are. The baleen whales however are as dumb as fungus.
Only in very specilaised areas. So for example they can be fooled into imprinting onto other animals and never unlearn it and they attack their own reflections in mirrors. This all seems tied to the weight limits place don birds. They’ve maintained an impressive intelligence with a small brain by hardwiring a lot of day-to-day tasks. That hardwiring makes them pretty dumb and inflexible in most fields compared to mammals, never mind primates.
But really, arguing over what creature would develop human style intelligence in our absence is pointless. It almost certainly wouldn’t have occurred before the next major extinction event. And after extinction events the most unlikely candidates always come out the winners. Just as nobody would have expected the mammals to win out after the last major event, the winner after the next event would likely have been something totally unexpected. The alternative intelligent creature could just as easily be a frog or kangaroo descendant as any of the obvious candidates nominated so far.
This question is similar to that debated by two giants of evolutionary biology. In one corner, the late Stephen Jay Gould, whose excellent book Wonderful Life espoused the view that the contingency of life on ‘random’ outcomes such as the survivors of local or global extinction events results in only a tiny probability of “super-intelligent” lifeforms appearing at all.
Conversely, Simon Conway Morris, ironically one of the “heroes” of Wonderful Life argues in the equally excellent Life’s Solution: Inevitable Humans in a Lonely Universe that the Earth’s environment constrains evolution such that the same “good tricks” are found over and over again, with the result that “humans” (not actual homo sapiens, but still some kind of large-brained biped with dextrous “hands” and two forward-facing eyes, say) would eventually acheive ‘super-intelligence’ any old how - maybe birds, maybe dinosaurs, maybe marsupials, who knows?
My guess is closer to Gould than SCM. Nature’s imperfect roam around evolutionary space might never have stumbled across so large a brain as ours again, any more than it has had another go at a wingspan as big as the 12 metres of the pterosaur Quezocoatlus or a neck as long as the 11 metres of the sauropod Mamenchisaurus, which both died out 65 million and 145 million years ago respectively. It would be interesting to see if “greater intelligence over time” is a rule which holds for ever in the absence of catastrophies, or whether it reaches a plateau from which an even larger, much more energy-hungry brain constitutes a risky evolutionary ‘gamble’. After all, homo sapiens might have come very close to extinction already. If they had died out completely, would a similar gamble by another species pay off in future? The odds don’t seem great to my admittedly amateur eyes.
How do we know this? How, for example, do we know how smart extinct creatures were? Is this merely a matter of measuring brain size - body size ratios? I rather suspect that, while indicative, such ratios may not necessarily tell the whole story.
Life, and large animals, have been around for a very long time. If the process were unidirectional in favour of increased intelligence, it seems odd that it never developed before. I suspect a great deal is chance.
How about Big Bird?
I realize that he is a bit dumb at times, but he is still as smart as many humans, and has a highly developed sense of empathy in common with many humans.
I’m going to vote for Raccoons as well. Seems they’re best poised to take over the primate niche if it were suddenly empty.
I’ve seen it suggested that cephalopods – specifically Octopoda – posess a surprising level of intelligence. Perhaps they could have evolved to a human level?
Being aquatic may be a disadvantage (no fire), and I don’t know that they are very social (I don’t think so). But they sure have the manual dexterity to go with their (relatively) big brains.
They already took over my garbage can and garage. They might as well have the primate niche, too. 
This graph shows a measure of how ‘far away’ certain species are from the average brain to body mass ratio of the animal kingdom, what’s called the encephalisation constant. Though it has its flaws, this simple graph might suggest candidates for future intelligence. Curiously, porpoises are closer to modern humans than many primates or even human ancestors in this respect.