[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).