There's no proof that humans have hit any such limits. But that's not how genes and evolution work. We got intelligent enough to adapt to the environment and conditions that were thrown at us to survive the ice age and through the many inter-tribal wars that have been fought since. But once a species hits a plateau in selection pressure there is little improvement, except via sexual competition.

I'm not sure the birth canal is a real limit. It's a limit on how wide your head can be when you are born.

That's not the same as a limit on brain size in adults. (Nor even directly a limit on brain size in newborns: up to a point they could always get longer heads instead of wider ones.)

> (And even if those biological limits coincidentally lined up with general limits on intelligence, just the fact that AGIs will be able to duplicate themselves or share knowledge directly would itself be a huge practical increase to their intelligence over us.)

Yes. A human with access to a calculator is much more capable than one without. An otherwise human-level AGI with hard-wired direct-'brain'-access would be even more capable---without having to manipulate the calculator with clumsy fingers and interpreting its outputs with a general visual system.

Add direct-'brain'-access to Google and Wikipedia and to huge amounts of raw storage, and your human-level AGI would already be super human.

"(Nor even directly a limit on brain size in newborns: up to a point they could always get longer heads instead of wider ones.)"

Coneheads confirmed? Surly I wasn't the only one to watch it.