[tuatoru]

> taken ourselves out already.

Neither nuclear war nor a climate apocalypse is existential. Some thousands of humans would survive the worst case of either or both.

That said I agree. Human brains arose through evolution: Evolution operates within iron laws of energy efficiency, on the material it has to hand; and it only does enough to provide an advantage in production of grandchildren.

There is no a priori reason to suppose that human-level intelligence is anywhere near being a limit, absent those mechanisms.