A neuron can use arbitrarily complex calculations, because evolution's only requirement is that it works.

Ultimately this is all we care about as well.

No we have another requirement: the state of the system must separate into relevant and irrelevant variable, so that we can easily speed up the process by only relying on relevant variables. Nature does not need to work this way. It might, but we only having experience with human-made computers, so we cannot be sure how much of the information can be disregarded.