This can be pretty easily seen in humans who have part of their cognitive processes knocked out, and how they’re not just slightly-less-good at cognition than other humans, but generally incredibly flawed at it.

Removing part of someone’s brain isn’t an evolutionary process. They are incredibly flawed at it partially because our mental processes evolved together within the same brain, IMO. The specific neural network that developed to support all of these processes is incredibly difficult to untangle after the fact, if possible at all.