
You're mixing 2 discussions here...
First, what was driving brain size before humans. My answer - things like stereo color vision and echolocation and dexterity. Kind of hard to live in trees without binocular vision. Easy to miss a branch.
Second, whether another wave of brain size increase, between monkeys and humans provided humans with some survival advantages. My answer is yes, see a post above.
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We have only 2 things to worry about: That
things will never get back to normal, and that they already have.