I don't think the brain is more than we need.
I think that we don't know how to fully take advantage of it.
As far as we can tell the brain has an infinite capacity.
If we knew how to do it we could learn in a way that we could stress the brain's capacity.
I think there is evidence that the ancients had this capability.
The great polymaths of history, people like Da Vinci, Hidegard of Bingen, Leon Battista Alberti, Thomas Young, et al were they born that way or did they learn how to be polymathic?
Baldassare Castiglione wrote a guide on how to become a polymath, The Book of the Courtier.
I helped develop a methodology for problem solving based on knowlege theory and we came to the conclusion that anyone can achieve Masters level competency in pretty much any topic if they know how to do it in about 3 months.
So, perhaps the issue isn't that we have capacity that we don't need, but rather we have capacity that humans have forgotten how to take advantage of and put to use?