To say we dont need our thumbs because of our brain power is stupid. We're one of the only species to have thumbs. And as cavemen without our thumbs we would be feeding off grass and trees and not using rocks, bones, and sticks to hunt. Hell, holding a baby with out thumbs running from predators is stupid when we can club it to death with our sure grips with sticks and stones. Which would later turn into hammers, clubs and spear. We wouldn't be able to properly throw, or even punch properly. The thumb on the human is one of the most important traits the specie has. Try not using your thumbs for a day or a couple.

Also you must look pretty stupid writing without your thumb. How do you hold the pen? With all fours or just two? You wouldnt be able handwrite as fast and efficiently. As I type on my iPod I'm using thumbs, would you type wig your index finga's?