I would not discard so easily that brains and fingers are not correlated, I know corvids, dolphins and other species show some level of intelligence but it's nowhere near humans so they are not in the same category to draw conclusions, so the only sample we have is humans, so until we have another sample I wouldn't make any affirmation one way or the other; also we know our advanced tool usage was not only develop due better brains but also due having opposable thumbs.