I think it's a matter of -- for lack of a better term -- a tipping point. Once the complexity of the brain a certain group of primates reached a certain point, a whole range of mental abilities -- including, perhaps, something to do with the soul -- developed.
Like my brother was telling me, it all comes down to memory. The amount that humans can memorize is insane, it`s just impossible. There is something else at work there. There are tons of animals who have bigger brains, more "folds" yet you don`t see them memorizing to the extent of humans. By the time you finish reading this, it will be one more in billions of things you have memorized, yet the brain you have is a tad larger than your clenched fist. It`s impossible, there is something else involved. Like you said, it`s the soul, something that is out of phsyical limitations that is responsible.