Perhaps This Explains Theism?

[Angrboda]

This study suggests that the DUF1220 may be the cause of evolution of brain size.
I think that a question equally if not more fascinating is why this mutation (increased brain size) gave enough survival advantage that it propagated through the generations.
Some have suggested that, in the hunter-gatherer world of our distant ancestors a larger brain size gave an individual an advantage in a highly social animal, perhaps using rudimentary language. In other words, smarter hominids were better at "chatting up" cave-women and so spread their genes. This would make the large brain a sort of "peacocks tail" with, at that time, no other purpose (yes I know evolution has no "purpose" but you know what I mean).
Agriculture, animal husbandry, writing, technology, etc. could all be just unforseeable side effects of this "peacocks tail".

Another aspect that I've heard, but don't have exact details on, happened somewhere in the 2-6 Mya time frame. During that time, Africa was undergoing significant climate instability, and the constant abrupt changes in climate served as a sieve for selecting out the most intelligent of the species.