Most biologists and anthrologists (?) believe that our brains got so large so that we could better understand and communicate with each other.
When I was in grad school some years ago my physical anthropology professor things larger brain size was a side effect.
He was of the opinion that the change to H. erectus involved a number of things, including more grassland adaptation, more bipedalism (and running), tool use, family structure, and that much of this came about in conjunction with persistence hunting.
Also included were loss of heavy body hair (for better cooling), much better memory and spacial sense (for tracking an animal and then remembering where you left the family after the hunt), and, to a lesser degree, communication (for the hunt and for explaining to the family if you were gone for days and didn't come back with anything).
Religious belief does not constitute scientific evidence, nor does it convey scientific knowledge.
Belief gets in the way of learning--Robert A. Heinlein
How can I possibly put a new idea into your heads, if I do not first remove your delusions?--Robert A. Heinlein
It's not what we don't know that hurts, it's what we know that ain't so--Will Rogers
If I am entitled to something, someone else is obliged to pay--Jerry Pournelle
If a religion's teachings are true, then it should have nothing to fear from science...--dwise1