I've always been highly sceptical of the notion that diet is the main driver 
behind the increasing hominid brain size in the Palaeolithic. Here's 1 article 
referring to a study claiming that bigger brain-size was actually partially due 
to the presence of an Ice-Age:-

http://www.newscientist.com/article/mg20327194.000-did-an-ice-age-boost-
human-brain-size.html

http://tinyurl.com/mb4qac

There may be some justification for this as the Inuit reportedly have the 
largest average human skull-size.

Geoff