New Scientist has as article arguing that the expansion in hominid brain size that occurred about two million years ago was due to the ice age which allowed an energy burning, heat generating brain to develop with sufficient environmental cooling.
Actually, it’s worth a read as it’s not as odd as it sounds, but it joins an ever growing list of theories that attempt to explain how our forebears ‘suddenly’ seemed to experience evolutionary brain expansion.
These include:
A diet high in meat.
A diet high in starch.
Social competition.
The development of cooking.
Essential fatty acids and schizophrenia.
An unspecified “special event”.
The trouble with some of these theories is that they typically assume that brain size is always related to greater intelligence, which is not necessarily the case, and they don’t always take into account the ratio of brain to body size, which seems to be more important than just brain size alone.
Interestingly, almost all the interest is on brain expansion and no-one seems particularly interested in the fact that the brain has shrunk about 10% since the Late Pleistocene, about 30,000 years ago.
You can have a great deal of fun coming up with evolutionary theories for that one. Just pick a key human activity that emerged around the same time.
Personally, I blame language, religion and karaoke.
Link to NewSci on brain science and the ice age.
Mind Hacks 
Hmm, so the brain has shrunk. Well that explains some things.
re:Essential fatty acids and schizophrenia.
“fat-rich Western societies experience more schizotypal illness than fat-poor Third World ones”
In my opinion the association of schizophrenia and fatty acids is false.
Third world living may be closer to nature compared to the stressful urban artificial environment of the western world, stressful world that can make a persons mind in conflict enough to hallucinate to listen to the suppressed unconscious mind.
pn.psychiatryonline.org/cgi/content/full/41/7/36
“Ever since 1939, a growing number of studies has demonstrated that the rate of schizophrenia in urban areas is higher than in rural ones.”
Ignoring physical causes for “schizophrenia” such as food allergy . gluten allergy. What about western world VS third world here?
Just shut em up with antipsychotics, never mind the cause.
I always assumed that the reason for shrinking brains is that large brains are not worth the cost. They may well make you more intelligent, but intelligence don’t necessarily equate with reproductive fitness. The fact that most animals don’t go down this route shows what a borderline proposition it is.
Our ancestors were competing with big-brained Neanderthals. Perhaps they did it by being more stupid but breeding faster!
John Hawks says “I think this paper may single-handedly destroy the credibility of [i]Climatic Change[/i]”
http://johnhawks.net/weblog/reviews/brain/climate-brain-expansion-new-scientist-2009.html