Paleodiet is the diet part of "evolutionary medicine".
One of the few books on that subject as a whole (though *not* dealing
with diet by some reason) is "Why we get sick" :
http://www.amazon.com/exec/obidos/ASIN/0679746749/qid=1016816697/sr=8-1/ref=sr_8_3_1/104-4530437-0386347
One of the authors is both Professor of Psychiatry and Professor of Psychology.
He suggests that there is an evolutionary advantage with havein the possibilty
for depressions, just like for example fever. He thinks that you get a depression
bekause there is a chnge in way of life needed, and it makes one stop up with
the current situation and find other ways in life.

- Hans