roybat -
I remember (vaguely my brain is another bad example), an Issue of New Scientist that discussed the effect on the brain and ability to stand upright of the "aquatic period" of human evolution.
Something to do with various nutrients found in the shell fish consumed.
Also the vegetarian animals on the plains would have a less difficult time adapting to changes in vegetation.
Homos had to be quick witted to work out new ways of staying alive as the climate changed quickly, they would have to be good at spotting new sources of food not seen before. Lack these skills and you died as a child before you could pass on any DNA.
HB