I may have misunderstood, however ...
Morgan posits a 2 million year lapse between bipedalism and expansion of the brain. Near the end of her trenchant essays, she suggests that a fish diet facilitated brain growth. The problem is that the fossils suggest that we hiked south before we got big headed. Did some of us take the walk while others stayed on Danakil and got bigger minds?
Curiouser and curiouser ...