For several millions years up to ~200,000 years ago there was a study increase in hominin cranial capacities. I say hominin because it seems that this increase was evident in all branches of the human lineage. Neandertals were increasing in cranial capacity, just as African humans were. Then there was a leveling off and stabilization. Finally, over the past 15,000 years or so there has been a decline, from a median of 1,500 cubic centimeters (cc) to 1,350 cc.
In her article, McAuliffe asks several paleoanthropologists (including John Hawks, Christopher Stringer, and Richard Wrangham) for an explanation and several hypotheses are offered. While all are plausible, the most provocative (courtesy of cognitive scientist David Geary) is that humans are becoming dumber. The most palatable, suggested by Wrangham, is that brain size decreases are due to selection for diminished aggression, and that as humans domesticated themselves, brain size has decreased. Both may be correct, as these hypotheses are not mutually exclusive.
A hypothesis I did not see, but which should be considered, is that of extended cognition. As Merlin Donald argues, cultural development allows for much greater storage capacity that is external to the mind — lots of information can be offloaded and accessed only when needed. Writing is of course a prime example, but such offloading would have begun much earlier with the advent of sophisticated oral traditions in which certain people were tasked with collective memory. As external storage systems became larger and more efficient, the need for additional gray matter (i.e., metabolically expensive brain tissue) would have decreased.
All three hypotheses — that living in stratified/specialized societies requires less brainpower, that humans living in such societies are effectively “domesticated” versions of their more wily and aggressive Paleolithic ancestors, and that humans have delegated much thinking/memory to external sources — are consistent with the shift from shamanic forms of supernaturalism to organized and systematic religions. Historically speaking, this shift corresponds to the profound cultural transition from foraging to agriculture and is known as the Neolithic Revolution.
Although he does not tightly link his ideas to this history, Harvey Whitehouse’s characterization of religiosity as either “imagistic” or “doctrinal” seems to track the movement from shamanic supernaturalisms to organized religions. It may well be the case that this domestication of supernatural experience corresponds to shrinking brains and diminished cognitive capacity.