Last week I wrote about the British neurobiologist Colin Blakemore’s assertion that human brain size suddenly increased by 30% about 200,000 years ago, and his idea that this increase was due to the fixation of a single macromutation. While I questioned this mechanism, John Hawks also questioned the data, claiming that brain size increase over time had been pretty steady, with no leap at the proposed time.
Well, an alert reader, anthropologist Ciarán Brewster from University College Cork, has done the proper statistical analysis of the brain-size data. Over at his website, Ad Hominin, Brewster shows that there is indeed an inhomogeneity in brain evolution, and that the pace of brain-size increase did indeed pick up about 200,000 years ago.
So Blakemore’s assertion about the acceleration was correct. But of course this says nothing about whether that acceleration reflected the fixation of a macromutation, which I consider very unlikely for reasons I’ve already mentioned. And Blakemore’s theory that any brain-size “macromutation” was initially neutral, and became advantageous only after it had already been fixed in our lineage by random processes (e.g., genetic drift), is pretty outlandish.
One thing that Brewster doesn’t discuss is whether that accelerated brain-size evolution was correlated with accelerated body-size evolution. If brains got bigger simply because bigger bodies produce bigger brains as a byproduct (this is what we geneticists call a “pleiotropic effect”), and if for some reason bodies were selected to get bigger around 200,000 years ago, then it’s not necessary to posit that there was something selecting for bigger brains per se. (Of course, increased body size could also be a byproduct of increased brain size!) I don’t know if this is likely given the data, but it’s something to consider.