A few days ago I posted about neurobiologist Colin Blakemore’s thesis that, about 200,000 years ago, human brain size underwent an instantaneous increase in volume by 30%. Blakemore suggested that this was the result of a macromutation sweeping to fixation, but not through natural selection (he suggests no alternative, but perhaps he meant genetic drift).
Over at john hawks weblog, Hawks shows that there was no sudden increase in human brain size. Here’s the chart that Hawks provides which plots brain volume over time (the data appear to come from Lee and Wolpoff, 2003, but I haven’t checked):
Fig. 1. Change in hominin brain size over time. Time (thousands of years before present) on x axis, brain volume (cc) on y axis.
Now none of us have done any statistics on the supposed 30% “leap” 200,000 years ago, and I haven’t fitted a line to this to see if the increase has accelerated over time. Hawks says that this:
As you can see, there’s no sudden jump 200,000 years ago, or at any other time. The data, such as they are, are consistent with a single pattern of increase over time, as pointed out by Sang-Hee Lee and Milford Wolpoff (2003).
Well, yes, there is certainly a “single” pattern of increase (whatever that means), but perhaps a hint that the rate of change became a bit faster 200,000 years ago. But it certainly wasn’t an instantaneous increase, as one would expect with the rapid fixation of a macromutation that increased brain size by 30%. There seem to be a fair few intermediates!
Lee S-H, Wolpoff MH. 2003. The pattern of evolution in Pleistocene human brain size. Paleobiology 29:186-196.