Significance of Evolutionary Lags in the Primate Brain Size/Body Size Relationship

Although brain size and body size co-evolves in primates, the correlation is far from perfect. This was originally interpreted as implying that evolutionary changes in brain size lag behind evolutionary changes in body size. Subsequent tests of the hypothesis, however, concluded that there is no meaningful lag. I reanalyse the original data taking socio-cognitive grades into account and show that there is, in fact, a very strong lag effect, but that the original “catch-up” hypothesis is not the explanation. Rather, the “lag” is part of an adaptive response to predation risk in which species initially respond by increasing body size, but later switch to increasing group size (with the latter made possible by a correlated increase in brain size). This adaptive response takes between 2 and 8 million years to fully implement, and is dependent on a switch to a more energy-rich diet. This trajectory can be clearly documented in the evolutionary history of fossil hominins over the past 5 My.