Why is human thought so flexible? Although many features of human brains can be attributed to selection for novel cognitive functions (e.g., for symbolic language), relaxation of selection on other attributes has additionally contributed to de-differentiation of certain brain functions. Here a parallel process is described by Terry Deacon (UC Berkeley) involving domestication and birdsong. Series: "CARTA - Center for Academic Research and Training in Anthropogeny" [Science] [Show ID: 24978]