Brain - the expensive tissue

It is possible that tool use and extractive foraging, particularly of animal carcasses, contributed to brain size increases in Plio-Pleistocene hominins, between 3 and 1 million years ago [22].

The evolution of large human brain size has had important implications for the nutritional biology of our species. Large brains are energetically expensive, and humans expend a larger proportion of their energy budget on brain metabolism than other primates. The high costs of large human brains are supported, in part, by our energy- and nutrient-rich diets and as we explained before also with the reduction in size of the gastrointestinal tract [11]. Among primates, relative brain size is positively correlated with dietary quality, and humans fall at the positive end of this relationship.

Figure 6. Plot of relative brain size versus relative diet quality for 33 primate species. Primates with higher-quality diets for their size have relatively larger brain size (r  0.63; P 0.001). Humans represent the positive extremes for both measures, having large brain:body size and a substantially higher-quality diet than would be expected for their size. [23]

In addition, humans are relatively “under muscled” and “over fat” compared with other primates, features that help to offset the high energy demands of our brains. Important changes in diet, body size and foraging behaviour occurred (as paleontological evidence indicates) with the emergence of Homo erectus 1.8 million years ago [23].

Is not totally right say that dietary change was the driving force behind major brain expansion during human evolution. Rather, the available evidence indicates that a sufficiently high-quality diet was probably a necessary condition for supporting the metabolic demands associated with evolving larger hominin brains [23].