Brain Evolution in the Human Species

Brain Evolution in the Human Species Introduction Hominid evolution is marked by a very significant increase in relative brain size. Because relative brain size has been linked to energetic requirements, it is possible to look at the pattern of encephalization as a factor in the evolution of human foraging and dieting (Foley et al., 1991). Major expansion of the brain is associated with the Homo species rather than the Hominidae as a whole, where the energetic costs are likely to have forced prolongation of growth rates and secondary altriciality (Foley et al., 1991). Paleontological evidence indicates that rapid brain evolution occurred with the emergence of Homo erectus 1.8 million years ago and was associated with important changes in diet, body size, and foraging behavior (Leonard et al., 2007). Energy Requirements Extensive energy is required for brain growth and functioning. Parker (1990) analyzes intelligence and encephalization from the perspective of life history strategy theory, which is based on the premise that evolutionary selection determines the timing of major life cycle events-especially those related to reproduction-as the solution to energy optimization problems. Foley and Lee (1991) analyze the evolutionary pattern of encephalization with respect to foraging and dieting strategies. In considering the development of human foraging strategies, increased returns for foraging effort and food processing may be an important prerequisite for encephalization, and in turn a large brain is necessary to organize human foraging behavior. Dietary quality is also correlated with brain size. Foley and Lee (1991) first consider brain size vs. primate feeding strategies, and note that folivorous diets (leaves) are correlated with smaller brains, while fruit and animal foods (insects, meat) are correlated with larger brains. Overall, the genetic costs of brain maintenance for modern humans are about three times that of a chimpanzee. The first dietary shift is seen beginning within the genus Hom o, which began to include meat in the diet. It may be argued that meat-eating represents an expansion of resource breadth beyond that found in non-human primates (Foley and Lee, 1991). Therefore, Homo and its encephalization may have been the product of the selection of capable of exploiting energy- and protein-rich resources as the habitat expanded. While the evolutionary causes of the enlarging human brain themselves are thought to have been due to factors that go beyond diet alone (increasing social organization being prime among the proposed factors usually cited), a diet of sufficient quality would nevertheless have been an important prerequisite. That is, diet would have been an important hurdle, or limiting factor, to overcome in providing the necessary physiological basis for brain enlargement to occur within the context of whatever those other primary selective pressures might have been. Leonard and Robinson (1994: add page numbers for direct quote) conclude: These results imply that changes in diet quality during hominid evolution were linked with the evolution of brain size. The shift to a more calorically dense diet was probably needed in order to substantially increase the amount of metabolic energy being used by the hominid brain. Thus, while nutritional factors alone are not sufficient to explain the evolution of our large brains, it seems clear that certain dietary changes were necessary for substantial brain evolution to take place.