In defense of the Expensive Tissue Hypothesis

Abstract

Modern humans have brains that are between three and five times the size that would be expected for average mammals of human body mass (Aiello & Wheeler, 1995, 1996; Aiello, 1997). Because brain tissue per unit mass has a basal metabolism that is over 22 times higher than the same amount of muscle tissue, a relatively large brain would be expected to have a significant effect on human energy budgets. In the recent literature on human evolution there has been considerable interest in the ways in which the metabolic costs of the large human brain may have either constrained or influenced adaptation and behavior. Focus has centered on how it is possible to grow such a large brain (Martin, 1996), on how adult humans might adjust their energy budgets to maintain their large brains (Aiello & Wheeler, 1995, 1996; Aiello, 1997), and on the implications of the metabolic aspects of brain growth and maintenance for human dietary evolution (Leonard & Robertson, 1992, 1994, 1997), life history evolution (Foley & Lee, 1991), social evolution (Key & Aiello, 1999, 2000), and symbolic evolution (Power & Aiello, 1997). One recent hypothesis has also suggested that the increase in relative brain size during the course of human evolution might be better explained by the metabolic resources available to mothers during gestation and lactation rather than by any specific behavioral feature (for example feeding ecology or complexity of social organization) that might be postulated to exert a selective pressure for a relative increase in brain size (Martin, 1996).