The human life history is unusual in having a childhood stage characterized by a prolonged period of exceptionally slow growth. In this talk, Prof. Kuzawa will discuss his team’s work quantifying the costs of the human brain during development, which has had a formative role in the evolution of the human life history. They find that the costs of the brain do not peak at birth, when relative brain size is largest, but at 4-5 years of age, when the brain consumes the equivalent of 66% of the body’s energy use at rest. This childhood peak in brain costs reflects the proliferation of energy-intensive synapses prior to experience-driven synaptic pruning, and accounts for more energy use than kids expend on physical activity at this age. Consistent with the hypothesis of a brain-body growth trade-off, maximal brain energy demands co-occur with slowest body weight gain and body weight growth rate is tightly, inversely related to brain energy demands from infancy until puberty. These findings illustrate the brain’s dominance of the body’s energy budget early in life which has constrained the human pattern of growth. They also reveal an intriguing paradox: children devote a lifetime peak of the body’s energy budget to a non-negotiable expenditure at the same age that energy stores (body fat) are at their lifetime minimum. This energetically precarious state is evidence for the hypothesized importance of social buffering -- cooperative childcare melded with food sharing -- to the evolution of human’s uniquely energy-intensive brain. The talk will conclude with some of the public health implications of these findings including work underway to incorporate the study of brain energetics into studies of child development.
Chris Kuzawa, Harvard University