Chris Kuzawa: Northwestern University

Humans are unusual in having a childhood stage characterized by a prolonged period of exceptionally slow growth. Many hypotheses have been proposed to explain why humans have evolved this life history stage. In this talk, Chris Kuzawa will discuss his recent collaborative work that quantifies the costs of human brain development and uses this information to shed light on the evolution of human life history. Compiling data from brain imaging studies, 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. Consistent with the hypothesis of a brain-body growth trade-off, body weight growth rate follows an inverse, linear relationship with brain glucose demands from infancy until puberty, and maximal brain glucose demands co-occur developmentally with the age of slowest body weight gain. These findings provide rare empirical evidence that humans evolved very slow body growth to free up energy for our unusually costly brain development. In addition, the finding that the peak in brain energy needs occurs after the age of weaning in most traditional small-scale societies shows that much of the energetic costs of human brain development are not provided by maternal metabolism, but by social provisioning and allocare.