Subcutaneous Fat in Humans
Humans are unusually fat as a species, and this trait is particularly evident at birth, when our newborns enter the world with a large layer of “baby fat.” One proposal to explain our large fat stores is that they evolved as a “brain battery,” or backup energy supply to buffer our costly brains, which are vulnerable to energy shortfall. Although humans are especially fat at birth, we lose much of our abundant fat stores after weaning, and human adiposity reaches its lowest level during childhood when brain costs are at their peak. If our large brains help explain the evolution of human baby fat, what accounts for these developmental changes in adiposity, and our relatively lean state by childhood? In this talk, I will argue that human body fat co-evolved not just with the energetically-demanding and vulnerable brain, but also with the cultural strategies that humans use to buffer offspring intake. The human infant’s need for ample baby fat traces to the fact that the main causes of nutritional stress at this age are infections, which force a reliance on onboard energy by reducing appetite and impairing digestion. As the child’s immune system matures, nutritional stress now traces to ecological factors that influence the group’s food availability, which are buffered through social and cultural strategies like food sharing, fall back foods, and cooperative childcare. Thus, our brainy newborns are forced to rely upon their own onboard energy reserves due to common infections, but by childhood we are less reliant upon this resource as a result of another uniquely human buffering system: food sharing and our cooperative strategy of caring for and feeding our young.