A newborn chimpanzee carries roughly 3% body fat. A human newborn arrives with about 15%—an extraordinary amount for a terrestrial mammal. Those chubby cheeks may reflect one of humanity’s most important evolutionary investments: the brain.
The human infant brain is exceptionally large and energetically expensive. Unlike muscles, it cannot simply stop working when food or oxygen becomes scarce. Fat provides a concentrated reserve that can be converted into ketones, an alternative fuel for the brain when glucose is limited.
The strongest evidence is comparative: human babies possess about five times the body-fat percentage of chimpanzee newborns, while human infants’ brains can consume a remarkably large share of their resting energy. Human body growth also slows when childhood brain-energy demands peak—evidence of an evolutionary trade-off between growing the brain and growing the rest of the body.
Well supported: humans are unusually fat at birth, and early brain development is extremely energy intensive.
Plausible but debated: natural selection specifically favored baby fat as emergency fuel protecting the developing brain. Insulation, immune demands and the loss of body hair may also have contributed.
Human baby fat, then, may be more than adorable padding. It could be part of the energetic safety system that made our exceptionally costly childhood brains possible.
