Among primates, brain size increases in relation to body mass. In humans, however, brain enlargement is much greater than in the great apes. The human brain is 5 to 6 times larger than would be expected in an average mammal of equivalent size. It is essentially over the last half-million years that this evolution has accelerated, with an increase in brain volume unrelated to any significant increase in body size. The brain is particularly costly in terms of energy. While in adults it represents only around 2% of total body mass, it consumes around 20% of basal metabolism. This proportion is much higher in young children. It peaks at over 65% of basal metabolism, around the age of five. At this age, somatic growth is still quite low, while the brain has already practically reached adult size. These extraordinary conditions in mammals were made possible in the Homo genus by a very marked improvement in the quality of food, with the consumption of meat and fat in particular.
In the course of human evolution, energy was also redistributed to the brain by reducing the energy costs of locomotion, feeding and reproduction. A very particular aspect of human development is the relatively small size of the brain at birth compared to its adult size. This adaptation is a response to obstetrical constraints and the energy constraints imposed on the mother. As a result, the developing brain of the young child is exposed to numerous external stimuli over a long period after birth. Its maturation lasts well beyond its volumetric growth, well into adolescence. Among fossil forms, it seems that a mode of development fully comparable to that of present-day humans only emerged relatively recently, and only in theHomo sapiens lineage.
