In their last review Jean-Jacques Hublin, Simon Neubauer and Philipp Gunz address the effects of hominins’ life histories in brain evolution. Encephalization in humans involved energetic costs that were sustained through changes in social structure and metabolic adaptations, including changes in the diet quality, as explained by the Expensive Tissue Hypothesis, and the ability to store energy in fat tissue, the primary source of energy for neonate brains. Because of the constraints of birth-giving associated with bipedalism, human brains develop mainly after birth. Also because of this prolonged development, the brain is exposed to a rich environment during its wiring process, with the child furthermore protected by the social community.
The study of fossil hominins’ brain development is only possible through the analysis of their endocranial casts, using cross-sectional samples. To establish their life histories it is necessary to attribute an age at death, generally according to the eruption timing of the teeth. Beyond variation in brain size, signs of brain reorganization can be investigated in fossil species to get insights about their cognitive capacities. Regarding Australopithecus, it is not yet clear how their brain development and life histories were more like that of humans or that of chimps. Homo erectus had body proportions and social structures closer to that of modern humans, but different brain organization and smaller brain capacity than that of latter Homo, pointing to a faster life history. Homo sapiens and H. neanderthalensis separately evolved similar brain capacities, although with different morphologies, and had different life histories, faster in the latter. Brain organization differs soon after birth, as modern humans undergo the so-called globularization phase, which does not exist in chimps or Neanderthals. This globular shape of the brain is characterized by the bulging of the parietal areas which may be linked to reorganization of the internal regions, like the precuneus.
There is still a debate about whether or not life histories in fossils can be investigated in terms of brain growth and development. The different brain morphology of Australopithecus when compared with African apes suggests that brain reorganization could have pre-dated encephalization. At last, our patterns of socio-cultural evolution might have been fundamentally responsible for the adaptive changes essential for the evolution of our big brains.