Dr. Philipp Gunz | What apes and Neanderthals can tell us about the evolution and development of our own brain

My group studies the evolution of the brain’s volume, shape and its organisation based on imprints of fossilised braincases. Using virtual skull reconstructions based on computed tomographic scans of our fossil ancestors and relatives we compare the evolutionary changes of brain development in our lineage. Moreover, we use in vivo ultrasound imaging of pregnant bonobos to compare fetal brain development in humans and our closest living relatives.

We document a uniquely modern human pattern of endocranial development that separates us from chimpanzees and Neanderthals. Analysing adult and subadult Neanderthals in a comparative developmental framework of recent modern humans and chimpanzees we show that many aspects of the developmental patterns are shared by the three groups. However, in the first year of life, during a critical phase for cognitive development, modern humans depart from this presumably ancestral pattern. Between birth and the eruption of the deciduous dentition, modern human brains change from an elongated to a more globular shape owing to both a relative expansion of the cerebellum and parietal bulging. The distinct globular shape of the braincase of adult modern humans is therefore largely the result of a “globularization phase” in the first few months of life, which is not present in chimpanzees and Neanderthals.

Our results suggest that endocranial shape changes are linked to the growth rate of the brain, as the overall shape of the brain becomes more globular during periods of rapid brain growth. In contrast to bonobos, human brains start becoming more globular around gestational week 27. This coincides with the onset of a cerebellar growth spurt in humans that continues until the first three postnatal months. The rapid prenatal and postnatal enlargement of the cerebellum is a key growth process underlying the modern human globularization phase.