The retention of typically juvenile features in adult life.
“The key to his argument is the concept of ‘neoteny’ — the retention of juvenile features into adult life. It is a commonplace of human evolution that the transition from Australopithecus to Homo and from Homo habilis to Homo erectus and thence to Homo sapiens all involved prolonging and slowing the development of the body so that it still looked like a baby when it was already mature. The relatively large brain case and small jaw, the slender limbs, the hairless skin, the unrotated big toe, the thin bones, even the external female genatilia — we look like baby apes.
The skull of a baby chimpanzee looks much more like the adult skull of an adult being than either the skull of an adult chimpanzee or the skull of a baby human being. Turning an ape-man into a man was simple matter of changing the genes that affect the rate of development of adult characters, so that by the time we stop growing and start breeding, we still look rather like a baby. ‘Man is born and remains more immature and for a longer period than any other animal,’ wrote Ashley Montagu in 1961.”(327, Red Queen)
“The combination of a slow change and long period of youthfulness means that as adults they have astonishingly large brains for an ape. Indeed, the mechanism by which ape-men turned into men was clearly a genetic switch that simply slowed the developmental clock. “(328, Red Queen)
The Red Queen by Matt Ridley