Neoteny in Humans

...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 genitalia—we look like baby apes.

The skull of a baby chimpanzee looks much more like the skull of an adult human being than either the skull of an adult chimpanzee or the skull of a baby human being. Turning an apeman into a man was a 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.

The evidence for neoteny is extensive. Human teeth erupt through the jaw in a set order: the first molar at the age of six, compared with three for a chimp. This pattern is a good indication of all sorts of other things because the teeth must come at just the right moment relative to the growth of the jaw. Holly Smith, an anthropologist at the University of Michigan, found in twenty-one species of primate a close correlation between the age at which the first molar erupted and body weight, length of gestation, age at weaning, birth interval, sexual maturity, life span, and especially brain size. Because she knew the brain size of fossil hominids, she was able to predict that Lucy would have erupted her first molar at three and lived to forty, much like chimpanzees, whereas the average Homo erectus would have erupted his at nearly five and lived to fifty-two.

Matt Ridley, The Red Queen: Sex and the Evolution of Human Nature (2003-05-01)

• evolution
• neoteny