Neoteny is the retention of juvenile traits well into adulthood. In humans, this trend is greatly amplified, especially when compared to non-human primates. Neotenic features of the head include the globular skull; (page 134), cited by:"In humans, neoteny is manifested in the resemblance of many physiological features of a human to a late-stage foetal chimpanzee. These foetal characteristics include hair on the head, a globular skull, ear shape, vertical plane face, absence of penal bone (baculum) in foetal male chimpanzees, the vagina pointing forward in foetal ape, the presence of hymen in neonate ape, and the structure of the foot. 'These and many other features', Bednarik says, 'define the anatomical relationship between ape and man as the latter's neoteny'". thinness of skull bones; the reduction of the brow ridge; the large brain; the flattened and broadened face; the hairless face; hair on (top of) the head; larger eyes; ear shape; small nose; small teeth; and the small maxilla (upper jaw) and mandible (lower jaw).
Neoteny of the human body is indicated by glabrousness (hairless body). Neoteny in humans is further indicated by the limbs and body posture, with the limbs proportionately short compared to torso length; longer leg than arm length; the structure of the foot; and the upright stance.
Humans also retain a plasticity of behavior that is generally found among animals only in the young. The emphasis on learned, rather than inherited, behavior requires the human brain to remain receptive much longer. These neotenic changes may have disparate roots. Some may have been brought about by sexual selection in human evolution. In turn, they may have permitted the development of human capacities such as emotional communication. However, humans also have relatively large noses and long legs, both peramorphic (not neotenic) traits, though these peramorphic traits separating modern humans from extant chimpanzees were present in Homo erectus to an even higher degree than in Homo sapiens, which means general neoteny is valid for the H.erectus to H.sapiens transition (although there were perimorphic changes separating H.erectus from even earlier hominins such as most Australopithecus). Later research shows that some species of Australopithecus, including Australopithecus sediba, had the non-neotenic traits of H.erectus to at least the same extent which separate them from other Australopithecus, making it possible that general neoteny applies throughout the evolution of the genus Homo depending on what species of Australopithecus that Homo descended from. The type specimen of A.sediba had these non-neotenic traits, despite being a juvenile, suggesting that the adults may have been less neotenic in these regards than any H.erectus or other Homo.
Neoteny and heterochrony
Heterochrony is defined as “a genetic shift in timing of the development of a tissue or anatomical part, or in the onset of a physiological process, relative to an ancestor”. Heterochrony can lead to a modification in shape, size and/or behavior of an organism through a variety of different ways. With heterochrony being more of an umbrella term, there are two different types of heterochrony where development timing is altered: paedomorphosis and peramorphosis. These terms refer to deceleration and acceleration of development, respectively. With neoteny (as described above) being defined as retention of juvenile features into adulthood, neoteny falls under paedomorphosis, as physical development of features is slowed.
Human evolution
Many prominent evolutionary theorists propose that neoteny has been a key feature in human evolution. Stephen Jay Gould believed that the "evolutionary story" of humans is one where we have been "retaining to adulthood the originally juvenile features of our ancestors". J. B. S. Haldane mirrors Gould's hypothesis by stating a "major evolutionary trend in human beings" is "greater prolongation of childhood and retardation of maturity." Delbert D. Thiessen states that "neoteny becomes more apparent as early primates evolved into later forms" and that primates have been "evolving toward flat face."
Doug Jones, a visiting scholar in anthropology at Cornell University, states that human evolution's trend toward neoteny may have been caused by sexual selection in human evolution for neotenous facial traits in women by men with the resulting neoteny in male faces being a "by-product" of sexual selection for neotenous female faces. Jones states that this type of sexual selection "likely" had a major role in human evolution once a larger proportion of women lived past the age of menopause. This increasing proportion of women who were too old to reproduce resulted in a greater variance in fecundity in the population of women, and it resulted in a greater sexual selection for indicators of youthful fecundity in women by men.
The anthropologist Ashley Montagu states that the fetalized Homo erectus represented by the juvenile Mojokerto skull and the fetalized australopithecine represented by the juvenile Australopithecus africanus skull would have had skulls with a closer resemblance to those of modern humans than to those of the adult forms of their own species. Montagu further listed the roundness of the skull, thinness of the skull bones, lack of brow ridges, lack of sagittal crests, form of the teeth, relative size of the brain and form of the brain as ways in which the juvenile skulls of these human ancestors resemble the skulls of adult modern humans. Montagu states that the retention of these juvenile characteristics of the skull into adulthood by australopithecine or H.erectus could have been a way that a modern type of human could have evolved earlier than what actually happened in human evolution.
The psychiatrist Stanley Greenspan and Stuart G. Shanker proposed a theory in The First Idea of psychological development in which neoteny is seen as crucial for the "development of species-typical capacities" that depend upon a long period of attachment to caregivers for the opportunities to engage in and develop their capacity for emotional communication. Because of the importance of facial expression in the process of interactive signaling, neotenous features, such as hair loss, allow for more efficient and rapid communication of socially important messages that are based on facially expressive emotional signaling.
Other theorists have argued that neoteny has not been the main cause of human evolution, because humans only retain some juvenile traits, while relinquishing others. For example, the high leg-to-body ratio (long legs) of adult humans as opposed to human infants shows that there is not a holistic trend in humans towards neoteny when compared to the other great apes. Andrew Arthur Abbie agrees, citing the gerontomorphic fleshy human nose and long human legs as contradicting the neoteny hominid evolution hypothesis, although he does believe humans are generally neotenous. Brian K. Hall also cites the long legs of humans as a peramorphic trait, which is in sharp contrast to neoteny.
On the balance, an all or nothing approach could be regarded as pointless, with a combination of heterochronic processes being more likely and more reasonable (Vrba, 1996).
Cooked food and protective genome simplification Based on calculations that show that more complex gene networks are more vulnerable to mutations as more conditions that are necessary but not sufficient increases the risk of one of them being hit, there is a theory that mutagens in food were more likely to be formed when food was burned while being cooked by human ancestors lacking modern cooking technology or the greater intelligence of modern humans. These commonly present mutagens thus selected against complex gene networks because longer genomes present a larger target for mutation. This theory successfully predicts that the human genome is shorter than other Great Ape genomes and that there are significantly more defunct pseudogenes with functional homologs in the chimpanzee genome than vice versa. While the protein coding portion of the FOXP2 gene is identical to that in Neanderthals, there is one point mutation in the regulatory part thereof (modern humans having a T where Neanderthals and all nonhuman vertebrates have an A). The observation that the effect of that difference is that the modern human FOXP2 gene does not interact with RNA from other genes while all other vertebrate including Neanderthal varieties did agrees with the idea that modern human origin was marked by the elimination (not formation) of complex gene networks, as predicted by this model. The researchers behind the theory argue that neoteny is a side effect of the destruction of gene networks preventing the firing of genetic activity patterns that marked adulthood in prehuman ancestors.
Growth pattern of children
In 1943 Konrad Lorenz noted that a newborn infant's rounded facial features might encourage guardians to show greater care for them, due to their perceived cuteness. He labeled this the Kewpie doll effect, because of their similarity to the eponymous doll.
Desmond Collins who was an Extension Lecturer of Archaeology at London University states that the lengthened youth period of humans is part of neoteny.
Physical anthropologist Barry Bogin states that the pattern of children's growth may intentionally increase the duration of their cuteness. Bogin states that the human brain reaches adult size when the body is only 40 percent complete, when "dental maturation is only 58 percent complete" and when "reproductive maturation is only 10 percent complete". Bogin states that this allometry of human growth allows children to have a "superficially infantile" appearance (large skull, small face, small body and sexual underdevelopment) longer than in other "mammalian species". Bogin states that this cute appearance causes a "nurturing" and "care-giving" response in "older individuals".
Milder punishment as a survival advantage One hypothesis of the premise that Stone Age humans did not record birth date but instead assumed age based on appearance holds that if milder punishment to juvenile delinquents existed in Paleolithic times, it would have imparted milder punishment for longer on those retaining a more youthful appearance into adulthood. This hypothesis posits that those who got milder punishment for the same breach of rules had the evolutionary advantage, passing their genes on while those who got more severe punishment had more limited reproductive success due to either limiting their survival by following all rules or by being severely punished.
Neotenous features elicit help The Multiple Fitness Model proposes that the qualities that make babies appear cute to adults additionally look "desirable" to adults when they see other adults. Neotenous features in adult females may help elicit more resource investment and nurturing from adult males. Likewise, neotenous features in adult males may similarly help elicit more resource investment and nurturing from adult females in addition to possibly making neotenous adult males appear less threatening and possibly making neotenous adult males more able to elicit resources from "other resource-rich people". Therefore, it could be adaptive for adult females to be attracted to adult males that have "some" neotenous traits.
Neotenous features elicits fitness benefits for mimickers. From the point of view of the mimicker, the neoteny expression signals appeasement or submissiveness. Thus, extra parental or alloparental care will more likely be administered because the mimicker appears to be more childlike and maybe ill-equipped to survive on its own. On the other hand, the recipient often faces aggression because of this signaled vulnerability.