Neoteny in humans
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; thinness of skull bones; the reduction of the brow ridge; the large brain; the flattened and broadened face; the hairless face; hair on the head; larger eyes; ear shape; small nose; small teeth; and the small maxilla and mandible.
Neoteny of the human body is indicated by glabrousness. Neoteny of the genitals is marked by the absence of a baculum ; the presence of a hymen; and the forward-facing vagina. 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 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. 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 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 said 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, said 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 said 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 said 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 said 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 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.
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. 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 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 said that the lengthened youth period of humans is part of neoteny.
Physical anthropologist Barry Bogin said that the pattern of children's growth may intentionally increase the duration of their cuteness. Bogin said 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 said that this allometry of human growth allows children to have a "superficially infantile" appearance longer than in other "mammalian species". Bogin said that this cute appearance causes a "nurturing" and "care-giving" response in "older individuals".
Genetic diversity, relaxed sexual selection and immunity
While upper body strength is on average more sexually dimorphic in humans than in most other primates, with the exception of gorillas, some fossil evidence suggests that male upper-body strength and muscular sexual dimorphism during human evolution peaked in Homo erectus and decreased, along with overall robustness, during the evolution of H.sapiens with its neotenic traits. The reduction in sexual dimorphism would suggest that taxa with high sexual dimorphism do not necessarily have an increased evolutionary advantage. This could be explained by the theory that sexual dimorphism could reduce genetic diversity in a population, i.e., if individuals are attracted to only highly masculine or highly feminine mates, then those without distinctly gendered features are excluded as potential partners, thus creating speciation.Neoteny in H.sapiens is explained by this theory as a result of relaxed sexual selection shifting human evolution into a less speciation-prone but more intraspecies adaptable strategy, decreasing sexual dimorphism and making adults assume a more juvenile form. As a possible trigger of such a change, it has been cited that while the Neanderthal version of the FOXP2 gene differed on only one point from the modern human version interacted strongly with other genes and was part of a gene regulatory network, the derived mutation that is unique to the modern human version of the gene knocked out the attachment to which RNA strains from other genes connected to it so that the gene was disconnected from its former genetic network.
It is suggested that since the FOXP2 gene controls synapses, its disconnection from a formerly complex network of genes instantly removed many instincts including ones that drove sexual selection. It is also suggested that it allowed more genetic variants that affect the phenotype to accumulate in humans, which in combination with increased synaptic plasticity made modern humans more able to survive environmental change and to colonize new environments and innovate. The theory that the origin of complex language was the most recent step in human evolution is considered unlikely as storytelling about past environments would be of little use in droughts with novel distributions of water while individual ability to make correct predictions would be useful and allow for differential survival that could eliminate the archaic version altogether, as opposed to selection for language in which some primitives could use imitation as long as there were enough storytellers in the group to keep the knowledge alive for long times which predicts that some individuals would have retained the archaic version if the modern version was for language.
H.sapiens is known from fossils to have had a mix of modern neotenic traits and older non-neotenic traits from its origin some 300000 years ago to the transition to early agriculture when the non-neotenic traits disappeared, which is theorized to be due to selection for the immune system adapting to survive a higher pathogen load caused by agriculture and men who retained more childlike traits being less burdened by weakening of the immune system from upper body musculature competing with the immune system over nutrients. It is argued that the genetic evidence of only a small part of the male population of the time of early agriculture passing on their Y chromosomes can be explained by the heredity of non-neotenic traits causing the male descendants of the non-neotenic men who were not killed by diseases in one generation to die from them in subsequent generations, leaving no Y chromosome evidence of their short term continuation of paternal bloodlines in present humans. Sexual selection for stereotypic masculinity causing most men to fail to breed is ruled out as it would have selected against neoteny, not for as the archaeological evidence shows.