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." 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. 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.
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. 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 and
mandible) as a function of age 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 (large
skull, small face, small body and sexual underdevelopment) 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 sexual 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 (not two points as the difference between chimpanzees and modern humans) 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.
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. Caroline F. Keating et al. tested the hypothesis that adult male and female faces with more neotenous features would elicit more help than adult male and female faces with less neotenous features. Keating et al. digitally modified photographs of faces of
African-Americans and
European Americans to make them appear more or less neotenous by either enlarging or decreasing the size of their eyes and lips. Keating et al. said that the more neotenous white male, white female and black female faces elicited more help from people in the
United States and
Kenya, but the difference in help from people in the United States and Kenya for more neotenous black male faces was not
significantly different from less neotenous black male faces. A 1987 study using 20 Caucasian subjects found that "babyfaced" individuals are assumed by both Korean and U.S. participants to possess more childlike psychological attributes than their mature-faced counterparts. As an additional part of the study, the asymmetric dominance paradigm was introduced, where a decoy option is presented to observe how it affects a person's decision on a certain matter. In the United States this asymmetric dominance paradigm induced a person to be more prone to a cuter item, whereas in Korea the opposite effect occurred. Cho concluded that this may be due to a different attitude toward cuteness, and so the advantages related to neoteny may be different in different countries. Aldo Poiani, an evolutionary ecologist at
Monash University,
Australia, said that he agrees that neoteny in humans may have become "accelerated" through "two-way
sexual selection" whereby females have been choosing smart males as mates and males have been choosing smart females as mates. Somel et al. said that 48% of the genes that affect the development of the
prefrontal cortex change with age differently between humans and chimpanzees. Somel et al. said that there is a "significant excess of genes" related to the development of the prefrontal cortex that show "neotenic expression in humans" relative to
chimpanzees and
rhesus macaques. Somel et al. said that this difference was in accordance with the neoteny hypothesis of human evolution. In terms of brain size differences, it has been noted that given the larger skull in neoteny humans, brain volume may be larger than an average human brain. It has been hypothesized that this is one mode of which the brains of
Homo sapiens grew as a species, as the prolonged development of neurons may have led to hypermorphosis, or excessive neuronal growth. Especially in the prefrontal cortex, brain pruning from childhood may be slower than usual, allowing for more time for neuronal maturation. This prolongs the transformation of otherwise very juvenile features.
Bruce Charlton, a
Newcastle University psychology professor, said what looks like immaturity — or in his terms, the "retention of youthful attitudes and behaviors into later adulthood" — is actually a valuable developmental characteristic, which he calls psychological neoteny. The ability of an adult human to learn is considered a neotenous trait. However, some studies may suggest the opposite of this idea of neoteny being beneficial. In general, the process of learning and developing new skills can be attributed to plasticity of neurons in the brain, especially in the prefrontal cortex for higher order decisions and activity. As neurons go through
ontogeny and maturity, it becomes more difficult to make new neuronal connections and change already present pathways and connections. However, during juvenile periods, cortical neurons are described to have higher plasticity and metabolic activity. In cases with neoteny, neurons are lingering in their more juvenile states since development is decelerated. On the surface this seems beneficial for the increased potential of younger cells. However, this may not be the case, as the consequences of the increased cellular activity must be taken into account. In general, oxidative phosphorylation is the process used to supply energy for neuronal processes in the brain. When resources for oxidative phosphorylation are exhausted, neurons turn to aerobic glycolysis in the place of oxygen. However, this can be taxing on a cell. Given that the neurons in question retain juvenile characteristics, they may not be entirely myelinated. Bufill, Agusti, Blesa et al. note how “The increase of the aerobic metabolism in these neurons may lead, however, to higher levels of oxidative stress, therefore, favoring the development of neurodegenerative diseases which are exclusive, or almost exclusive, to humans, such as Alzheimer's disease.” Specifically through various studies of the brain, aerobic glycolysis activity has been detected at high levels in the dorsolateral prefrontal cortex, which has functionality regarding the working memory. Stress on these working memory cells may support conditions related to neurodegenerative diseases such as Alzheimer's Disease. == Physical attractiveness ==