User:Psbheb/sandbox

Self-domestication is the process of adaptation of wild animals to life with humans, without direct human selective breeding of the animals. The term has also been used to refer to biological processes in the evolution of hominids, particularly humans and bonobos, toward collaborative ("tamer") behavior and control of reproduction. Self-domestication is generally characterized by a reduction in aggressive behavior, including interspecific and intraspecific antagonism. Spandrels, or evolutionary byproducts, also accompany self-domestication, including depigmentation, arrested development, and reduced sexual dimorphism.

In animals
Wild animals may self-domesticate when tame behaviour enhances their survival in the vicinity of human beings. Tolerating or even enjoying the close proximity of humans in order to feed near them, and a lessening of natural adult aggression, are two aspects of tameness. An environment that supports the survival of tame animals can lead to other changes in behaviour and appearance as well.

Smaller skulls on tame animals have been noticed in other species. Noticing that a dog's skull looks like that of a juvenile wolf, British primatologist Richard Wrangham goes on to say that "this leads to the thought that species can self-domesticate." However, the evolutionary biologist Abbey Drake has found that "dogs are not paedomorphic wolves."

Other characteristics that are associated with juvenility such as barking and meowing (sounds used by wolf cubs and kittens of large felines, respectively, to communicate with their parents), increased playfulness and reduced aggression, may also be seen in tame animals.

Cats
Self-domestication is described by biological anthropologist Richard Wrangham as being in an environment where lessening of aggression was beneficial for survival. As grain plants and livestock became domesticated 9,000 years ago in the Fertile Crescent, wild cats adapted to living with humans, hunting rodents in grain stores and "abandoning their aggressive wild-born behaviors", which led to today's house cats.

Dogs
While humans may have intentionally domesticated wolves into dogs, an alternate hypothesis is that wolves effectively domesticated themselves by establishing a mutually beneficial relationship with prehistoric humans. They scavenged on the remains of the prey animals left by the prehistoric people at the human settlements or the kill sites. Those wolves that were less anxious and aggressive thrived, continued to follow the prehistoric humans, and colonized the human-dominated environments, generation after generation. Gradually, the first primitive dogs emerged from this group.

Foxes
In 1959, geneticist Dmitry K. Belyaev initiated an experiment to expedite the rate of fox domestication, an over thousand year process, into the time constraints of an experiment. Belyaev had noticed that domesticated animals had undergone numerous changes from their wild selves, including body size reductions, depigmentation, curling tails, and drooping ears. On a farm in Estonia, Belyaev began with raising 30 male foxes and 100 vixens, evaluating the foxes for tameness for which he believed variance arose from genes. In the span of 40 years, researchers had facilitated 35 generations of selection, and they identified many traits supporting domestication. For example, domesticated foxes show a delay in plasma corticosteroids, meaning a delayed response of fear. Additionally, domesticated foxes demonstrated white facial patterns, floppy ears, and curled tails.

The sixty-year experiment is still ongoing, withstanding various funding issues.

Bonobos (Pan paniscus) and chimpanzees (Pan trogolodytes)
The evolutionary anthropologist Brian Hare proposes that bonobos (Pan paniscus) have undergone self-domestication, in the hominid evolutionary sense. Despite their close relation to chimpanzees, bonobos exhibit different levels of aggression. They participate in calmer displays than chimpanzees, and researchers have never observed a bonobo male occupying a group's highest rank. In addition to observed behavior, morphological evidence supports the self-domestication hypothesis. Bonobos, who exhibit a much less aggressive demeanor, have a cranial reduction up to 20%, flattening of facial project, smaller tooth size, and diminished sexual dimorphism. Bonobos also have a white tail-tuft and pink lips, coloration that does not change with age. In chimpanzees, this depigmentation is only observed in younger apes. Thus, this depigmentation observed in bonobos acts as morphological evidence in favor of extended periods of juvenilized traits. The cognitive traits that have caused these phenotypic differences to arise are not fully clear; however, cognitive differences between bonobos and chimpanzees have been established in the orbitofrontal cortex, motor cortices, and hippocampus.

Marmoset monkeys (Callithrix jacchus)
The neuroscientist Asif A. Ghazanfar revisited the self-domestication hypothesis in marmoset monkeys, a previously undocumented species in application to the self-domestication hypothesis. When marmoset parents provide more vocal feedback to their offspring, the juvenile marmosets correspondingly grow a larger size of a species-distinctive white facial fur patch. Their pleiotropic linkage may be neural crest cells, as the white facial patch lacked melanocytes, which originate from neural crest cells. With the reduction in pigmentation, a common byproduct of self-domestication, marmoset monkeys provide further support for the hypothesis.

Origins of self-domestication
Self-domestication theories describe how humans developed and evolved. "Contemporary reproductive technologies such as selective abortion and genetic screening are typical examples where our self-domestication is most directly apparent", writes philosopher Masahiro Morioka, who also says that "Through domesticating ourselves like cattle, people began civilization."

Gregory Stock, director of the UCLA School of Medicine's Program of Medicine, Technology and Society, describes self-domestication as a process which "... mirrors our domestication [of animals] ... we have transformed ourselves through a similar process of self-selection ... our transformation has been primarily cultural, but it has almost certainly had a biological component."

Bonobos
Clark & Henneberg argue that during the earliest stages of human evolution a more paedomorphic skull arose through self-domestication. This assertion is based upon a comparison of the skull of Ardipithecus and chimpanzees of various ages. It was found that Ardipithecus clustered with the infant and juvenile species. The consequent lack of a pubertal growth spurt in males of the species and the consequent growth of aggressive canine armoury was taken as evidence that Ardipithecus evolved its paedomorphic skull through self domestication. As the authors state, comparing the species with Bonobos:

"Of course A. ramidus differs significantly from bonobos, bonobos having retained a functional canine honing complex. However, the fact that A. ramidus shares with bonobos reduced sexual dimorphism, and a more paedomorphic form relative to chimpanzees, suggests that the developmental and social adaptations evident in bonobos may be of assistance in future reconstructions of early hominin social and sexual psychology. In fact the trend towards increased maternal care, female mate selection and self-domestication may have been stronger and more refined in A. ramidus than what we see in bonobos. "

Richard Wrangham, an eminent biological anthropologist, further built upon this body of research, addressing bonobos and chimpanzees could elucidate development of aggression in humans. Reconciling both the Hobbes-Huxley and Rousseu-Kropotkin hypotheses, Wrangham posits that humans demonstrate both proactive and reactive aggression. Proactive aggression is defined as an attack that was planned, motivated by achieving an end goal. Reactive aggression, much more closely associated with anger, is characterized as an immediate response to a threat—the human equivalent being "bar fights." Both proactive and reactive aggression are commonly observed in chimpanzee behavior. Similar to chimpanzees, humans engage in proactive aggression. However, humans do not have such a high propensity for reactive aggression, modeling more so after bonobos. This lends further evidence to supporting the self-domestication hypothesis, of which decreased reactive aggression is a central trait.

Further research has confirmed that Ardipithecus possessed paedomorphic cranial base angulation, position of the foramen magnum as well as vocal tract dimensions. This was interpreted as not only evidence of a change in social behavior but also a potentially early emergence of hominin vocal capability. If this thesis is correct then not only human social behavior but also language ability originally evolved through paedomorphic skull morphogenesis via the process of self-domestication.

Marmoset monkeys
Ghazanfar's study with marmoset monkeys provided further evidence of the self-domestication hypothesis, which has also emerged in humans. He proposed that the common denominator, and thus a likely influence on domestication, between both marmosets and humans was cooperative breeding. In marmosets, cooperative breeding was a mating system driven by the their production of dizygotic twins, whereas in humans, it may be driven due to the extensive amount of parental care that goes into an offspring's early years of development.

Human evolutionary history
The most comprehensive case for human self-domestication has been proposed for the changes that account for the much later transition from robust humans such as Neanderthals or Denisovans to anatomically modern humans. Occurring between 40,000 and 25,000 years ago, this rapid neotenization has been explained as the result of cultural selection of mating partners on the basis of variables lacking evolutionary benefits, such as perceived attractiveness, facial symmetry, youth, specific body ratios, skin tone or hair, none of which play any role in any other animal species. This unintentional auto-domestication, coinciding with the introduction of imagery of female sexuality, occurred simultaneously in four continents then occupied by hominins. It led to rapid changes typical for domestication, such as in cranial morphology, skeletal architecture, reduction in brain volume, to playful and exploratory behavior, and the establishment of thousands of deleterious conditions, syndromes, disorders and illnesses presumed absent in robust humans. This hypothesis effectively replaces the Replacement Hypothesis (known as "African Eve theory") and explains the relatively rapid transition as a culturally induced domestication process still continuing today. It also explains the rise of exograms and their role in selecting for competence in the use of external memory traces.

Theoretical criticism
The self-domestication hypothesis has been met with some degree of criticism. Some researchers have argued that the human brain is peramorphic, instead of paedomorphic. Wrangham puts forth that these arguments do not address the evolution of Homo sapiens from their most recent ancestor, instead focusing too heavily on a direct contrast between apes and humans.

Social implications
The idea of self-domestication was used by early Social Darwinism which, according to psychiatrist Martin Brüne in an article "On human self-domestication", developed from the idea that humans could perfect themselves biologically