Self-domestication is the process of adaptation of wild animals to humans, without direct human selective breeding of the animals. The biological processes in the evolution of humans and human culture are also referred to by the term "self-domestication".
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, Richard Wrangham goes on to say that "this leads to the thought that species can self-domesticate." 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.
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.
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.
Research done beginning in 1959 by the Soviet geneticist Dmitry Belyayev on silver foxes selectively bred only for tameness revealed that a whole range of other physical and behavioural features, such as neoteny, also appeared along with the tameness, characteristics that were not specifically the result of selective breeding by humans. White spots on their fur, floppy ears, rolled tails and smaller skulls were seen in the tame foxes, and the foxes were also described as "incredibly endearing."
Belyayev and his successors also selectively bred wild rats for tameness, with similar results. These results with selective breeding suggest that the natural process of self-domestication of animals can occur within a single human generation.
It has been argued that bonobos (Pan paniscus), relatives of the chimpanzee, have undergone self-domestication.
Self-domestication theories describe how humans developed and evolved. 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.
"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."
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.
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.
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.
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