A longstanding evolutionary process of human sociosexual selection for reduced reactive aggression, leading to correlated behavioral, physiological, and morphological trait changes consistent with “domestication syndrome” as seen in domesticated animal lineages.
Domesticated animals are known to show a range of correlated trait changes in common when compared to their wild ancestors or relatives (Leach 2003; Wilkins et al. 2014; Zeder 2015). These traits have been experimentally demonstrated to emerge in response to sustained breeding selection for less reactive aggression (Trut et al. 2009). Modern humans show similar changes when compared to fossil remains of earlier Homo sapiens (Cieri et al. 2014). This evidence, coupled with high levels of cooperation and sociability, has led multiple authors to diagnose a process of “self-domestication” in human evolution (Cieri et al. 2014; Hare 2017; Leach 2003; Thomas and Kirby 2018). Human self-domestication is thought to have substantially enhanced our capacity for amicable social interaction, promoting language acquisition, shared culture, and technological capacity.
Domesticated lineages are known to show a syndrome of correlated traits in common when compared to their non-domesticated ancestors or relatives (Leach 2003; Wilkins et al. 2014). These traits typically include behavioral docility(especially dampened reactive aggression); reduced sexual dimorphism; smaller brain sizes; lower skeletal robusticity; smaller teeth; floppy ears; pedomorphic morphology, physiology, and behavior; earlier sexual maturity; nonseasonal estrus; less prognathism; and altered pigmentation (Leach 2003; Trut et al. 2009; Wilkins et al. 2014).
Long-standing experimental breeding of farmed silver foxes has shown these traits will emerge in response to selection for lower aggressive reactivity suggesting the existence of underlying correlatory mechanisms (Trut et al. 2009). Similar behavior-based selection has been shown to have the same effect in other vertebrates, including rats, mink, and chickens. Given most traits of domestication syndrome can be associated, directly or indirectly, with various roles of embryonic neural crest cells, the most widely accepted proximate explanation for this collection of traits is mildly altered function in this cell lineage (Wilkins et al. 2014).
Neural crest cell contributions to the autonomic nervous system, especially the adrenal medulla, and the pituitary suggest an influence on bodily endocrine regulation, which could explain both moderated behavior and shifted developmental processes (Trut et al. 2009; Zeder 2015). As such, major heritable shifts could be accomplished via altered neural crest cell function with only minor genetic mutation. Secondary influence from endocrine change could compound any direct effect of altered neural crest cell behavior on specific tissues, organs, or structures. For example, dampened adrenal reactivity might directly moderate aggressive responses while an extended developmental period could maintain juvenile sociability and cognitive flexibility and both would promote enhanced sociability in adults (Wilkins et al. 2014).
Interestingly, a form of “self-domestication” has been described in wild bonobos (Pan paniscus) whose features, in comparison to their near relative, the chimpanzee (P. troglodytes), strongly resemble those attributed to domestication syndrome (Hare et al. 2012). In this case, aspects of the bonobo’s socioecology – especially relatively elevated female social status – are thought to have supported sociosexual selection in favor of less reactively aggressive males.
Interestingly, multiple authors have suggested that modern humans show signs of domestication syndrome when compared to fossil remains of ancestral Homo sapiens (e.g. see: Hare 2017; Leach 2003; Thomas and Kirby 2018; Wrangham 2018). Stated evidence for human domestication includes reduced sexual dimorphism; reduced cranial capacity; smaller tooth sizes; lower skeletal robusticity; reduced craniofacial masculinity and prognathism; and enhanced sociability and social complexity (Cieri et al. 2014; Hare 2017; Leach 2003). Given that selection for lower aggressive reactivity has been experimentally demonstrated to cause domestication syndrome in other vertebrates, similar selection appears likely in human evolutionary history (Cieri et al. 2014; Hare 2017; Wrangham 2018).
Mechanisms explicitly proposed to drive human self-domestication via selection against aggressive reactivity include: (1) social benefits derived from dampened aggressive tendencies which may enhance the fitness of cooperative and sociable group members (Cieri et al. 2014); (2) ostracism or capital punishment of particularly aggressive individuals by cooperative coalitions, which would dampen the fitness of aggressive behavioral traits (Wrangham 2018); and (3) female mating preferences which might select against aggressive males in favor of more sociable individuals with elevated capacity for paternal investment (Cieri et al. 2014; Gleeson and Kushnick 2018). Other modes of selection against reactive aggression, though not previously associated with self-domestication, could include the reproductive benefits of increased alloparenting ability, and selection for enhanced group collaboration during intergroup conflict.
Human self-domestication holds substantial implications for the emergence of human sociability and the study of mechanisms which contributed to it, at individual and collective levels (Cieri et al. 2014; Hare 2017; Thomas and Kirby 2018). If, as suggested by Cieri et al. (2014), self-domestication has operated by dampening expressions of physiological masculinity, the process would be highly relevant to expanded studies of human social evolution and factors affecting social interaction today. For example, self-domestication research might increase our understanding of sexually differentiated aggression and antisocial behavior witnessed cross-culturally across most human groups.
Domestication syndrome is a heritable physiological condition experimentally demonstrated to emerge in a range of vertebrates under breeding selection for diminished aggressive stress reactivity. Fossilized morphological evidence, along with noted capacities for cooperation and social complexity, points to an evolutionary process of human self-domestication occurring via sociosexual selection for reduced aggression. This phenomenon suggests a cumulative evolutionary influence of past social interaction and implies significant fitness benefits from enhanced social capacity in humans.
Broadly speaking, domestication-related research has a long pedigree of useful contributions to evolutionary knowledge. It has enhanced understanding of relevant selective mechanisms and the underlying physiology of inheritance. An expanding literature on this topic suggests ongoing research into the genetic, physiological, and selective factors associated with vertebrate domestication, and human self-domestication, will continue to provide useful insights for cultural and evolutionary anthropology in future.
- Hare, B. (2017). Survival of the friendliest: Homo sapiens evolved via selection for prosociality. Annual Review of Psychology, 68(1), 155–186. https://doi.org/10.1146/annurev-psych-010416-044201.CrossRefPubMedGoogle Scholar