The study draws on evidence from such diverse fields as archaeology, evolutionary genomics, neurobiology, animal behaviour and clinical researcher on neuropsychiatric disorders. The authors hypothesize that a reduction of reactive aggression, resulting from the evolution and process of self-domestication of our species, could have led to an increase in our language complexity. According to the authors, this development would be caused by a reduced impact on brain networks of stress hormones, neurotransmitters that activate in aggressive situations, and which would be crucial for how we learn to speak. To show this interaction, the researchers analysed the genomic, neurobiological and singing-type differences between the domesticated Bengalese finch and its closest wild relative.
Looking for keys of the human language evolution in birdsong
A central aspect of the authorsʼ approach in the study is that the aspects that make language special can be elucidated by comparing them to the communication systems of other animals. “For instance, take how children learn to speak and how birds learn to sing: unlike most animal communication systems, juvenile birdsong and the child speech only develop properly in presence of adult tutors. Without the vocal support from adults, the great range of sounds available to humans and songbirds does not develop properly”, the researchers note.
Moreover, although speech and birdsong evolved independently, the authors suggest both communication systems are associated with similar patterns in brain connectivity and are negatively affected by stress: “Birds that are regularly under stress during their development sing a more stereotypical song as adults, while children with chronic stress problems are more susceptible to developing repetitive tics, including vocalizations in the case of Tourette syndrome”.
In this context, Kazuo Okanoya, one of the authors of the study, has been studying the Bengalese finch for years. This domesticated songbird sings a more varied and complex song than its wild ancestor. The study points out that the same happens with other domesticated species. The Bengalese finch has a weakened stress response and is less aggressive than its wild counterpart. In fact, according to the authors, there is more and more “evidence of multiple domesticated species having altered vocal repertoires compared to their wild counterparts”.
The impact of domestication in stress and aggressiveness
For the researchers, these differences between domestic and wild animals are “a central piece in the puzzle of the evolution of human language”, since our species and domesticated species share particular physical changes related to their closest wild species. Modern humans have a flattened face, a rounded skull and reduced tooth size compared to our extinct archaic relatives, the Neanderthals. Domesticates have comparable changes to facial and cranial bone structures, often accompanied by the development of other traits such as skin depigmentation, floppy ears and curly tails. Finally, modern humans have marked reductions in measures of stress response and reactive aggression compared to other living apes. These similarities do not stop at physical traits since, according to researchers, the genomes of modern humans and multiple domesticated species show changes targeting the same genes.
In particular, a disproportionate number of these genes downregulate the activity of the glutamate neurotransmitter system, which drives the brain response to stressful experiences. Authors note that “glutamate, the brainʼs main excitatory neurotransmitter, interacts with dopamine, in birdsong learning, aggressive behaviour, and the repetitive vocal tics of Tourette syndrome”.
Alterations in stress hormone balance in the striatum
In the study, authors show how the activity of glutamate tends to promote the release of dopamine in the striatum, an evolutionary old brain structure important for reward-based learning and motor output. “In adult songbirds, the increase in dopamine release in this striatal area is correlated with the learning of a more restricted song, which replaces experimental vocalizations typical of young birds”. “Regarding human beings and other mammals” —-the authors add—, “dopamine release in the dorsal striatum promotes restrictive and repetitive motor outputs, such as vocalizations, while other more experimental and exploratory behaviours are supported by the dopaminergic activity of the ventral striatum”.
According to the study, many of the glutamatergic signaling genes that have changed in recent human evolution code for receptors that reduce the excitation of the dorsal striatum. Meanwhile, these receptors tend not to reduce activity, and even promote, the dopamine release, in ventral striatal regions.
The authors say these alterations in the balance of stress hormones in the striatum were an important advance in the evolution of vocal learning in the lineage of modern humans. “These results suggest the glutamate system and its interactions with dopamine are involved in the process in which humans acquired their varied and flexible ability to speak. Selection against reactive aggression in our species would have altered the interaction of these neurotransmitters promoting the communicative skills of our species. These findings shed light on new ways for comparative biological research on the human language faculty” conclude researchers.
Thomas O'Rourke, Pedro Tiago Martins, Rie Asano, Ryosuke O. Tachibana, Kazuo Okanoya, Cedric Boeckx, “Capturing the Effects of Domestication on Vocal Learning Complexity", Trends in Cognitive Sciences, 2021, ISSN 1364-6613, https://doi.org/10.1016/j.tics.2021.03.007