These are exciting times for research in language evolution! Never before have we been so close to solving this evolutionary puzzle, and you can also be part of this exciting moment in science history!

It has been suggested that many human-specific traits, such as language, evolved as humans found new ways of controlling their environment. Examples of this include developing collective ambushing strategies to hunt for stronger animals; showing altruistic and inclusive behaviors towards the elderly, children, and disabled individuals, which minimizes the fitness consequences of their vulnerability; and taking part in traditional learning and division of labor, which allows for the development and optimization of agriculture, tool-making, and hunting, among other fundamental human activities.

Our ability to manipulate the environment to our own advantage would have developed as we evolved a more gregarious lifestyle, which helped us avoid many sources of natural selection such as predation and foraging efforts. In this context, human sociality would have entered a feedback loop, reducing common environmental selective pressures, but making itself the main source of selection in our species. Individuals showing more tolerance to social stress, and more cooperative, instead of aggressive, behaviors toward each other would have been granted selective advantages, and a number of the unique traits that define our species would have evolved as direct or indirect effects of these selective pressures for peaceful and cooperative living.

This process has been termed “self-domestication”, because, in addition to the acquisition of traits that breeders desire their organisms to evolve, one of the salient features of domestication is the replacement of commonly found environmental sources of selection by sources of selection that are imposed by socialization with individuals of a same or different species.

The relaxation of natural selection allows the domesticated species to evolve a so-called “domesticated phenotype”, which is characterized by a suite of traits mostly linked to the retention in adulthood of traits typically found at younger ages in the ancestral species, a phenomenon called “neoteny”. For example, domesticated animals tend to have smaller teeth, jaws, and faces, and are less aggressive than their wild ancestors - all of which are juvenile traits. Interestingly enough, these are all traits that humans show when compared to reconstitutions of hominid ancestors.

Most subscribers to the self-domestication hypothesis agree that relaxation of natural selection would have freed us from constraints that were limiting our behavioral repertoire, thus allowing for the evolution of greater behavioral flexibility and complexity. This means that instead of having specific behaviors innately “hard-wired”, humans could have learned complex new behaviors from personal experience, and by observing others. However, exactly which mechanisms influence human evolution is a subject of intense debate among advocates of the self-domestication hypothesis. One possible theory, posed by so-called “neutralists,” argues that the mere accumulation of mutations in genes that were previously constrained would be enough to create behavioral flexibility and complexity in our species; alternatively, “selectionists” are reluctant to accept such a simple and random process, and look for additional adaptive advantages associated with the evolution of flexible and complex behaviors.

What does it take for language to evolve?

When analyzed within the framework of the self-domestication hypothesis, the human language would have evolved following a relaxation of the environmental pressures for stereotyped vocalizations. Cooperation against predation, increased overall intelligence, and dependency on non-vocal forms of communication (e.g. gestures) are all factors that could have contributed to reduce the need for stereotyped vocalizations in our species.

Neutralists support the idea that once self-domestication removed the natural forces that had previously kept human vocalizations simple, brain areas formerly committed to diverse and independent functions could begin working together, ultimately leading to the evolution of language. Suspicious of this almost autogenous narrative, selectionists argue that relaxation is only the beginning, and not the end of the story, and maintain that positive selection is necessary to explain the evolution of human language. For example, voluntary control over vocalizations is one of the first and most significant steps in the evolution of human language. While other primates can only produce emotionally charged vocalizations (angry territorial calls, frightened alarm calls, etc.), humans can produce anything from a melodramatic speech, to a nonchalant comment about the weather. Neutralists posit that self-domestication would have allowed for the accumulation of mutations, and the subsequent degeneration of strictly emotional vocalizations in humans. Once free from the tyrannical control of emotional brain centers, vocal behavior could become subject to voluntary control. Selectionists agree that the degeneration of emotional dominance over vocalizations allowed for new brain synergies relating to voluntary vocal control to accidentally emerge. They, however, argue that these brain synergies could not have been kept and further optimized if individuals had not been targeted by strong selection, like that for efficient communication.

Okay, this all may sound pretty complex and speculative, but these ideas are testable! One of the ways to determine which of these hypotheses is more correct is to turn to comparative animal research. The self-domestication hypothesis has been investigated mainly by comparing wild species to their domesticated counterparts (e.g. wolfs vs dogs), in order to characterize the evolutionary forces and trade-offs underlying behavioral and morphological changes, and then evaluate whether or not the same evolutionary patterns would apply to humans.

Here at the Brain Evolution Laboratory in the Department of Anthropology at UC Berkeley, we chose to investigate the self-domestication hypothesis, and its relation to human language evolution, by studying the genes of a songbird species. Our model system is composed of two songbird strains: the domesticated Bengalese finch and its wild ancestor, the white-backed munia. Even though it was never bred for its singing ability, the Bengalese finch evolved a much more flexible vocal behavior than its wild ancestor. Once we understand the genetic differences between the wild and the domesticated songbirds, we can look for similar patterns of genetic change between humans and other primates.

You may think that comparing birds to humans is the same as comparing apples to oranges… Why not just compare human genes to chimpanzee genes? Find out more by watching our video, and help us uncover the mysteries of the human language!

Madza Y Farias-Virgens and Yevgeniya Sosnovskaya work in the Brain Evo Lab at UC Berkeley's Department of Anthropology.

Title image used with permission of CARTA/Jesse Robie.