Jarvis, E. D., Duke University Medical Center, Durham, USA, jarvis@neuro.duke.edu

Evolution of brain systems for complex behavioral traits: song in birds and spoken language in humans.

Darwin’s theory of evolution by natural selection heavily influenced many fields of science, including neuroscience. In this regard, an understanding of the evolution and mechanism of how the brain controls complex behavioral traits has been a mysterious question for generations. One such complex trait is vocal learning, a critical behavioral substrate for song in song learning birds and spoken language in humans. A common feature of species that have this trait (songbirds, parrots, hummingbirds, and humans) is that they have forebrain to brainstem systems for vocal control, whereas those that produce only innate sounds have only the brainstem vocal system. Using behavioral, molecular, electrophysiological, and anatomical approaches we have found that the song learning systems of distantly related birds are embedded within a motor system also present in vocal non-learning birds that is involved limb and body movements. The song learning and adjacent motor systems share many properties in common, including motor-driven gene expression cascades and connectivity into two sub-networks ­ an anterior pathway that in songbirds is necessary for song learning and a posterior pathway that is necessary for song production. Comparative analyses suggest parallels with the spoken language brain system in humans. Based on these and other findings, we propose a motor theory for the origin of vocal learning, where unique brain systems used to learn and produce song and spoken language evolved out of a pre-existing vertebrate system that controls movement and motor learning, such as learning how to walk or fly. We propose that the pre-existing system is a fundamental design of the vertebrate brain, which consists of the two motor sub-pathways (anterior and posterior) that during embryonic development form parallels systems to control different muscle groups, and are innervated by sensory systems for feedback control of different motor behaviors. When vocal learning evolves, this pre-existing motor system is then connected to muscles of the vocal organ (syrinx in birds, larynx in humans) to control a specialized form of learned movement control - song and speech. In this manner, the evolution of brain pathways for vocal learning may have evolved independently of a common ancestor, but dependent on a pre-existing motor learning pathway used a scaffold. We suggest that this could be a possible mechanism for evolution of complex behavioral traits in birds and mammals beyond vocal learning.

Session #:K1
Date: 08-14-2009

Presentation is given by student: No

Presenting authors are underlined.