The antiquity and evolution of the neural bases of rhythmic activity

Abstract

The evolution of the anatomy and neural circuits that regulate the rhythm of speech can be traced back to the Devonian age, 400 million years ago. Epigenetic processes 100 million years later modified these circuits. Natural selection on similar genetic processes occurred during the evolution of archaic hominins and humans. The lungs and larynx—anatomy that produces the rhythmic fundamental frequency patterns of speech—have a deep evolutionary history. Neural circuits linking the cortex, basal ganglia, and other subcortical structures plan, sequence, and execute motor as well as cognitive acts. These neural circuits generate the rhythm of speech, singing, and chanting. The human form of the transcription factor FOXP2 increased synaptic connectivity and plasticity in basal ganglia circuits, enhancing motor control and cognitive and linguistic capabilities in humans as well as Neanderthals. The archeological record also suggests that Neanderthals passed spoken language. Homologous circuits existed in amphibians. In songbirds, the avian form of FOXP2 acted on similar neural circuits allowing birds to learn and produce new songs. Current studies point to natural selection on genetic events enhancing these and other neural circuits to yield fully human rhythmic speech, and motor, cognitive, and linguistic capabilities, rather than the saltation proposed by Noam Chomsky.

Speech rhythm as naturally occurring and culturally transmitted behavioral patterns

“…Complex behaviors cannot evolve without being functional, and intermediate stages in the evolution of each component of rhythmic cognition should exhibit adaptive value and gradual development in all genera (see also Lieberman). At the same time, strict parallel or consecutive development of components is not a prerequisite for the emergence and evolution of a specific component of rhythmic cognition.…”

“…They focused on the interaction of slow, delta (<4 Hz) and theta rhythms (4–8 Hz), as well as fast, gamma (30–40 Hz) rhythm emerging in the left and right hemispheres during speech perception to understand the nested coupling of neural oscillations at different frequencies, and the entrainment of the whole system to the speech signal. Lieberman provides an extensive literature review to give an overview of the genetic underpinning of neural substrates and networks involved in rhythm processing. He advocates the claim of evolutionary continuity in proximate mechanisms of rhythmic cognition.…”