From Food of the Gods by Terrance McKenna
The evolutionary advantages of the use of speech are both obvious and subtle. Many unusual factors converged at the birth of human language. Obviously speech facilitates communication and cognitive activity, but it also may have had unanticipated effects on the whole human enterprise.
Some neurophysiologists have hypothesized that the vocal vibration associated with human use of language caused a kind of cleansing of the cerebrospinal fluid. It has been observed that vibrations can precipitate and concentrate small molecules in the spinal fluid, which bathes and continuously purifies the brain. Our ancestors may have, consciously or unconsciously, discovered that vocal sound cleared the chemical cobwebs out of their heads. This practice may have affected the evolution of our present-day thin skull structure and proclivity for language. A self-regulated process as simple as singing might well have positive adaptive advantages if it also made the removal of chemical waste from the brain more efficient.