Evolution of the mind

We thus see that the normal development of the brain depends on a critical interaction between genetic inheritance and environmental experience. The genome provides the general structure of the central nervous system, and nervous system activity and sensory stimulation provide the means by which the system is fine-tuned and made operational. But this fine-tuning does not depend on adding new components and connections in the way that a radio is assembled in a factory, but rather it is achieved by eliminating much of what was originally present. It is as if the radio arrived on the assembly line with twice as many electrical components and connections as necessary to work. If such an overconnected radio were plugged in and turned on, nothing but silence, static, or a hum would be heard from its speaker. However, careful removal of unnecessary components and judicious snipping of redundant wires would leave just those components and connections that result in a functioning radio. This snipping is analogous to the elimination of synapses in the human brain as part of its normal development.