The tool-making source
The human brain is not only large relative to human body size, it is also lateralized, that is, it has specialized functions in each of the two hemispheres. (More details are presentedinChapter12.)Thosefunctionsthatcontrolthemotormovementsinvolvedin complex vocalization (speaking) and object manipulation (making or using tools) are very close to each other in the left hemisphere of the brain. It may be that there was an evolutionary connection between the language-using and tool-using abilities of humans and thatboth wereinvolved inthe development of the speaking brain.