People make gestures when speaking because ancient fish first developed the brain circuity that coupled moving their pectoral fins to enhance communication while vocalizing according to research presented by Professor Andrew Bass of Cornell University at the July 3, 2013, session of the annual meeting of the Society for Experimental Biology.
Fish are the first animals known to have developed networks in the brain that coordinate the movement of their pectoral fins with vocalizations to enhance communication between individuals. Pectoral fins make an additional set of sounds that add to fish communication abilities.
The combination of vocal and pectoral appendage mechanisms of social signaling among birds and mammals have their ancestral origins in a single compartment of the hindbrain in fishes. Birds and fishes were the first species to develop this added communication skill.
When men came along, evolutionarily speaking, their brains already had the circuity to combine gesturing with vocalizations to enhance the effect and understanding of their intent while communicating with others.
Human language as it is known today may not have ever developed without the tiny part of the brain that couples gesturing with vocalization in fish. Men have taken the simple combination of gesturing and vocalization to the level of an art form that is a separate branch of scientific study.