The development of the interpersonal functions of language is a key step in language ontogeny. Archaeological evidence of hominids moving raw materials across the landscape
suggest that changes in the interpersonal communication abilities of hominids represent major events in human language evolution. The earliest hominids moved raw materials short distances, suggesting home-range sizes, social complexity and interpersonal abilities similar to those of primates. A transition from primate communication
to a protolanguage is indicated by a large increase in raw-material transfer distances at about 1.2 million years ago. The increase in transfer distances results from the ability to pool social and environmental information using a protolanguage. The transition to human language is suggested by the emergence of long-distance exchange
networks during the African Middle Stone Age. The operation of exchange networks requires the full panoply of human interpersonal communication abilities, such as the use of symbols in social contexts, expression of displacement, the expression of multiple degrees of intentionality and recursiveness. The results of computer simulations show
that this transition from protolanguage to full language may have resulted from language adapting itself rather than any specific biological or cultural mutation.