The emergence of shared symbol systems is considered to be a pivotal moment in human evolution and human development. These changes are normally explained by reference to changes in people's internal cognitive processes. We present 2 experiments which provide evidence that changes in the external, collaborative processes that people use to communicate can also affect the structure and organization of symbol systems independently of cognitive change. We propose that mutual-modifiability--opportunities for people to edit or manipulate each other's contributions--is a key constraint on the emergence of complex symbol systems. We discuss the implications for models of language development and the origins of compositionality.