Functional MRI was used to test predictions from a theory of the origin of human language. The gradual theory suggests that human language and tool-use skills have a similar hierarchical structure, and proposes that tool-manipulation skills are related to the origin and evolution of human language. Our results show an overlap of brain activity for perceiving language and using tools in Broca's area. The location of this overlap suggests that language and tool use share computational principles for processing complex hierarchical structures common to these two abilities. The involvement of monkeys' homologous region during tool use suggests that neural processes for computation of complex hierarchical structures exist in primates without language, and could have been exapted to support human grammatical ability.