A remarkable characteristic of human language is its hierarchical structure. It is suggested that this structure is produced via a recursive combination operation. Adopting the hypothesis that a recursive combination of objects is a precursor of a syntactic operation, a computational study was conducted to clarify the formation of the adaptive value of recursive combination in human evolution. Previously, the authors identified two adaptabilities of recursive combination: the diversification of production methods and the diversification of products. In this paper, the ways the environment is formed in which the diversification of products effectively functions is analyzed. Based on the assumption that an increase in the severity of the environment, which occurred during human evolution, encourages the evolution of recursive object manipulation, a fitness function was designed that reflects resource competition among agents. It is shown that a recursive combination could evolve in a large product space and under strong resource competition. The evolution of recursive combination favors generalists when resource competition is very strong. Through discussion on the functions of recursive combinations during communication and planning as important adaptabilities of language, it is reasonable to suggest that action planning causes the evolution of the recursive combination operation rather than information transmission during communication.

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