The acceleration phenomenon of infant word acquisition can not be explained by simple neural learning mechanism of brain. To explain it, we have to assume some prior knowledge or structure in brain that is specific for language. However, the assumption is not plausible from evolutional view. It is natural to assume that the mechanism also exist in animal brain and is used for different purpose originally, and it is re-used for language in evolutional process. To examine possibility of the idea, we apply memory model PATON for the simulation of infant primary word acquisition process, and propose a model of its acceleration process. From the result, we discuss on how language is specific for human by examining the back grounding brain mechanism that enable the rapid word acquisition.