The draft of chimpanzee genome was published recently (Nature September 2005). It has been known that chimpanzees share more than 98% of our DNA and almost all of our genes. In addition to the striking genetic closeness, the studies on chimpanzees in both laboratories and natural habitats have revealed that they share with us many cognitive abilities (Tomasello & Call 1997; Hauser 2005) , and exhibit complex social behaviors (de Waal 2005) and rich cultural traditions which are transmitted

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... gh social learning (Whiten 2005). In particular, chimpanzees have demonstrated cognitive abilities which are considered crucial for learning and using language, including manipulation of symbols, understanding of abstract concepts, intention reading and attention sharing, the ability of imitation, and so on. While chimpanzees share a strikingly high degree of similarity with humans, the question about language origin become more intriguing: if chimpanzees are so close to humans in cognitive abilities and social behaviors, why can't they invent a complex communication system with compositionality, hierarchy, and recursion similar to humans? Elman (2005) points out that "language sits at the crossroads of a number of small phenotypic changes in our species that interact uniquely to yield language as the outcome" (p114). It is these small phenotypic differences between human and chimpanzees that result in a communication means of a totally different nature. The study of complex nonlinear systems has shown abundant examples of such small quantitative differences leading to phase transitions, i.e.