Human language is qualitatively different from animal communication systems in at least two separate ways. Human languages contain tens of thousands of arbitrary learned symbols (mainly words). No other animal communication system involves learning the component symbolic elements afresh in each individual's lifetime, and certainly not in such vast numbers. Human language also has complex compositional syntax. The meanings of our sentences are composed from the meanings of the constituent parts (e.g. the words). This is obvious to us, but no other animal communication system (with honeybees as an odd but distracting exception) puts messages together in this way. A recent theoretical claim that the sole distinguishing feature of human language is recursion is discussed, and related to these features of learned symbols and compositional syntax. It is argued that recursive thought could have existed in prelinguistic hominids, and that the key step to language was the innovative disposition to learn massive numbers of arbitrary symbols