Steven Pinker and his colleague Paul Bloom, now at the University of Arizona, argue that it must have evolved gradually, shaped by natural selection. At least a partial answer is that language emerged not from vocalization, but from manual gestures, and switched to a vocal mode relatively recently in hominid evolution, perhaps with the emergence of H. sapiens . It has not found general favor among linguists or anthropologists, perhaps because it lacks parsimony and because there is no direct evidence that any of our hominid ancestors gestured rather than spoke. Even so, argument in its favor has continued to grow. Consider first some facts about primate evolution.
Primates are largely visual animals, perhaps as a result of a common adaptation to visual predation.
In people as in monkeys, vision is much more highly developed than any other sense modality, including hearing. Further, with the exception of humans, primates have much better cortical control over movements of the hands than over vocalization, which is largely restricted to emotionally based sounds controlled by subcortical structures. This means that the early hominids would have been much better preadapted for expressive, voluntary communication using the hands, and it perhaps explains why attempts to teach chimpanzees a version of sign language have been much more successful than attempts to teach them anything resembling human vocal language. Reciprocity of gesture, as a precursor to language, may go back even further, to our common origins with apes and monkeys perhaps 25 or 30 million years ago. These cells are in an area of the monkey cortex that appears to be homologous to Broca's area in the human brain, which is critically involved in the programming of human speech.