Human language may have evolved from manual gestures, which survive today as a "behavioral fossil" coupled to speech
In 1934 the behavioral psychologist B. F. Skinner found himself seated at the dinner table with the eminent philosopher Alfred North Whitehead, and proceeded to explain to Whitehead what behaviorism was all about. Obliged to offer a challenge, Whitehead uttered the sentence "No black scorpion is falling upon this table" and then asked Skinner to explain why he might have said that. Skinner attempted a reply more than 20 years later in an appendix to his 1957 book Verbal Behavior. He proposed that Whitehead was unconsciously expressing a fear of behaviorism, likening it to a black scorpion that he would not allow to intrude into his philosophy. The skeptical reader may be forgiven for concluding that the reply owed more to psychoanalysis than to behavioral principles.
Be that as it may, Whitehead had articulated one of the properties of language that seems to distinguish it from all other forms of communication, its generativity. Whereas other forms of communication among animals seem to be limited to a relatively small number of signals, and restricted to limited contexts, there is essentially no limit to the number of ideas or propositions that we can convey using sentences. We can immediately understand sentences made up of words that we have never heard in combination before, as Whitehead's sentence illustrates. Language also allows us to escape from the immediate present and to refer to events in other places and at other times. We can use language to fantasize, to describe events that have never existed and never will. This remarkable flexibility is achieved at least in part through the human invention of grammar, a recursive set of rules that allows us to generate sentences of any desired complexity. The eminent linguist Noam Chomsky has attributed this to a unique human endowment that he calls universal grammar. All human languages, he suggests, are variants on this fundamental endowment.
There has nevertheless been considerable progress in teaching something resembling language to captive apes. For example, a young pygmy chimpanzee called Kanzi, studied by Sue Savage-Rumbaugh of Georgia State University, has shown an impressive ability to use symbols on a computerized keyboard in a language-like way and even to understand moderately complex commands spoken in English. Nevertheless, the "utterances" that Kanzi produces typically consist of no more than two or three symbols strung together, sometimes in novel combinations, demonstrating a grammatical capacity that approximates that of a two-year-old human. Children go on to acquire a sophisticated, recursive grammar that is far beyond anything Kanzi, or any other ape, has mastered. There is little doubt that Kanzi and other great apes (and perhaps other species such as dolphins) can use symbols to represent actions and objects in the real world, but they lack nearly all of the other ingredients of true language. As Steven Pinker, a psychologist at the Massachusetts Institute of Technology, remarks in his 1994 book The Language Instinct, the apes just don't "get it."
Since the common ancestor of human beings and chimpanzees lived some 5 million years ago, it is a reasonable inference that grammatical language must have evolved in the hominid line at some point following the split from the line that led to the modern chimpanzee. There has been much disagreement as to when this might have happened. Some linguists, such as Derek Bickerton of the University of Hawaii, have supposed that it is impossible to conceive of grammar as having been formed incrementally; it must therefore have evolved as a single catastrophic event, probably late in hominid evolution. Indeed, Bickerton and others have suggested that it may have coincided with the emergence of Homo sapiens in Africa a mere 150,000 or so years ago. It might explain why H. sapiens came to dominate and ultimately replace all other hominid species, such as the Neanderthals in Europe or Homo erectus in Southeast Asia. Philip Lieberman of Brown University has also argued, on the basis of fossil evidence, that the vocal apparatus necessary to support articulate speech did not emerge until late in hominid evolution, and that even the Neanderthals, who survived until about 30,000 years ago, would have been severely challenged vocally. In his 1998 book Eve Spoke: Human Language and Human Evolution, he too argues that it was language that distinguished our own species from all other hominids. Attempts to trace present-day languages back to the original mother tongue (also known as Proto-World) also suggest a recent origin rather than one that antedates H. sapiens.
The position that language is a recent invention begs the question of whether the vocalizations of living primates—the alarm calls of monkeys and the hooting of apes, for example—are in some form related to human spoken language. Presumably our ancestors were also capable of such vocalizations, so why couldn't these calls have evolved into what we recognize as language? The strongest argument against this scenario is that human language and primate vocalizations are fundamentally very different phenomena. As Chomsky observed in his 1966 book Cartesian Linguistics, human speech is unbounded in its capacity to express thought and in its freedom from stimulus control, whereas animal communication systems either consist of a fixed number of signals or a fixed number of "linguistic dimensions," each associated with a nonlinguistic dimension. Peter MacNeilage of the University of Texas at Austin has also noted that primate vocalizations are "holistic," containing a message in themselves, whereas human vocalizations can be combined in novel ways to create a message. In my view, it seems more likely that the call-like vocalizations of our ancestors have persisted in the emotional cries of modern human beings—such as crying, laughing and screaming—rather than in speech.
Yet it is difficult to accept that an accomplishment as complex as human language could have evolved as an all-or-none event—a "big bang," as it were—late in the evolution of our species. Steven Pinker and his colleague Paul Bloom, now at the University of Arizona, argue that it must have evolved gradually, shaped by natural selection. Some primatologists, such as Richard W. Byrne of the University of St. Andrews, argue that the cognitive prerequisites of language (such as the ability to adopt the mental perspective of another individual) are present in the great apes, and they therefore antedated the split of our hominid ancestors from the chimpanzee line, probably by several million years.
How are we to reconcile these alternative perspectives? 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. This idea was suggested by the 17th-century French philosopher Étienne Condillac and revived in the 1970s by the American anthropologist Gordon W. Hewes. 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.