Monkey vocal tracts are speech-ready

Abstract

Despite repeated attempts, no nonhuman primates have ever been trained to produce speech sounds, not even chimpanzees raised from birth in human homes (1). Humans appear to be the only primates with a capacity to flexibly control their vocalizations and to integrate respiration, phonation, and vocal tract movements in an intricate manner as required for speech (2–4). Since Darwin’s time, two hypotheses have been considered to be the likely explanations for this fact. The first “neural” hypothesis is that other primates lack the brain mechanisms required to control and coordinate their otherwise adequate vocal production system; Darwin favored this hypothesis, and it was widely accepted until the 1960s (5). The second “peripheral” hypothesis, in contrast, identifies the basis of primate vocal limitations as the anatomy and configuration of the nonhuman primate vocal tract. This hypothesis is widely accepted today, largely due to a seminal 1969 Science paper by Lieberman et al. (6), which used a computer program to explore the phonetic capability of a rhesus macaque and, by extension, other nonhuman primates. They concluded that “the vocal apparatus of the rhesus monkey is inherently incapable of producing the range of human speech” [(6), p. 1187]. Later work used the same methods and reached the same conclusions for chimpanzees (7), and thus inaugurated the reign of the “peripheral” hypothesis, which today remains a widely accepted “textbook fact” concerning human speech (8–13). For example, “early experiments to teach chimpanzees to communicate with their voices failed because of the insufficiencies of the animals’ vocal organs” (9). This now-traditional hypothesis has an important implication for the evolution of human language: that the broad phonetic range used in modern human speech required key changes in peripheral vocal anatomy during recent human evolution. Here, we present new data, based on x-ray images from living monkeys, that sharply challenge this hypothesis and thus its implication concerning language evolution.