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A fundamental problem for understanding the evolution of human language has been the lack of significant parallels among nonhuman primates. Most researchers have focused on vocal plasticity—that is, the ability to learn novel sounds or modify call structure in response to social or environmental variables. Although songbirds, whales, dolphins, and some other mammals have this ability, nonhuman primates have appeared not to have it (1). Other studies found that nonhuman primates do not have a vocal tract that would allow them to produce the sounds of human speech (2) and that primates cannot take turns, a critical aspect of human conversation (3). All three points have been challenged by recent research (see the table), suggesting that nonhuman primates may after all be valuable models for understanding the evolution of speech and language.