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Walking onto the Great Ape Wing at Yerkes Regional Primate Research Center (YRPRC) of Emory University in Atlanta, Georgia and walking into a noisy school room filled with excited children can have similar effects; both are guaranteed to make you wish you had your ear plugs. While the kids will usually outgrow their inability to contain their excitement, the chimpanzees that are housed at YRPRC will most likely never outgrow theirs. The primates may be a link to our evolutionary roots, but they are anything but quiet about it, especially when they are excited about meeting someone new. Their initial cacophony of screams, hoots, and grunts are loud enough to make the inventor of Tylenol your best friend, but before researchers reach for their medicine bottles, they have begun to ask themselves what the chimpanzees' vocalizations are all about and how they relate to the evolution of human speech.
While the noise produced by an excited group of kids may have the same effects on the cochlea as the noise produced by an excited group of chimpanzees, the motives for producing these sounds by each of the groups may or may not be so similar. Investigators of speech production typically recognize two related but very different types of sounds produced by the human voice box. The first are affective intonations that are produced primarily as an expression of an emotional state (i.e. screams, laughing, crying, etc.). Brain researchers have found that these noises seem to originate primarily in a sub-cortical region of the brain known as the limbic system. This is the same system that is commonly recognized as controlling various aspects of human emotion. The second type of sounds are those that make up the human speech repertoire — all of the vowel and consonant sounds capable of being produced by the human vocal tract. We as humans combine these sounds in various ways to produce words corresponding to some specific idea. Unlike the affective intonations originating in our limbic system, these semantic sounds seem to originate in the cortical areas of our brain, areas associated with more complex cognitive processes.
The common argument is that the affective sounds we produce are more primitive than our semantic sounds, as some equivalent of screams seems to exist in many lower species. The question, therefore, is when and how exactly our species evolved the capacity to control our affective vocalizations and began to intentionally form them into semantic utterances. Presumably, the first step in this process would have been the recognition that others hear our vocal output and that this vocal output can therefore be used to manipulate an audience. As our closest living evolutionary ancestors, chimpanzees could provide valuable insight into when this ability developed; thus, researchers in the Living Links Lab at YRPRC have recently begun to investigate chimpanzees’ use of their vocalizations and whether or not they appear to understand the effects that their vocalizations have on an audience.
The most recent experiments in this line of research have found that chimpanzees produce vocalizations faster when their audience is not looking at them than when their audience is looking at them, a phenomenon which the head of the research team, Dr. William Hopkins, describes as “simply incredible.” While the evidence may be incredible, there is nothing simple about such chimpanzee behavior as it seems to imply that the primates are using their vocalizations as an intentional means of gaining their audience’s attention before making an effort to communicate through hand gestures. This suggests not only that they can voluntarily control their vocalizations, but also that they understand their vocalizations as being a medium of communication or, at least, as an attention-getting tool.
While the evidence does seem to suggest that one of the first necessary steps in the development of human speech (a mental link between the production of vocalizations and their effect on an audience) exists in chimpanzees, it would be presumptuous to conclude that this means that chimpanzees use their vocalizations as an intentional means of communication all of the time and that the cacophony produced when a new person walks onto the Great Ape Wing means something beyond a simple expression of excitement. It seems quite clear that chimpanzees do not possess the ability to produce vocalizations other than affective intonations. The physiology of their vocal tracts simply makes it virtually impossible for them to produce the complicated series of sounds required for human speech. Even chimpanzees that are trained to use sign language or some other form of symbolic language to communicate with humans are incapable of producing more than a few distorted attempts at human speech. Presumably, therefore, at the same time that the ability to control vocalizations evolved, the physiology of the vocal tract was also undergoing necessary changes to make these vocalizations more capable of being manipulated into discrete phonetic sounds.
What such evidence regarding chimpanzees’ intentional use of these affective sounds could imply, however, is that chimpanzee vocalizations are not controlled exclusively by the limbic system but instead involve the cortex to some degree as well. Exactly how vocal control evolved to include cortical areas is still unclear, but further investigation of our noisy relatives, the chimpanzees, could provide helpful clues. Just as human children grow up and refine their ability to control their excitement and the mindless chatter that often accompanies it, the human species may have “grown up” from a primitive ancestor by gaining more and more cortical control of our vocalizations until we eventually developed our most unique human capacity: speech.
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