A Review of:
Nishimura, Takeshi; Mikami, Akichika; et al. 2006. Descent of the Hyoid in Chimpanzees: Evolution of Face Flattening and Speech. Journal of Human Evolution 51 (2006) 244-254.
The human's ability of speech is an extraordinary adaptation in the evolutionary spectrum. But why do we have the ability of speech, when our closest known relatives, the chimpanzees, do not? This study, completed by Nishimura, attempts to find an explanation for this phenomenon.
Three chimpanzees were studied from the time they were four months old to the time they were five years old. These ages were compared to humans being studied from one month of age to nine years of age. The ages being studied in both chimpanzees and humans were based on the stages of dental development in each species and grouped into three vital periods; early infancy, late infancy, and the juvenile period. The studies during the juvenile period were stopped halfway through for both organisms. The researchers took magnetic resonance images (MR images) in order to get measurements from each of the three chimpanzees at set intervals of the supralaryngeal vocal tract (SVT). This tract, along with the vocal folds, produces human speech sound and many primate vocalizations when vibrations occur. Both parts of the SVT, the vertical pharyngeal and the horizontal oral cavities, were measured. In order to get these measurements in the same way each time, the chimpanzees were anesthetized and then placed with the exact same positioning of the body and head, relative to the neck. These measurements were then compared to the corresponding measurements from the previously studied humans.
The studies show how the chimpanzees' SVT and humans' SVT differ, by comparing both the horizontal and vertical cavities to the corresponding cavities in the other species. Resulting from the comparison of the SVT between species, it clearly shows, in figure 4b on page 248, that the chimpanzees' horizontal oral cavities continually increase through age five without leveling at any point, whereas the human's horizontal oral cavity increases at first, but then starts leveling at approximately four (comparable to age two in chimpanzees) years of age with only slight wavering thereafter. In the same figure, the comparison between the vertical pharyngeal cavities illustrates that, in both species, the length increases gradually at about the same rate with the chimpanzees' always being smaller in length then the human's. The differences in the growth rates between the horizontal oral and vertical pharyngeal cavities, in both species, lead to the ending results being significantly dissimilar. Therefore the human, at age nine, has both cavities of the SVT being extremely close together, with a resulting ratio of one to one, as seen in figure 4a on page 248. In contrast, the chimpanzees at age five (comparable to human age nine) have the cavities of the SVT significantly farther apart, with an averaging ratio of 2.10, which is seen on the same figure. The human larynx is also shown in this study to descend much more then the larynx in chimpanzees, thus making the human hyoid also descend more. This difference is probably caused by the chimpanzees' growth of a laryngeal air sac that humans do not have and also possibly because of evolutionary facial flattening. Regardless of this difference however, both species' velum loses contact with the epiglottis, suggesting that it was evolutionarily advantageous, even though it is still not known whether or not it plays a significant role on the development of human speech. At the conclusion of this study, not much was proven for the evolutionary spectrum. The arrangement of the SVT and laryngeal descent in humans are not the main causes of language expansion, but with further studies it could be established that many features, including the unique SVT and larynx, all work together to result in the development of human speech and language.
The scientific explanation for how the human speech developed is just beginning and this study is just one more step in the right direction. It may not have determined the exact organ or evolutionary trait that characterizes are language abilities, but it presents an idea that, with further studies, could play a vital role in the quest for the answers to our countless questions. Many organs aid humans is the ability to communicate vocally, including the SVT, larynx, hyoid, vocal folds, and numerous others not even mentioned in this study. This article demands us, as a human population, to dig deeper into the roots of evolution with high hopes of pulling out some sort of explanation as to how we came to be. Chimpanzees are our closest known relatives, yet they do not have the ability to vocalize as we do. They have their own ways of communicating, but language is something that is unique to the human culture. As shown in Nishimura's study, chimpanzees have vocal organs that are very similar to our own. The difference in some of them is caused by the different ways that they develop in chimpanzees compared to humans, like the SVT, hyoid, and larynx for instance. Other scientists and researchers, of course, have other views. Majority of these views, as in this article, can only explain some of the vocal ability of humans, but never quite gets to the main cause of it. This article presents the fact that evolutionary facial prognathism could have caused the descent of the larynx and hyoid in humans. Also, the study proposed that the reason human speech is not developed in chimpanzees and other primates is that the epiglottis is separate from the velum. After closely examining the MR images, though, it is clearly seen that chimpanzees also have their epiglottis separate from the velum, just not to the extent that it is seen in humans. It is obvious to see that certain characteristics that led to the development of the human language are not present in chimpanzees, but there are some characteristics that are similar just with different developmental patterns. In either case, this study explains that for some reason or another, we as human beings are unique and have unique abilities that other primates do not have.