Language Related Anatomy Outside the Brain

The idea that apes' brains are capable of sophisticated communication but that their peripheral anatomy does not allow them to speak has underlain the many studies in ape sign language. Washoe, the chimp, and Koko, the gorilla, for example, can both communicate in simple sentences by signing with their hands, but they cannot articulate words. By looking at the anatomical differences between humans and apes and comparing these findings with the fossil record, paleoanthropologists have come up with some interesting insights into the evolution of spoken language.

The anatomical position of the voice box, the larynx, has a lot to do with whether a hominoid can speak or not. Jeffrey Laitman at Mount

Sinai Medical School in New York and his colleagues have shown that the larynx is located high up near the base of the skull in apes and modern infants.5 In adults the larynx descends, allowing a column of air above it to be manipulated to produce the controlled verbalizations of language. Laitman looked at fossil hominids to determine when in human evolution speech may have begun. He inferred the position of the larynx from the degree of flexion of the base of the skull—open and with an obtuse angle between the underside of the face and the base of the back of the skull in apes, and acutely angled in modern people. All the earliest hominids in Laitman's analysis are apelike in their skull base flexion and thus incapable of speech. Early Homo sapiens, now termed Homo heidelbergensis, were the first hominids to show enough basicranial flexion for Laitman to accept that they could speak. This analysis puts the Longgushan people on the nonspeaking side of the linguistic divide in human evolution.

The tongue also has a lot to do with language production. Richard Kay and his colleagues at Duke University looked at the size of the nerve to the tongue (the hypoglossal nerve, or cranial nerve 12) in apes and modern people and determined that it was relatively larger in people.6 If the motor nerve to the tongue muscles is relatively larger, a greater number of nerve fibers must be coursing through it to innervate the tongue and control its fine movements. Since ape and human tongues are nearly the same size, and the other activities of the tongue—helping manipulate food while chewing, closing off the throat while swallowing, and getting out of the way while breathing—are the same in all hominoids, Kay and his colleagues reasoned that the relative increase in the size of the human hypoglossal nerve must be related to the fine motor control needed for language. The bony hypoglossal canal in the base of the skull must be preserved in fossils for this trait to be analyzed. Kay and his colleagues concluded that the australopithecines and earliest Homo from Africa had small, apelike hypoglossal nerves and thus were incapable of language. Earliest Homo sapiens had the enlarged hypoglossal nerves characteristic of modern people, and according to this analysis were the first to speak. All of the Longgushan skulls, as we have seen, lacked the skull base and thus the hypoglossal canal, and could not be included in Kay's study, but other Homo erectus fossils had small canals like their ancestors. Kay's conclusions were called into question by a study conducted by David DeGusta and his colleagues in 1998, who contended that they did not see the same increase in size of the hypoglossal canal from australopithecines to Homo.7 However, the sample sizes are small and the postulated changes in canal diameters are difficult to measure precisely. Further study is needed, but for the moment there is some evidence that tongue anatomy tends to agree with laryngeal anatomy in indicating that Homo erectus was incapable of speech.

A final and unexpected contribution to the anatomy of speech came from the discovery and analysis of the early Homo erectus skeleton from the northern Kenyan site of Nariokotome. A study by anatomist Ann MacLar-non surprisingly revealed that this young male adolescent had a vertebral canal only three-quarters the size expected for a modern boy of the same age.8 The vertebral canal is formed by the holes that run through vertebrae stacked one on top the other in the spine, and in life it contains the spinal cord. Overall body size, however, was within the range of modern humans. MacLarnon's controversial interpretation was that Homo erectus did not have the fine control of its respiratory chest muscles needed for language. Although this study agrees with other anatomical indicators of Homo erectus's language ability, there are also a number of other nerve cells that run through the upper spinal cord, such as nerves for motor control to the hand and arm. The small diameter of Homo erectus's upper spinal cord could as likely be related to less motor-nerve innervation to the hand and arm as to a decreased innervation to the muscles running between the ribs, used as accessory muscles in breathing. It is important to remember that the diaphragm, the major muscle used in breathing, is innervated from spinal levels between the third neck vertebra and the fifth neck vertebra, thus higher than the preserved fossil vertebral column from Nariokotome (which starts at the seventh cervical vertebra). Thus, we believe that citing the narrow vertebral canal of Homo erectus is a weak argument for the inability of the Longgushan people to speak, and that it may also (or instead) point to a lack of manual dexterity in Homo erectus, a topic to which we now turn.

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Responses

  • MortimerPanic
    Homo erectus did not have a small spinal cord. Meyer (2005) found that new fossils had a fully human sized cord. There was no limitation on their ability to produce spoken language.
    8 years ago

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