The mammalian *hypoglossal canal transmits the *nerve that supplies
the *motor innervation to the tongue. Hypoglossal canal size has been used
to date the origin of human-like speech capabilities to at least 400,000
years ago, and to assign modern human vocal abilities to *Neandertals.
These conclusions are based on the hypothesis that the size of the hypoglossal
canal is indicative of speech capabilities. ... ... D. DeGusta et al (3
authors at 2 installations, US) now present the results of a study to test
the hypothesis that hypoglossal canal size is indicative of speech. The
authors report they measured the following: a) the hypoglossal canals of
75 nonhuman primates and 104 modern humans; b) the hypoglossal canal in
specimens of the early *hominid *taxa *Australopithecus afarensis and *Australopithecus
boisei; c) both the nerve and canal diameter and estimated nerve axon number
in a sample of human cadavers. The authors report the following results:
a) Many nonhuman primate specimens have hypoglossal canals that are absolutely
and relatively within the size range of modern humans. b) The hypoglossal
canals of Australopithecus afarensis, Australopithecus boisei, and *Australopithecus
africanus are also within the modern human size range. c) The size of the
hypoglossal nerve and the number of axons it contains do not appear to
be significantly correlated with the size of the hypoglossal canal. The
authors conclude: "The size of the hypoglossal canal is not a reliable
indicator of speech. Therefore the timing of the origin of human language
and the speech capabilities of Neandertals remain open questions." [*Note
Editor's note: The authors present this report essentially as a refutation of a paper by R.F. Kay et al, a summary of which appears in the background material below. Also, for more generally related material, see the SW Focus Report "Anthropology: Human Evolution" at URL [http://scienceweek.com/swfr017.htm].
D. DeGusta et al: Hypoglossal canal size and hominid speech.
(Proc. Natl. Acad. Sci. US 16 Feb 99 96:1800)
QY: David DeGusta [email@example.com]
... ... *hypoglossal canal: This canal, at the level of the brainstem, is a passageway through bone for the XII cranial nerve, the nerve bundle that innervates the tongue.
... ... *nerve: In general, the term "nerve" refers to a bundle of nerve axons (nerve fibers; neuron axons), the nerve usually visible to the naked eye. Nerves can contain large number of individual axons: the optic nerve in humans, for example, contains approximately 1 million nerve fibers. The hypoglossal nerve, the cranial nerve of relevance in this report, contains mostly nerve axons whose cell bodies are in the hypoglossal nucleus in the brainstem (efferent fibers carrying information to activate the muscles of the tongue), and perhaps some axons carrying information from sensory receptors in the tongue to the central nervous system (afferent fibers).
... ... *motor innervation: This refers to the anatomical connections of nerve fibers to muscle cells, the electrical activity of the nerve axons resulting in the activation of the muscle cells.
... ... *Neandertals: (Neanderthals) About 10 kilometers east of Dusseldorf in Germany, in the valley of the Dussel, there is a little town called Neander. One hundred and forty-one years ago, in the summer of 1856, some workmen broke into a cave to get at the limestone inside and discovered a set of ancient bones. Most of the bones were smashed to bits by the workmen, but some of the bones, including part of the skull, survived, and the skeleton was soon recognized by anthropologists as belonging to an ancient race of men who came to be known as the Neanderthals. A Neanderthal fossil had actually been discovered some years earlier in Gibraltar, but not recognized as such. Neanderthal- like fossils have also been found in France, Spain, Italy, Yugoslavia, Iraq, China, Java, and Israel. For more than a century, one of the central questions in paleoanthropology has been whether modern man evolved from this race.
... ... *hominid: The term "hominid" refers to any primate in the human family (Hominidae) of which Homo sapiens (modern man) is the only living specimen.
... ... *taxa: In general, a grouping defined in terms of shared similar characters.
... ... *Australopithecus afarensis: The first record of human footprints, of hominids walking upright, was discovered at Laetoli in East Africa, and has been dated at 3.6 million years ago. This ancestor, Australopithecus afarensis, probably weighed 25 to 50 kilograms (60 to 120 lbs.) as an adult.
... ... *Australopithecus boisei: Discovered by Mary Leakey in the Olduvai Gorge in Tanzania, this fossil has been dated at 1.75 million years ago.
... ... *Australopithecus africanus: Apparently derived from Australopithecus afarensis were several species, including Australopithecus africanus, a species which is believed to have appeared approximately 3 million years ago. ... ... *Note #1: Although the focus in this report is on the role of the neural innervation of the tongue in human speech, it must be emphasized that the organization of information and motor output necessary for speech apparently occurs concomitantly in several localized region of the cerebral cortex, and the evolution of these regions of the brain most likely played a significant role in the appearance of speech in humans. Essentially, the hypoglossal nerve merely transmits information originating in the brain, and both the origin and transmission of this information must be considered in any analysis of the evolution of human speech. Unfortunately, the brain is soft tissue and is not preserved in fossils; what we have is bone, and the data provided by bone and relevant archeological entities.
ORIGIN OF HUMAN VOCAL BEHAVIOR: AN ANATOMICAL CONSIDERATION
It can be argued that language is the most important behavioral attribute that distinguishes humans from other animals, and one of the important problems in anthropology and human evolution is to demarcate as narrowly as possible the time frame during which language in humans first appeared. Such demarcations have been based on either apparent anatomical correlates (e.g., bone and soft tissue analysis) or apparent archeological correlates (e.g., analysis of apparent symbolic behavior), with no firm specific consensus among specialists. One of the important anatomical features related to language is the nerve supply controlling the muscles of the tongue. The mammalian hypoglossal canal is a bony canal that contains the trunk of nerve fibers that constitute this nerve supply. This canal is absolutely and relatively larger in modern humans than it is in the African apes. ... ... Kay et al (3 authors at Duke University, US) report a study of the cross-sectional areas of hypoglossal canals in adult skulls of contemporary humans, African apes, and several key fossil hominids. They propose that hypoglossal canal size in fossil hominids may provide an indication of the motor coordination of the tongue and reflect the evolution of speech and language. What they report is that the hypoglossal canals of gracile Australopithecus, and possibly Homo habilis, fall within the range of extant African apes, and are significantly smaller than those of modern Homo. The canals of Neanderthals and an early "modern" Homo sapiens (Skhul 5), as well as of African and European middle Pleistocene Homo (Kabwe and Swanscombe), fall within the range of contemporary Homo and are significantly larger than those of Pan troglodytes (a chimpanzee species). In summary, the authors suggest these anatomical findings indicate the vocal capabilities of Neanderthals were the same as those of humans today. The authors further suggest that the vocal abilities of Australopithecus were not advanced significantly over those of chimpanzees, whereas those of Homo may have been essentially modern by at least 400,000 years ago, which is consistent with the evidence for accelerated encephalization rates in middle Pleistocene Homo. The authors conclude: "Thus, human vocal abilities may have appeared much earlier in time than the first archeological evidence for symbolic behavior."
QY: Richard F. Kay (Rich.Kay@baa.mc.duke.edu)
Proc. Natl. Acad. Sci. US 28 Apr 98 95:5417
Science-Week 19 Jun 98
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