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
#1].
-----------
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 [degusta@uclink.berkeley.edu]
-----------
Text Notes:
... ... *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.
-------------------
Related Background:
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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