Hello.
The mandible is the lower jaw.
And, yes, human mandibles have "retracted" in the sense that they are
smaller and thinner than they used to be. There is evidence that human
ancestors who lived hundreds of thousands of years ago had jaws that
were much larger and thicker than our jaws now.
Consider the relatively masssive lower jaw of the Australopithecus
afarensis
that live approximately 3.5 million years ago.
See: "Australopithecus afarensis: Composite Reconstruction," hosted by
the Smithsonian:
http://www.mnh.si.edu/anthro/humanorigins/ha/afarcomp.htm
Now compare this to the mandible of the Homo heidelbergensis that
lived about 500,000 years ago. See "The Mauer Mandible," hosted by
the Smithsonian:
http://www.mnh.si.edu/anthro/humanorigins/ha/mauer.htm
Now consider the lower jaw of Homo sapiens of 90,000 years ago. Notice
how much smaller the lower jaw is relative to the skull compared with,
say, the Australopithecus. See "Skhul V," hosted by the Smithsonian.
http://www.mnh.si.edu/anthro/humanorigins/ha/skhul.html
Also notice how the modern lower jaw of homo sapiens is somewhat
smaller than the mandibles of the Neanderthal and Cro-Magnon of
100,000 years ago or so. See "Homo sapiens," hosted by Geocities.com:
http://www.geocities.com/palaeoanthropology/Hsapiens.html
Here are some citations that document this phenomenon:
"Human adults are less efficient at chewing because the roof of the
mouth and the lower jaw have been reduced compared with non-human
primates and archaic hominids. This reduced palate and mandible crowd
our teeth and may lead to infection because of impaction--a
potentially fatal condition before modern medicine."
Teeth and Jaw: Structure and Mastication Muscles
http://www.beyondveg.com/billings-t/comp-anat/comp-anat-6b.shtml
"Our faces changed as well, with smaller mandibles and maxillaries,
smaller teeth, a mouth set back further into the face, and prominent
chins."
What Bones Tell Us about Human Evolution
http://www.saitama-kenpaku.com/jinrui/special/english/number/07_E/factor01_03.htm
"Thickened mandibles can act to resist extreme stresses associated
with transverse bending (that is, "wishboning") and torsion. Because
wishboning stresses decline towards the back of the corpus, torsion is
likely a more important explanation. Corpus torsion can result from
bite force and muscle activity during mastication. Therefore, it may
be that australopithecine mandibular morphology reflects elevated
stresses associated with unusual mechanical demands. Daegling and
Grine (1991) suggest that australopithecines may have eaten fibrous,
coarse foods that required repetitive loading. While this fails to
explain why colobines do not have thick corpora, it does suggest a
fundamental difference between australopithecines and living great
apes that may reflect a shift in diet in the early hominids."
Source: "A paleontological perspective on the evolution of human
diet," hosted by uark.edu
http://www.cast.uark.edu/local/icaes/conferences/wburg/posters/pungar/satalk.htm
As indicated in the preceding article, the larger jaw was associated
with a diet of "fibrous, coarse foods." The general view on this
subject is that as the human diet evolved toward eating softer foods
and meat (and, later, cooked food), the evolutionary advantages of
such a large jaw were reduced. This view is displayed in articles
such as the next one:
" A change from hard, rough foods to a diet of softer, processed food
requires less chewing force, leading to smaller jaws and jaw muscles.
One disadvantage of a shrinking face turns out to be the painful
growth of wisdom teeth. Genes largely control tooth size, while
lessening the mechanical load of chewing is responsible for shrinking
our jaws. As a result, our jaws have run out of room for these big
teeth to grow in easily. "
Source: "Heads Up: Looking at the changing face of humanity," hosted
by harvard.edu
http://www.news.harvard.edu/gazette/2001/10.25/03-lieberman.html
As indicated, many observers view the phenomenon of impacted wisdom
teeth to be the direct result of the evolution of smaller jaws:
"There is some evidence that our jaws are getting smaller. However,
the size of our teeth have not changed as much. Consequently, many of
us do not have room for our 3rd molars. Mankind has evolved over the
last half million years without having 3rd molars removed."
http://faculty.washington.edu/quarn/3rds.html
Interestingly, there is also a school of thought that asserts that the
changes in the human mandible are not only the result of changes in
diet but also the byproduct of the evolution of human speech.
"We have to come to accept that the mandible is undergoing a change in
function. It is no longer designed for chewing, but for speech."
source: BEYOND CHEWING
http://www.voicetraining.co.uk/pdfs/doc_chewing.pdf
So, how does the chin fit in to all this?
Well, despite the decline in overall mandible size and thickness, a
protruding chin evolved. The distinctive chin is one feature that
anthropologists use to distinguish anatomically modern humans (and the
Cro Magnon) from the earlier hominids including the relatively recent
Neanderthals. There does not appear to be general consensus on the
evolutionary function of the chin.
"a small face and a protruding chin: there is no clear consensus for
the functional significance of the chin. It may be due to a reduction
in face size, to reduce chewing stress of the mandible, or it may be
an adaptation to speech production"
See: "Counter Archaic Homo Sapiens Narrative Background Notes and
References," hosted by geocities.com
http://www.geocities.com/xerexes/archomo.html
"The modern form of Homo sapiens first appeared about 100,000 years
ago. This species is distinguished by ... a prominent chin, and
lighter bone structure than H. heidelbergensis."
sourece: "Origins of Humankind," hosted by PBS.org
http://www.pbs.org/wgbh/evolution/humans/humankind/o.html
search strategy:
mandible, evolved, homo sapiens
mandible, evolution, origins
"lower jaw" evolved, homo sapiens,
jaw, evolved, "wisdom teeth"
chin, evolved, "homo sapiens"
I hope this helps, and thank you for the question. I have been
wondering about some of these things myself for quite some time. About
a decade ago, I took an introductory anthropology course in college. I
noticed that all of the ancient hominid jaws had very straight teeth
(despite the lack of orthodontics back then!). I asked my instructor
about this, and he basically said that human jaws have become smaller,
and, as a result, teeth have become overcrowded. This puzzled me back
then, but this question gave me the opportunity to revisit this
subject, so thanks again! |
