Some fascinating news regarding the separation of the human lineage from
other primates.
R
---------------------------- Original Message ----------------------------
Subject: [Fwd: Mutation Leading to Human Evolution? Article]
From: "Cheryl Wistrom" <cherylw@saintjoe.edu>
Date: Wed, March 24, 2004 1:53 pm
To: "Kurt Shoemaker" <kurts@saintjoe.edu>
"Roger Olson" <rogero@saintjoe.edu>
"Neal Haskell" <nealh@saintjoe.edu>
"Mark Seely" <marks@saintjoe.edu>
"stang >> William Stang" <williams@saintjoe.edu>
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-------- Original Message --------
Subject: Mutation Leading to Human Evolution? Article
Date: Wed, 24 Mar 2004 13:50:53 -0600
From: Cheryl Wistrom <cherylw@saintjoe.edu>
To: bobb@siantjoe.edu, cherylw@saintjoe.edu
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March 24, 2004
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Bite makes way for brain
A single mutation may have caused gross anatomical changes that spurred
human evolution | * By Brendan A Maher*
<mailto:bmaher@the-scientist.com>
A pile of evidence from disparate disciplines indicates that a single
change in a single gene—/MYH16/—may be responsible for significant
morphologic differences between humans and other primates, including
possibly the three-fold increase in brain size that set the earliest
species of /Homo/ apart from their kin. This is the first protein
disparity between humans and chimps that can be correlated to drastic
anatomical changes seen in the fossil record, according to a group of
University of Pennsylvania researchers who published a letter in the
March 25 issue of Nature <http://www.nature.com>.
“This is a brilliant piece of detective work that has enormous
implications for medicine, biology, evolution, molecular genetics, [and]
the human genome,” said Frederick S. Kaplan
<http://www.uphs.upenn.edu/ortho/physicians/faculty/kaplan.html>, a
professor of orthopedic molecular medicine at Penn who was not part of
the study. “By relegating a gene to the evolutionary garbage heap, we
were able to lift the constraints to the development of human
complexity,” he told /The Scientist/.
Powerful muscles for biting practically dominate the cranial structures
of most primates, including gorillas, chimps, and extinct
/Australopithecus /and/ Paranthropus/ species. A gene responsible for a
majority of jaw musculature was lost from human ancestors, presumably 2.4
million years ago, according to the study. Drastic reductions in these
muscles may have lifted significant physical constraints on braincase
volume, allowing primates with weak jaws and big brains to eventually
think about their origins.
A research team headed by Hansell H. Stedman
<http://www.uphs.upenn.edu/surgery/fac/hhs.html> unearthed a previously
unannotated human gene, /MYH16/, using degenerate polymerase chain
reaction. The gene encodes a myosin heavy chain protein—or rather it
would, if it didn't have a frameshift deletion at codon 660. But the
group needed to make sure that the deletion was more than an artifact. So
they looked and found that it was conserved in at least six
geographically distinct human populations. Moreover, they found
full-length orthologs in at least seven nonhuman primate species. Humans
and macaques both transcribe mRNA from the gene, but apparently only in
muscles of mastication. The experiments suggested that indeed this fixed
mutation causes a proteomic difference between humans and other
primates. At a press briefing in Philadelphia last Friday (March 19),
Stedman called the find a “slam dunk.”
Closer inspection of the anatomical structures affected by such a
mutation revealed clues as to what such a change might have meant for
human predecessors. Jaw muscle samples from both macaques and humans
revealed that humans have a relative dearth of so-called fast twitch
muscle. The hypertrophy of type II fibers, the authors write, actually
resembles that seen in a rare limb-muscle myopathy also caused by a
myosin gene mutation.
The comparatively colossal jaw muscles in apes certainly have
evolutionary advantages in feeding. And they could impart other
important survival traits, such as the ability to fatally bite a
competing male, said Nancy Minugh-Purvis, coauthor on the paper and
director of advanced gross anatomy at Penn.
Loss of such muscle mass would affect survival, and also development, as
the forces muscles exert have been largely implicated in sculpting bone
structure. Stedman cited, as an example, studies involving specific
knockout mice, which have doubled muscle mass
<http://www.biomedcentral.com/pubmed/9139826> accompanied by altered bone
structure <http://www.biomedcentral.com/pubmed/10962344>. “This is
built-in precedent for a causal relationship,” Stedman said. Kaplan
offered the effects on children afflicted with polio as an example. “They
have normal genes, but they have abnormal muscle pull because the nerves
to the muscles aren't working,” he told /The Scientist/. “By deactivating
the muscle, you can see dramatic effects on the skeleton.”
In nonhuman primates, as in some other animals, attachment points for
masticatory musculature—particularly the temporalis muscle—run along a
prominent sagittal ridge, a bony protuberance where plates of the skull
are fused. At the briefing, the authors suggest that stress from the
temporalis muscle could cause the sutures—the spaces between brain plates
that, in humans, remain flexible until adulthood—to fuse early in
development.
“Very possibly, the massive muscles of mastication could have been
constraining the brain by limiting the flexibility of growth plates in
the skull,” said Minugh-Purvis. For humans, the rather thin slivers of
muscle running up the temples would have lifted constraints on brain
size, such that the group jokingly named the mutation /RTF/ for “room for
thought.”
The change appears to correlate nicely with features in the fossil record
<http://www.mnh.si.edu/anthro/humanorigins/ha/a_tree.html>.
/Australopithecus /and/ Paranthropus/ appear to have had large
masticatory muscles and small brain cases, whereas /Homo/ species
appearing 1.8 to 2.0 million years ago seem to have smaller musculature
and increased cranial capacity. Alignment of /MYH16/ with orthologous
vertebrate sequences reveals an approximate date for the human mutation
at 2.4 million years ago. “Two million years ago, the human brain
essentially tripled in size,” said Minugh-Purvis.
Cliff Tabin, professor of genetics at Harvard Medical School told /The
Scientist/ that this work may change some preconceptions about human
evolution: “Even though in the fossil record you have a fairly abrupt
transition… one might have expected that even if it's an instant in
geologic time, it would be something that took place over tens of
thousands of years through selective pressure.” He added, “Potentially,
this was quite a dramatic and sudden change.” The situation, he noted, is
fairly unique—hinging on the existence of a jaw-specific myosin as well
as other myosins expressed in the jaw that could take up the slack for
the lost human muscle.
The findings may open doors to developmental and disease studies. Kaplan
said that studies have shown
<http://www.biomedcentral.com/pubmed/12687003> that fibroblast growth
factor receptors (FGFRs) control the regulation of bone morphogenetic
protein (BMP) in fusing skull sutures. Such factors may be involved in
craniosynostoses, diseases in which sutures fuse prematurely.
Kaplan said he will be collaborating with Stedman's group to investigate
how changes in skull biomechanics due to musculature may alter the
relation between BMPs, FGFRs, and the other players involved. And, of
course, such research will likely lead to ponderings about how a
slack-jawed, bigheaded primate may best compete for food and mates in a
population presumably dominated by males with a tough bite.
The group is not the first to make such conjectures about the
morphologic differences between human and ape. “In the adult male
anthropomorphous ape, as Rutimeyer, and others, have insisted, it is
precisely the effect which the jaw-muscles by their great development
have produced on the skull that causes it to differ so greatly in many
respects from that of man,” Charles Darwin wrote in 1871
<http://www.infidels.org/library/historical/charles_darwin/descent_of_man/index.shtml>.
For some, however, the findings may be hard to swallow. “Even if it's a
scientifically based just-so story, it's still a little bit of a just-so
story… We can't really prove it, but I think there are enough
morphological characters that changed in concert that could all be
explained by this mutation that there really is a parsimony to it that is
quite striking and makes it quite believable,” Tabin said.
Links for this article
H.H. Stedman et al., “Myosin gene mutation correlates with anatomical
changes in the human lineage,” /Nature/, 428:415-418, March 25, 2004.
http://www.nature.com
Frederick S. Kaplan
http://www.uphs.upenn.edu/ortho/physicians/faculty/kaplan.html
Hansell H. Stedman
http://www.uphs.upenn.edu/surgery/fac/hhs.html
A.C. McPherron et al., “Regulation of skeletal muscle mass in mice by a
new TGF-Beta superfamily member,” /Nature/, 387:83-90, May 1, 1997.
[PubMed Abstract <http://www.biomedcentral.com/pubmed/9139826>]
M.W. Hamrick et al., “Femoral morphology and cross-sectional geometry of
adult myostatin-deficient mice,” /Bone/, 27:343-349, September 2000.
[PubMed Abstract <http://www.biomedcentral.com/pubmed/10962344>]
Early Human Phylogeny
http://www.mnh.si.edu/anthro/humanorigins/ha/a_tree.html
S.M. Warren et al., “The BMP antagonist noggin regulates cranial suture
fusion,” /Nature/, 422:625-629, April 10, 2003.
[PubMed Abstract <http://www.biomedcentral.com/pubmed/12687003>]
C. Darwin, /Descent of Man,/ 1871:144.
http://www.infidels.org/library/historical/charles_darwin/desce
nt_of_man/index.shtml
<http://www.infidels.org/library/historical/charles_darwin/descent_of_man/index.shtml>
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