Researchers at Johns Hopkins found that use over time and not just
genetics informs the structure of jaw bones in human populations.
The researchers say these findings may be used to predict the diet
of an ancient population, even if little evidence exists in the
fossil record. It can also make it easier for scientists to
pinpoint the genetic relationship between fossils.
Their results were published online June 23 in the American
Journal of Physical Anthropology.
"Our research aimed to see how much of the mandible's—or jaw
bone's—shape is plastic, a response to environmental influences,
such as diet, and how much is genetic. We used archaeological jaw
bones from two different regions to answer that question," explains
Megan Holmes, graduate student at the Johns Hopkins Center for
Functional Anatomy and Evolution, and lead author of the paper.
"Before we can make inferences about what the shape of a bone tells
us, like what environment the individual lived in, who it's related
to or what it ate, we have to understand what creates that shape.
The idea that function influences the shape of jaw bones is great
for the archeological record in terms of discovering the diet of a
population, and it's also really useful for reconstructing the
fossil record—finding which fossils are related to which, and
how."
The group chose to study the Arikara and Point Hope American Indian
populations, since they were genetically isolated from other groups
and ate different diets. They investigated bones from the regions
dating back to the 1600s and 1700s, times for which the diets are
known from other records. The Point Hope population in Alaska ate a
"hard" diet that included tough dried meat. They also used their
teeth for a variety of nonfood-related tasks, such as stripping
leather. The Arikara, from the Dakota area of the United States,
ate a softer diet, which consisted of farming supplemented with
light hunting.
The researchers precisely measured jaw bones from 63 members of the
Point Hope population and 42 individuals from the Arikara
population using an X-ray gun as well as calipers, and they used
those measurements to extrapolate the proportions of the entire
jaw. "The jaw bones were similar in children before they were old
enough to start chewing, but different in adulthood, which implies
that this divergence is likely a functional result of their diet
and the use of their jaw, rather than genetics," says Holmes.
The changes to the jaw bones were explained using a theory drawn
from engineering, which directly relates the geometry of a bone to
the stresses put on it during use. The team was able to investigate
very specific parts of the jaw bones and relate them to specific
dietary habits. In the Point Hope population, for example, they
found round, wide jaw bones—a result of having to exert more force
to chew a harder diet. The Arikara, on the other hand, did not show
this expansion, which they attributed to the lighter chewing
requirement of a softer diet.
"Genetics creates a blueprint of the bone, but a lot of things
influence the bone's construction," says Holmes. "Mechanical
pressure from muscle stress and strain from day-to-day activities
can remodel the bone's surface and internal structure. Knowing how
much the shape of a mandible we find is related to diet and how
much genetically connects it to fossils found elsewhere can really
help us parse out the family tree."
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Johns Hopkins Medical Institutions