HUMAN FACES ARE SO VARIABLE BECAUSE WE EVOLVED TO LOOK UNIQUE
The amazing variety of
human faces -- far greater than that of most other animals -- is the result of
evolutionary pressure to make each of us unique and easily recognizable,
according to a new study by University of California, Berkeley, scientists.
Our
highly visual social interactions are almost certainly the driver of this
evolutionary trend, said behavioral ecologist Michael J. Sheehan, a
postdoctoral fellow in UC Berkeley's Museum of Vertebrate Zoology. Many animals
use smell or vocalization to identify individuals, making distinctive facial
features unimportant, especially for animals that roam after dark, he said. But
humans are different.
"Humans
are phenomenally good at recognizing faces; there is a part of the brain
specialized for that," Sheehan said. "Our study now shows that humans
have been selected to be unique and easily recognizable. It is clearly
beneficial for me to recognize others, but also beneficial for me to be
recognizable. Otherwise, we would all look more similar."
"The idea that
social interaction may have facilitated or led to selection for us to be
individually recognizable implies that human social structure has driven the
evolution of how we look," said coauthor Michael Nachman, a population
geneticist, professor of integrative biology and director of the UC Berkeley
Museum of Vertebrate Zoology.
The study will appear
Sept. 16 in the online journal Nature Communications.
In the study, Sheehan
said, "we asked, 'Are traits such as distance between the eyes or width of
the nose variable just by chance, or has there been evolutionary selection to
be more variable than they would be otherwise; more distinctive and more unique?'"
As predicted, the
researchers found that facial traits are much more variable than other bodily
traits, such as the length of the hand, and that facial traits are independent
of other facial traits, unlike most body measures. People with longer arms, for
example, typically have longer legs, while people with wider noses or widely
spaced eyes don't have longer noses. Both findings suggest that facial
variation has been enhanced through evolution.
Finally, they compared
the genomes of people from around the world and found more genetic variation in
the genomic regions that control facial characteristics than in other areas of
the genome, a sign that variation is evolutionarily advantageous.
"All three
predictions were met: facial traits are more variable and less correlated than
other traits, and the genes that underlie them show higher levels of
variation," Nachman said. "Lots of regions of the genome contribute
to facial features, so you would expect the genetic variation to be subtle, and
it is. But it is consistent and statistically significant."
Using Army data
Sheehan was able to
assess human facial variability thanks to a U.S. Army database of body
measurements compiled from male and female personnel in 1988. The Army
Anthropometric Survey (ANSUR) data are used to design and size everything from
uniforms and protective clothing to vehicles and workstations.
A statistical comparison
of facial traits of European Americans and African Americans -- forehead-chin
distance, ear height, nose width and distance between pupils, for example --
with other body traits -- forearm length, height at waist, etc. -- showed that
facial traits are, on average, more varied than the others. The most variable
traits are situated within the triangle of the eyes, mouth and nose.
Sheehan and Nachman also
had access to data collected by the 1000 Genome project, which has sequenced
more than 1,000 human genomes since 2008 and catalogued nearly 40 million
genetic variations among humans worldwide. Looking at regions of the human
genome that have been identified as determining the shape of the face, they
found a much higher number of variants than for traits, such as height, not
involving the face.
Prehistoric origins
"Genetic variation
tends to be weeded out by natural selection in the case of traits that are
essential to survival," Nachman said. "Here it is the opposite;
selection is maintaining variation. All of this is consistent with the idea
that there has been selection for variation to facilitate recognition of
individuals."
They also compared the
human genomes with recently sequenced genomes of Neanderthals and Denisovans
and found similar genetic variation, which indicates that the facial variation
in modern humans must have originated prior to the split between these
different lineages.
"Clearly, we
recognize people by many traits -- for example their height or their gait --
but our findings argue that the face is the predominant way we recognize
people," Sheehan said.
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