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. Credit: UC Berkeley

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.

Source: University of California - Berkeley

March of the penguin genomes

Adélie penguins.
Credit: David Lambert

Two penguin genomes have been sequenced and analyzed for the first time in the open access, open data journal GigaScience. Timely for the holiday season, the study reveals insights into how these birds have been able to adapt to the cold and hostile Antarctic environment.

Antarctic penguins are subject to extremely low temperatures, high winds, and profound changes in daylight. They have developed complicated biological systems to regulate temperature and store energy for long-term fasting. Most studies have focused on the physiological and behavioral aspects of their biology, but an international team of researchers has now analyzed the DNA of two Antarctic penguins (Adélie and emperor) relative to other bird species, revealing the genetic basis of their adaptations and their evolutionary history in response to climate change.

Using the historical genetic record within the DNA across bird species, the researchers estimate that penguins first appeared around 60 million years ago. The study shows that the Adélie penguin population increased rapidly about 150,000 years ago when the climate became warmer, but later declined by 40% about 60,000 years ago during a cold and dry glacial period. In contrast, the emperor penguin population remained stable, suggesting that they were better adapted to glacial conditions, for example, by being able to protect their eggs from freezing temperatures and incubate them on their feet.

Cai Li, Team Lead at BGI-Shenzhen, China, said: "These different patterns in historical population change also suggest that future climate change may have impacts on the two penguin species. For example, the fact that emperor penguins didn't experience the same population boom as Adélie penguins in warm climates means that they could suffer more from global warming, and this needs to be considered in conservation efforts in Antarctica."
Both penguins were found to have expanded genes related to beta-keratins -- the proteins which make up 90% of feathers. They also had at least 13 genes responsible for a single type of beta-keratin, which is the highest number compared to all other known bird genomes. This would explain their importance in ensuring that penguin feathers are short, stiff and densely packed to minimize heat loss, remain waterproof and aid underwater flight. Likely to be responsible for penguins' thick skin, the team also identified a gene called DSG1, which is known to be involved in a human dermatological disease characterized by thick skin on the palms and soles.

Fat storage is critical for penguins to withstand the cold and survive long fasting periods -- up to four months in emperor penguins. The two penguins were found to have exploited different adaptations for lipid metabolism in the course of their evolution, which may also provide insight into their contrasting abilities for coping with climate change. The researchers found eight genes involved in lipid metabolism in the Adélie penguin, and three in the emperor penguin.

During their evolutionary history, the wings (or forelimbs) of penguins changed profoundly for wing-propelled diving in the water. The team identified 17 forelimb-related genes in the penguin genomes that had unique changes. One of the genes in particular, EVC2, showed a larger number of genetic changes compared to other birds. Mutations of EVC2 in humans cause Ellis-van Creveld syndrome, characterized by short-limb dwarfism and short ribs.
Guojie Zhang, Assistant Professor at the University of Copenhagen and Associate Director at China National GeneBank, BGI-Shenzhen, China, said: "Penguins show distinct evolution relative to other bird species. They can't fly, have specialized skin and feathers, degenerated wings, and live in a cold environment in which most other birds could not survive. 

Comparative genomics is a powerful tool for providing answers on the molecular basis of these evolutionary changes and how organisms deal with the conditions they are exposed to. Our study has revealed several of these secrets for the two penguins."

David Lambert, Professor of Evolutionary Biology at Griffith University, Australia, said: "Although Adélie and Emperor penguins both breed on the Antarctic continent, they do so in very different ways. By sequencing the genomes of two penguin species we have been able to compare many of the genes that are responsible for these different abilities to do the same thing -- namely to survive and breed in Antarctica. This study is particularly important because it now provides us with the opportunity to conduct large scale evolutionary studies of both species."

These papers are part of a series of reports from the Avian Phylogenomics Project that are being published in concert in multiple journals. The authors of several Science papers will unveil new genomic results related to the avian tree of life, and a number of papers are also published in BMC Genomics, BMC Evolutionary Biology and Genome Biology.

Avian Genome Collection on BioMed Central: http://www.biomedcentral.com/series/avian