Why does a songbird sing? To attract a mate, most of us would immediately answer, and to separate himself from the many other suitors trying to win the heart of that pretty female bird. Animals often use vocalization to distinguish themselves from the pack, while other species use smell, making distinctive facial features more or less irrelevant to a great number of animals, especially those that roam after dark. But we humans, well, we’re a little different.

“There are many situations where an individual can be confused with another,” Dr. Michael J. Sheehan, a postdoctoral researcher and evolutionary biologist, told Medical Daily. “If everyone looked really similar, costly mistakes would be made so humans as a species have evolved to avoid that.” Specifically, we’ve developed unique faces. A wide variety of facial features, Sheehan’s new study conducted at UC Berkeley finds, is the result of evolutionary pressure to make each of us singular and therefore easily recognizable on sight, with an accompanying region of the brain specialized to recognize this.

Army of One

To study the evolution of facial variation, Sheehan began in what you might believe to be an odd place: the U.S. Army, renowned or notorious, depending on your point of view, for wanting to curb, not encourage, individuality. In particular, Sheehan relied upon a database of body measurements known as the Army Anthropometric Survey (ANSUR), which compiled measurements from male and female personnel in 1988. ANSUR data have been used to design and size everything from uniforms and protective clothing to vehicles and workstations.

From this database, Sheehan and his colleague, Dr. Michael Nachman, a population geneticist and director of the UC Berkeley Museum of Vertebrate Zoology, made a statistical comparison of the facial traits of Army personnel (such as forehead-chin distance, ear height, nose width, and the distance between pupils) with other body traits (such as height, forearm length, and height at waist.) The comparison showed that human facial traits are, on average, more varied than other human features. “And the most variable traits on our faces are within the triangle of the eyes, mouth, and nose,” Sheehan explained.

Taking their investigation one step further, Sheehan and Nachman next accessed 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 the human genome and focusing on the regions identified as influencing our facial characteristics, they discovered a much higher number of variants than in other areas of the genome influencing other traits not involving the face, suggesting this variation is evolutionarily advantageous.

Significantly, each facial trait is independent of all other facial traits, unlike most body measures. A tall person, for instance, usually has longer arms and legs, yet someone with a wide nose does not necessarily have widely spaced eyes. "Lots of regions of the genome contribute to facial features, so you would expect the genetic variation to be subtle, and it is,” Nachman said. “But it is consistent and statistically significant." Ultimately, then, all three of findings in the study — that facial traits are more variable, that facial features are more independent than other traits, and that the underlying genes of facial characteristics show higher levels of variation — suggest our facial variation has been enhanced over time through evolution.

Natural Selection

Considering peacocks with their great feather fans are more colorful than peahens, do men show greater facial variety than women? According to Sheehan, the variation in facial traits is probably more or less the same. “In the ANSUR database the sample sizes [of men compared to women] were so different, it would be difficult to draw a robust conclusion,” Sheehan told Medical Daily.

Since natural selection is founded in survival — with the more advantageous traits becoming more common in a population over time — why is it important to our survival to have a distinct and recognizable face? “It’s good for us to let everyone know who we are and be identified,” Sheehan said. “If you’re committing crimes, you probably want to blend in but otherwise, it’s important to be recognizable and being able to tell everyone apart helps us do that.” Sheehan noted how humans may use personal “recognition without even knowing how often we use it,” citing how important it is for children to identify their mothers and a previous study, which suggested more aggression occurs in groups of people when individuals are less recognizable to one another. If we all looked alike, it would also be more difficult to take credit and reward from our individual work, he said.

So, which came first — the great diversity of facial features or that specialized region of our brains able to recognize individual faces? “We don’t have a clear answer to that just yet,” Sheehan said. “In order for specialization in the brain to have occurred, there must have been some level of individuality to start with and then further distinctions arose from there. We are interested in pursuing this further, and finding out.”

Source: Sheehan MJ, Nachman M. Morphological and population genomic evidence that human faces have evolved to signal individual identity. Nature Communications. 2014.