Your identity is written all over your face. Few animals have individual faces as distinctive as ours — even other species like crows, pigeons, dogs and bees can tell one human face from another.
This may seem obvious, as plain as the nose on your face. But have you ever wondered why? Scientists from the University of California-Berkeley did, and in a new study published in Nature Communications, they suggest our facial diversity is the result of evolutionary pressure to make sure every human is easy for other humans to recognize.
"Humans are phenomenally good at recognizing faces; there is a part of the brain specialized for that," lead author Michael Sheehan explains in a press release. "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."
Lots of animals rely on smell or sound to identify each other, leaving little need for distinctive facial features. But humans are a highly visual and a highly social species, so it makes sense we focus on the face. It's like we've evolved a name tag, which is more important than it might initially sound. Being recognizable has many advantages — it may prevent an angry stranger from confusing you with his enemy, for example, or ensure that someone with a similar face doesn't claim a reward meant for you.
Like most animals, king penguins have little facial variation, instead using voices to recognize each other. (Photo: Brian Gratwicke/Flickr)
To see whether our faces vary by chance or by natural selection, Sheehan and co-author Michael Nachman sifted through anatomical statistics from a U.S. Army database as well as the 1000 Genomes Project. They found that facial features are far more variable than other bodily traits, and that each facial feature varies independently from the others, which is unusual for most body measurements. People with longer arms typically also have longer legs, for example, yet people with broader noses or widely spaced eyes don't necessarily have longer noses or larger eyes, the researchers note.
On top of that, Sheehan and Nachman found more variation in the genomic regions that control facial features than in other parts of the genome. Traits like forehead-chin distance, ear height, nose width and eye spacing vary more than non-facial traits in European-Americans as well as African-Americans, the study shows. The most variable traits are found within the triangle of the eyes, mouth and nose.
"Lots of regions of the genome contribute to facial features, so you would expect the genetic variation to be subtle, and it is," Nachman says. "But it is consistent and statistically significant."
The researchers even looked back at the DNA of a Neanderthal and a Denisovan, two extinct human species who showed a degree of facial variability similar to us. That suggests the adaptive advantage of a unique mug evolved before modern humans did. Most humans can now identify faces with more than 97 percent accuracy, although a small number suffer from a condition called prosopagnosia, aka "face blindness," that renders them unable to recognize other people's faces as well as their own.
Most non-human animals have relatively uniform faces, since they use other cues like scent or voice to know who's who. But a few species share our variability — in a 2011 study, for example, Sheehan and University of Michigan biologist Elizabeth Tibbetts showed that paper wasps have facial diversity and recognition skills similar to ours, a finding Tibbetts called "surprising and sort of bizarre."
We use more than just faces to tell each other apart, of course. But we're so hard-wired to identify faces that we do it without thinking — even when a face isn't really there, a quirk known as pareidolia.
"Clearly, we recognize people by many traits — for example their height or their gait," Sheehan says, "but our findings argue that the face is the predominant way we recognize people." And beyond that, Nachman adds, the idea that social interaction leads to natural selection for more distinctive faces also "implies that human social structure has driven the evolution of how we look."