You probably don't spend much time thinking about how curious it is that your mother looks dramatically different from your boss, whose face doesn't at all resemble your mail carrier's, let alone your fourth grade teacher's or your prom date's or that barista's at the coffee shop.
But in the grand scheme of things, the massive variation among human faces is quite extraordinary when compared to animals that pretty much all look the same.
As it turns out, evolutionary pressures for individuals to be easily recognizable pushed us toward having widely different faces, according to a new study published in Nature Communications and funded by the National Institutes of Health.
"Individual recognition is really important, in some ways so important that sometimes we don't realize how we recognize individuals," said study co-author Michael Sheehan, a postdoctoral fellow at University of California, Berkeley's Museum of Vertebrate Zoology. "It's so ingrained within us."
The study found that there's more variation in human facial traits -- such as the distance between eyes or the length of a nose -- than there is for other body traits. And facial traits aren't connected to each other the way other body traits are; someone with long legs tends to have long arms. But you can have close-set eyes and either a wide nose or a small nose. Faces are unpredictable like that.
Researchers turned to two data sets to discover the variance: The Army Anthropometric Survey, which includes measurements of men and women and is often used to design things like clothes, and the 1000 Genomes Project, which has mapped out the genomes of nearly 1,000 people.
While the relationship between our genes and certain traits (like height) are quite reliable, the researchers found the genes that influence facial features to be less straightforward. The study's authors also tested genetic sequences from Neanderthals and found that some of the variants are millions of years old, "and predate the origins of humanity itself," Sheehan said.
The ability to distinguish between individuals is not unique to humans, Sheehan said. Penguins, for example, have widely different vocal calls, even if their faces pretty much all look the same. Humans, on the other hand, have the cognitive ability to recognize specific faces and connect them to information about the individual.
Without this ability, we never would have selected for more diverse faces over time. But that selection also occurred because distinguishing between individuals is an important thing for human interactions, Sheehan said. Imagine all the cases of mistaken identity: Awkwardly waving at a stranger you thought was a friend! Being wrongly accused of committing a murder!
For some animals, being identifiable as an individual isn't even all that important. That's not the case for humans.
"You can imagine, if everyone looked more similar, we would have more problems," Sheehan said.
We're people, after all, not penguins.