Researchers from the University of California, Berkeley analyzed human faces and the genes that code for facial features.
They found a high variability that could only be explained by selection for variable faces, probably because of the importance of social interactions in human relationships and the need for humans to be recognizable.
Our highly visual social interactions are almost certainly the driver of this evolutionary trend, said behavioural 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,"
Sheehan said.
"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, professor of integrative biology and director of the UC Berkeley Museum of Vertebrate Zoology.
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, researchers 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.
The study 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.
The study will appear in the journal Nature Communications.

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