01/12/2012 08:56 am ET Updated Jan 12, 2012

Facial Evolution: Biologists Find Faces Of Primates Depend On Group Size

A new evolutionary study on face color in primates may tell us more about ourselves and why humans' faces are so dull compared to the bright colors on primates.

UCLA biologists studied the faces of 129 adult male primates from Central and South America and published their results in Proceedings of the Royal Society B. The researchers say the faces of these primates evolved over the course of 24 million years.

A surprising finding of the study was that faces are simpler -- less color and complexity -- in species that live in larger groups.

Lead author Sharlene Santana said, "Humans have pretty bare faces, which may allow us to see facial expressions more easily than if, for example, we had many colors in our faces."

"Species that live in larger groups live in closer proximity to one another and tend to use facial expressions more than species in smaller groups that are more spread out. Being in closer proximity puts a stronger pressure on using facial expressions," said Santana.

Another finding of the research is Santana's claim that the idea of evolutionary change is irreversible is potentially wrong. The contested theory suggests once a primate species evolves to have a specific color, it is irreversible -- that species cannot revert back to a previous color in its evolutionary history.

Click here for the full article text on Proceedings of the Royal Society B.

Below, see a slideshow of primates used in the study.
Caption From PotRSB: "Warmer colors indicate higher complexity in facial color patterns (i.e. faces with higher number of areas, as illustrated in figure 2b, that are uniquely different from other facial areas based on hair and skin color)."

Primate Faces