Why can we see color? It's cause for blushing

WASHINGTON — Primates may have evolved color vision not to find the ripest, tastiest fruit but rather to detect that telltale blush on someone else's rump, researchers reported Thursday.

The cone structures in the eye that help detect color seem exquisitely tuned to skin tones, the team at the California Institute of Technology in Pasadena, Calif., reported.

"For a hundred years, we've thought that color vision was for finding the right fruit to eat when it was ripe," said Mark Changizi, a neurobiologist and postdoctoral researcher at Caltech who led the study.

Instead, Changizi and colleagues report in the current issue of the journal Biology Letters, the system seems adapted especially to find the colors prevalent in primate skins, notably changes due to how much oxygenated hemoglobin is in the blood.

The three-cone system can help a primate detect if a potential partner is having a rush of emotion in anticipation of mating and if an enemy's blood has drained out of his face due to fear.

"Also ... when you're more oxygenated, you're in better shape," Changizi said. That may be why humans value rosy cheeks, he said.

The clincher: Changizi said old-world primates that have the three-cone vision are also all bare-faced and bare-butted.

"There's no sense in being able to see the slight color variations in skin if you can't see the skin," Changizi said. "This could connect up with why we're the 'naked ape.' "