Recognizing a fellow human’s face is about more than just identifying a nose or mouth. It’s believed we use something called configural recognition to process the entire facial structure altogether, which is why there’s often a little bit of a lag time when we see a face upside-down (humans have an easier time recognizing other objects, like cars or houses, that have been flipped).
Researchers now believe chimpanzees have something similar to configural recognition. Only they use it to recognize each other’s butts.
In a paper published in the journal PLOS One, researchers from the Netherlands and Japan observed chimps as they examined photographs of primate buttocks and played a variation on the “match” game, coupling two identical butts together on a touch screen. They appeared slower to recognize posteriors when the images were rotated 180 degrees, indicating they rely on the same configural clues humans do. The researchers also carried out experiments on humans, who (as expected) took a longer time to process images of human faces flipped upside-down, but whose reaction time didn't change significantly when they were presented with upside-down images of human behinds.
It’s believed chimps have evolved to focus on butts due to their proximity to them while moving in groups. Walking on four legs, they’re often (literally) faced with a rump ahead of them. Since ovulating females usually have red, swollen rear ends, male chimps benefit from being able to identify them. What’s more, chimps can typically separate an ovulating non-relative from a relative, preventing inbreeding.
The paper concludes, "The findings suggest an evolutionary shift in socio-sexual signalling function from behinds to faces, two hairless, symmetrical and attractive body parts, which might have attuned the human brain to process faces, and the human face to become more behind-like."