Humans may have evolved to have faces that look uncannily similar to asses, according to new research on chimpanzee butt-recognition.
Researchers out of Leiden University and Kyoto University used experiments based on the "inversion effect" for facial recognition. When chimpanzees are experts at recognizing something, such as other chimp-faces or human faces, they're able to pick it out even if it's flipped upside-down. Previous recognition studies, they write in their study published in PLOS, have examined every body part "except the most obvious one," the butt. Obviously!
"The findings suggest an evolutionary shift in socio-sexual signaling 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." Our faces, our asses, our selves.
Butts contain multitude social cues for chimpanzees, one of the strongest related to mating: When female chimps are ovulating, their backsides turn shiny, swollen and red, cueing to males that it's go-time. But as we evolved into upright-walking creatures, things began to change.
We no longer showed off when we were in heat with a literal booty call, hiding it to encourage fewer one-night-stands (or what the chimpanzee version of that would be), and more long-term mates that would share offspring-rearing duties. "Permanent" adipose tissue replaced our red-alert behinds.
As we evolved, faces, especially in females, grew to have redder, thicker lips and fattier cheeks. Our faces even blush as a signal for emotional or hormonal response, and can be interpreted as a socio-sexual cue.
There's not much literature on how humans process behinds, they note. So get out there, in the name of science.