Why would this be obvious? Because rear ends serve a big purpose in the chimp world. Female chimps’ buttocks grow redder and swollen when they are ovulating, signaling to males that it’s business time. And it’s important to know whose bottom it is, in part to prevent inbreeding. The buttocks have, in scientific parlance, a “high socio-sexual signaling function.”

But when we began walking upright, our bottoms became fleshier and no longer broadcast our ovulation status, possibly to discourage casual hookups in favor of pairing up and sticking together for the children’s sake. On the other hand, humans — “especially females,” the researchers write — developed ruddier and thicker lips, as well as fattier faces.

Bottoms and faces are both symmetrical, they add, and interpreting what a butt is saying is crucial to chimpanzees’ reproductive success, just as interpreting facial messaging is important to human mating.

“Thus,” the authors write, “human faces share important features with the ancient primate behind.”