Have you ever stopped to consider your face? Compared to most of the rest of the animal kingdom, the human face has at least one really peculiar feature: it's almost completely devoid of hair. Sure, some people grow beards or moustaches, but even a full pirate's beard would leave quite a bit of skin showing. They don't call us "the hairless ape" for nothing. How did we come to be so bare-faced?

The reasons we lost our body hair are still debated. Some researchers think that we lost our fur to free ourselves of parasites, such as lice. This might have made us more attractive to the opposite sex; bare skin would advertise our lack of parasites. Others have suggested that we lost most of our hair to facilitate cooling as we moved from the shady forests to the hot savannah. Still others wonder whether nakedness is one of a number of juvenile traits that humans retain throughout their lifetimes; humans are thought to be, in one sense, juvenilised apes, who mature slower and live longer than our ape cousins.

Yet Mark Changizi, a neurobiologist at 2AI Labs in Boston, has an intriguing alternative explanation for why we don’t have hair on our faces, at least when we compare ourselves to other primates. It's because we're walking, talking, breathing ‘mood rings’.

Colour reveal

Mood rings were a short-lived fad of the mid 1970s. The idea behind wearing one was that it would act as a sort of emotional barometer, betraying your innermost feelings to anyone who took a glance at the jewellery. In truth, mood rings were little more than thermometers, designed to change colour according to body temperature. But the idea – that colour betrays emotion – isn't actually all that farfetched. The idea is pervasive in human culture. We become "green with envy". We turn red when angry or embarrassed. Sadness is referred to as "the blues". The truth may not be so far off.