Think before you tweeze, eyebrows are essential

A clue to the point of eyebrows, courtesy the 18th century: According to a report in the journal Perception, wealthy women in western Europe in the 1700s, desperate for thicker brows, glued thin strips of mouse hide to their foreheads.

Why? Eyebrows, like eyelashes, accentuate the eyes and make us more attractive to other people. Even from a distance, we check out other people's facial expressions - especially the position and shape of their eyebrows - to figure out their mood.

Our twin-caterpillar eyebrows are one of evolution's more clever uses of body hair. The underlying brow bone juts out protectively above our vulnerable eyes. Its two hairy strips are multitaskers, acting as both physical filters and expert conveyers of emotion.

Eyebrows are remnants of the hair that once covered our distant ancestors' bodies, tiny survival aids protecting our vision. Eyebrows can warn of an insect approaching the eye, since a crawling bug causes hairs to move to and fro like grass. The hair strips also catch dust and dirt before it can enter the eyes.

On a hot day, a sweating forehead helps keep the head and brain from overheating, as sweat evaporates into the air. But when sweat trickles down, eyebrows help trap some of the salty, stinging liquid before it drips into our eyes.

The shape and structure of eyebrows actually hints at their function. Mr. Spock aside, the average eyebrow arches up and then down again, and most of the brow's hair growth follows this slant. Like rain gutters, the slanting brows channel liquids, such as raindrops, toward the outside of the eyes (and also down the side of the nose).

You could do your own eyebrow experiment, shaving off your brows and then standing in a downpour. But the scientists who conducted the 2002 eyebrow study published in Perception probably wouldn't recommend it. Eyebrows, they say, have even more to do with conveying emotion and identity than with protecting us from the elements.

Even an eyebrow "flash" conveys emotion, signaling welcome, agreement or appreciation. And then there's high eyebrow drama: Your approaching boss' eyebrows set in a stern frown, your eyebrows knit in concentration or raised in surprise (why is he coming to my desk?).

How important are eyebrows to the face? It's easy to identify Mr. Spock by brows alone, but the MIT eyebrow study found that our brows identify the rest of us, too. Researchers showed volunteers faces of celebrities, with and without eyes or eyebrows. The result: People were better at identifying celebrities missing eyes than those missing eyebrows. Hmm. . . . Who is that bland, sad face with the rather beady eyes? But the same face with full, bushy brows - and no eyes at all? Piece of cake. Richard M. Nixon!