In addition to being helpful in treading water, eyebrows are essential in portraying many of our moods and emotions. They are a key to facial expressions and basically tell us about how a person regards or feels about something. That is also a reason the Mona Lisa is so famous, her expression is always questioned, is she sad, happy? The answer to these questions lie above our eyes, alas the Mona Lias has no eyebrows and therefore her mood is a hard guess.
The online mailbag item to which the original post refers is “What is the purpose of eyebrows? (22-Mar-2000)”