This has always–– well, not always, as I was a really shy kid–– still, it has long puzzled me why public speaking is one of the greatest fears people suffer. "Most people fear public speaking worse than they fear death," or so I have heard. Why is public speaking so frightening? I have very little fear of public speaking, but then again, I have had years of giving class reports, preaching at church groups, and teaching a wide range of students, so maybe I have gradually become inoculated to the fear and have forgotten my anxiety years before. In any case, I'll tell you my hunch why most people fear public speaking. I claim no originality for this explanation, but if I happen to strike upon something good, I demand royalties that reach the heavens!
In terms of our evolutionary cognitive background, we are highly sensitive to eye/head figures. To my knowledge, reactivity to eyes-upon-us reaches as "deep back" as the gecko brain. This is why geckos often start and stop as they scurry along walls. They pause–– nay, freeze–– when we look at them and then "thaw" enough to make a short dash forward, only to be seized by oculophobia again. According to Jerome Kagan in Three Seductive Ideas (page 27), "a head containing eyes with a pupil-to-eye ratio that is slightly over 0.5 produces immobility in chickens." The fear that many and/or large eyes instinctively generate in animals is, presumably, the selective advantage of the eye-like pattern on a peacock's peafowl and the enlarged eye-shapes on many fishes' bodies. I'm sure there are other example, probably even on plants and flowers. Humans certainly share this sensitivity to eyes around us. Indeed, this sensitivity is probably much of what's behind the uncanny "feeling of being watched." We are hyper-sensitive to eyes and therefore may subconsciously and peripherally detect eyes long before the unease generated by them causes us to feel watched.
So now imagine you are standing in front dozens of other animals all looking at you–– with their many eyes of course! No wonder people feel scared to speak in front of a crowd. Consciously, we know our words are aimed at their ears, but subconsciously, instinctively, we know their eyes are aimed at us. Further, if you've ever spoken before large groups, you know just how bad most listeners look: bored, irritated, confused, tired, anxious, etc. Psychologically, we are waiting to see how the listeners will respond to our speech, which is rather like trying to interpret eyes in the bush as either wanting to eat us or wanting to flee from us. Add to this the draining cognitive (computational) burden of prolonged, advanced language production and you have a sure recipe for fear of public speaking.