Why are we scared of public speaking?

Are there any reasons why majority of the people are terrified of speaking in public? Why is this such as huge fear?
I was asked asked this question on another website and I thought it would serve well as an introduction to the fear of public speaking.

I have taught people who are scared of public speaking for the last ten years and there are some core answers to this question. (There are of course individual experiences which add to this)

1) We over-think before, during and after about our "performance". So self-consciousness is at its peak. We see it as a performance so it feels like we have to pressure on ourselves to be better than we normally are
2) We have a 500 million old flight and fight system kicking in (language is only 120,000 years old and powerpoint late 1980's!)
3) Our brain is evolutionary biassed towards the negative - on the look out for threats and great at recalling previous threats. ("is that a stick or a snake?" type of decisions were very important for most of our evolution)
4) We don't understand the psychological difference between conversation and public speaking and that gets us into trouble - big time!

And if I had to choose where to start fixing this fear - I would start with point 4. And this might sound strange place to start. But I really think its a fundamental factor we need to get our head around.
The one-to-one conversation skills we all know: nodding, eye contact, affirming by little sounds, asking questions, smiles etc. Wth a listener offering some of these we know that we are doing ok/well in a conversation.
When we publicly speaking two things are happening
1) the audience move from active listening and drop those micro-approval signs that we normally give. So the individual does n't look after you any more. The social pyschology of a group takes over. So instead of the normal smiles and nods the speaker get the "dreaded" blank faces.
2) If the speaker hasn't thought about changing the skills she is using then we are still looking for approval signs. And because they have become an audience - a group - they don't give us approval. They give us blank faces. A blank face in a normal conversation is tantamount to being disliked, judged, humiliated perhaps. So we are facing what we think of an audience that doesn't like us. Not a great place to be as we want their approaval.

So we need to change our heads around a little and just see blank faces as just listening faces. Full stop. Blank faces in an audience don't mean attack - its just how people listen. So lets just get used to blank faces as normal. Understanding point 4 helps to make shifts with points 1-3 as well. We no longer see the audience as a threat but just as a whole bunch of listening faces.
And then public speaking becomes a lot less threatening.