On Being Thrown Under the Bus

Anyone have a clue where the phrase “threw him under the bus” derives from? It apparently means to reveal information about a friend for personal gain, usually the friend does not deserve the treatment.

If I understand cliches I believe the original statement has to have actually meant something and quite possibly be a valid insight. Then through overuse it loses its original firm meaning.

So I ask, who exactly was thrown under the first bus and why?