The legend stems from where all legend stems from - something in the distant past happened, and we embelished it beyond belief.
I regret that I don't have a straight answer (you'll find scant many of those when dealing with the majority of topics here) for you, but I can give
you my thought on the whole Lizard thing - crap. A whole load of snake crap. Personally, I'm more frightened of the actual people running the world
than a group of possibily ficticious lizard men taking it over.