Why do so many cultures have myths about Dragons?

     In order for dragon legends to be so widespread, there must have been a catastrophic event that was common to all areas of the Earth. One theory states that "the only thing that could be seen by every person in the world, and be able to cause global destructive effects, would be an impact on the Earth by a comet."  

     If this did happen at some time in history, every culture would have seen the comet (dragon) moving across the sky, and each night it would have gotten bigger and bigger. This monstrous flying serpent monster would have terrified everyone who saw it.  Probably not very many people would have seen the comet, but when the Earth was hit later by burning rocks falling from the sky, they would have noticed!  The sky would have grown dark for months, crops would have died, and people would have starved (thus the BAD dragons!). Makes sense, doesn't it?!