Dragon evolved from pterodactyls. When the meteor that ended the rest of dinosaurs hit the earth, the dragon ancestors then seculded themselves into deep cavernous lava caves. They lived within these lava caves for millions of years. They only started resurfacing during the middle ages in Europe. Since the cave systems in which the dragons live could not support very high populations, there were not very many at the time of surfacing. Knights then promptly eliminated the entire dragon population. It is still unknown if there are more dragons in the caves today. To answer your question, dragons were most likely first encountered by humans in the 600's in Europe. It is unknown when Asian dragons were first discovered.