At one point in the game you run into a scholar who's studying evolution. He calls himself a psarologist. Psaro was the Big Bad of the fourth game and was searching for the Secret of Evolution.
The DS remake of DQVI has an NPC in Reaper's Peak explicitly state that DQIV is the near future of the same world, and DQV is the far future of it, and the Zenith Dragon hatches from its egg in the ending of that game.
No Export for You: The original SNES version didn't get a overseas release, it stayed only in Japan, probally because the franchise was still rather unknown outside its country of origin, Dragon Warrior had no chance against Final Fantasy overseas, while in Japan Dragon Quest triumphed over Final Fantasy quite a lot back then. Then comes the PS2Remake, in 2004 the franchise already had some ground outside Japan but this version still didn't get an international release. Only in 2008 the rest of the world could truly enjoy DQV officially with its Nintendo DS release, before that only Fan Translations were available.