This is a proposed timeline.
BE: Before Eruption - AE: After Eruption
~10-100 million years BE: In close succession: three meteorites hit the Earth and two more crash on the moon. The direct and indirect effects of the incident bring about the extinction of a big number of species in the following period. The ones that survive, though, will eventually populate the planet in full, returning life to it. The meteorite in the ocean will later start poisoning the water around it and mutating the residents of the sea. Even later hints of it will wash off in the western continent's coast. At this point, it's unclear when exactly.
~350,000 years BE: Homo Sapiens first appears.
~100,000 years BE: The land surrounding the second meteorite crash site is first inhabited by humans. They will slowly come under its influence.
~4000 BE: The abilities of the special bronze found in the land neighbouring the second crash site are first discovered (although not by the broad public).
~300 BE: At the third meteorite's crash site, in the southernmost island, alchemy using yellowstone is developed.
1 AE: The Volcanoes erupt. Much destruction ensues. You know the story. At that point, civilisation should be at about an early medieval (or antiquity) phase. After the isolation, separate civilisations advance or regress in their own pace and direction.
1-653 AE: A mild Ice Age comes upon the planet.
649 AE: Southern-Continent Ships (re-)discover the Northern-Continent. Limited contact is established in the next years.
703 AE: Youmoto finally resolves its internal conflicts with the rivaling nations of its continent ("China", "Korea") and having established itself as the reigning power in its land mass, it now looks to the rest of the world.
741 AE: The first official contact between non-central landmasses occurs when a Youmoto aircraft lands in Northern-Continent's land.
745 AE: The first global congress is held in "Junction", with representatives hailing from all major countries present, to discuss world politics to maintain the peace and stability of the planet.
874 AE: The first Grand Tournament of Magicians is held and our story begins.