Sometimes islands spread apart, sometimes they shove together.
Indeed, the land of the Earth used to be a single mass, Pangaea. Before that, the land of the Earth was a collection of islands and continents that eventually shoved themselves together to form Pangaea. Before that, the land of the Earth was a single mass, Pannotia. Before that, the land of the Earth was a collection of islands and continents that eventually shoved themselves together to form Pannotia. Before that, the land of the Earth was a single mass, Rodnia...
http://en.wikipedia.org/wiki/Supercontinent_cycle
The monsters we see in the game do not line up with Akaneia history, and the level of technology appears above that seen in FE1/3. Furthermore, the game must be separated from FE1/3 by at least a few hundred years for so many differences to exist, but Marth and anyone relevant anything like him were not even born a few hundred years before the events of FE1/3, leaving a few hundred years after FE1/3 as the only logical possibility for why some "form" of Marth would show up now.