The Phantasy Star Compendium book has a timeline that claims the Earthmen arrived in Algo in AW 822, sometime between PSI and PSII. But, it seems to me that there is some potential evidence suggesting that they might have been around before that.
PSI's world has a lot of connections to Earth. There are burgers and cola, which are very Earthy things. Flashes are basically flashlights (Generation 1 even shows that they look like real flashlights). There's the Perseus/Medusa thing that mirrors the Greek legend. One of Lassic's histories says he converted to a religion brought to Algo by outsiders, who made his armor. The Earthmen and their Dark Force could have certainly fit that bill.
So my question is, do you think the Earthmen could have arrived even earlier and begun influencing the development of Algo before PSI?