In the scenario editor, the system looks at the rule.txt file and within that there are groupings civs that show up when you play many opponents and choose to select your opponents. This is why if you learn how to mod this, you can play specific groups of seven civs on a map.
Ever wonder why you can't play with the Indians if you play with another civ? The groupings are set to a specific slot so you can't play with two designated from the same slot.
Then there is a parameter for not restarting as well because while the new area may be distant enough to allow them to thrive, they start out so weak that it is nearly futile for that respawned civ.
But looking at the rule file, what can happen is say the Spanish and the Zulu are both in group seven, then if the Zulu are eliminated,the Spanish may show up. If configured well, then as civs are eliminated then others vastly far away may start, say in New World.