The Thirteenth's time line flow (POSSIBLE SPOILERS)

[Iscah]

A thousand thousand is a million (1000x1000=1000000)

A "thousand thousand" is also used poetically in the game to mean a large but non-specific number. And to "precede" is to come before. Therefore, the span of time prior to the first calamity, which at the point of writing was a mystery.

The exact line is speculating the "evolution of [mankind] in the thousand thousand years preceding the first calamity", so setting up the question of an evolutionary vs mythical origin for the human races - although in this case the mythical seems to have won out.