Good point you brought up here, Monkeys. I can't say with any more certainty than you on this, but it is possible that humans existed before the date in which the lore states. Aside from the Extra-dimensional beings, we have six races deemed intelligent enough to have a minimum of an oral record: Centaurs, Tengu, Charr, Humans, Dwarves, and Forgotten. Written records serve as more reliable proof to oral traditions, and I assume that out of the six, only Humans (one of the youngest of the races), Dwarves, and possibly Forgotten have written traditions. It is possible that humans have been in the world long before the date stated in the lore, but lacked a written tradition and were not noticed by the other races. Therefore, they could have travelled to the desert even before the Forgotten got there and Ascended. So it is still possible that the Mursaat may have once been human.