My theory is that ascension would kill any pure-blood humans and the reason ascension is possible is because you need a magical body to reach ascension, since vastayans have magical bodies I believe that the ascended had a vastayan ancestor no matter how far back it was.
OR ascension is how vastayans came to be, vastayans were maybe come into existence BECAUSE of ascension and the Ascended had children which became the first of that particular vastayan species...but I'm pretty sure ascended cant have children right?
I know what you're thinking "but what about Xerath, did he have a rock as a vastayan ancestor?"
I believe xerath was a special case, since he was a mage before ascension, that had something to do with it even though his body wasn't magic...which is probably why his body wasn't morphed but instead completely destroyed.. I believe since he was a mage but his body wasn't a magical body his body didn't ascend but his magic did and in some way he moved his conscience over from his body to his magic, not his soul but his actual magic maybe the rock and magic form he has now means he had some rune on him or something at the time and it was some sort of catalyst that made it possible for that to happen.
Azir is also another special case like xerath, If my theory is correct he either has a vastayan ancestor in him or something else happened, I don't believe he had a vastayan ancestor because he didn't take on any animalistic traits (his helmet that looks like a bird doesn't count, it's not a part of his physical body)... if my theory is correct what do you guys believe is going on with azir being able to ascend? why is it so closely related to sand?