But the first humans to come to Thedas already had magic, did they not?
It's a theory.
My memory is fuzzy and tends to remember details fitting my ideas and put aside those which don't ...
But anyway what make me believe this is :
Somehow it seems humans started to have dreamers when they landed in Thedas.We know ancient elves had dreamers , so the only way I can imagine catching that "dreamer" disease is by breeding with elves.
In WOT1 (page 26) you can read the Neromanian believe to be the first human in Thedas (future Tevinter) were also the first human to practise magic .Now they could have had mages before of course , but I find that suspicious.
Those first humans , first to practise magic , also discovered blood magic after arriving in Thedas , on top of getting Dreamers after landing in Thedas.
They also have an history of tension with the elves from the very beginning.
Anyway what I assume happened is humans and elves didn't start fighting right away.The elves were in a bad shape , but humans were tribal without magic .Not a threat.
Then they started trading , breeding...and they figured out two things:
First humans got magic , yeah for humans!
Second elves were getting humans babies in the bargain.Erf for elves.
Elves decided to withdraw , Tevinter decided they loved that sweet elven blood for their future kids and blood magic...