Protheans. Thessia naturally possessed a large amount of Eezo, which is a powerful mutagen. Thus life on Thessia would have had to evolve a method of dealing with it safely which the protheans would merely needed to subtly alter to allow for the use of biotics. The reasoning? Who the hell knows, since if the Reapers found the protheans doing something as obvious as a genetic uplift on a strategically important world (Fucktons of Eezo) they probably would have killed them all and reverse or alter the changes to their uses (or just kill them too).

Protheans. Thessia naturally possessed a large amount of Eezo, which is a powerful mutagen. Thus life on Thessia would have had to evolve a method of dealing with it safely which the protheans would merely needed to subtly alter to allow for the use of biotics. The reasoning? Nobody really knows, since if the Reapers found the protheans doing something as obvious as a genetic uplift on a strategically important world (Large amounts of Eezo) they probably would have killed them all and reverse or alter the changes to their uses (or just kill them too).