User blog comment:Sethril/Destiny: Lore...of sorts..../@comment-107.200.106.28-20141022150738

If you look at the class items the speaker sells in the tower there is a Titan Mark (Deep Stone Crypt) that has the description "A badge illustrating the infamous subroutine which seeded the first Exo consciousness."

It looks as if the Exo came about randomly from a glitch in coding. Perhaps they started off as standard human built machines that began to self produce with their new consciousness?