It seemed that Desmond is able to relive both Ezio Auditore Da Firenze and Altair Ibn La`Ahad because both are ancestors to Desmond, but they’re not related to each other. One is from the mother’s side and the other’s from the father’s side. So Desmond has a strong assassin background, which can explain why he’s the chosen one. Hopefully that’ll clear off the complex storyline of Assassin’s Creed, or are you madder now?