Man of Science, Man of Faith

Jack & Desmond / Jack & Shannon Via St. Sebastians

Jack meets Desmond when he was running the Tour de Stad on the stadium steps. Jack injures his foot and Desmond stops to help him.  Later in the episode he is  revealed as the one living inside the hatch .  

In Orientation, Jack and Desmond face off in the jungle and Desmond finally remembers where he's seen Jack before, running in the stadium in L.A.

Jack is called to the emergency room where he needs to tend to two car crash victims.  The EMT says one victim is "Adam Rutherford".  Jack decides to tend to Sarah's wound which resulted in the death of Mr. Rutherford.  In Abandoned we find out this is indeed Shannon Rutherford's father.