In the alternate time line the island was sunk by the bomb in 1977. This means Desmond never went to the island and never caused Oceanic 815 to crash. As to why he was on the plane from Sydney to LAX has not been explained. (EDIT) I'm sorry but the bomb did not sink the island. I wish people would stop saying that without facts to back that statement up. If the bomb was the cause then why were the buildings still erect underwater. Wouldn't the blast from a hydrogen bomb scorch the island. I'm just saying.