Fr. Roderick posted a very interesting theory over at the SQPN website called The Fountain Of Youth. Sounds very good, and might be pretty close to what is happening on the island. It would also explain why the six that went back want to protect the folks that stayed !
I am not sure about the whole time traveling thing though, there is ofcourse something off, as we have seen in the last episode, but if it’s to the extreme as this theory suggests remains to be seen.