Im watching lost again from the beginning, some things are becoming more clear, one thing that has popped in my mind we are now seeing dharma in the 70s and on the island in 2007, i was thinking maybe they were set to start on the island during the 70s working like they are now, the purge comes but they survive, in the meantime somthing happens like they all get amnesia and they end up back on the main land during 80s or there abouts, they carry on their lives then not knowing eachother they cross paths again, the plane crashes and start a fresh not knowing they have been there before and they are scattered around the island. It sort of makes sence as to why some are in the 70s and locke is in current time.
I know its very wacky and far fetched but its just another random theory... well we all know anything is possible in this program lol