adam and eve were 2 skeletons jack found in the caves way back in season 1. jack found a pair of white and black stones in their pockets or something.
the producers have said that that by the time lost ends, adam and eve being in season 1 will be evidence that they had certain things planned all along, which basically means before lost ends a male and female character in the past are going to get a hold of black and white stones, go to the caves and die and everybody will go "ohhhh so thats who adam and eve were"