They are skeletons found in the caves discovered by Jack in the episode "". They have not been seen since.
The bodies themselves were not discovered until the episode "", when Jack returned to the caves to collect water.
They have been dead at least 40-50 years according to the deterioration of their clothing.
They were carrying one black and one white stone. Jack took these stones and neglected to mention them when Locke asked what had been discovered. See also: Black and white.
The religious nickname is taken from Adam and Eve of the Bible.
In an EW.com article, Damon Lindelof and Carlton Cuse answered the fan question, "What is the meaning or significance of the two skeletons that Jack and Kate found in the cave of season 1?" (potential spoilers)
In an Underground Online Article the producers talk about how the skeletons may give insight into the nature of the show and evidence that certain elements were planned early on. (potential spoilers)