In "House of the Rising Sun", Jack discovered two bodies in the cave. On these bodies, he found a small bag containing one white and one black rock. Locke gave the bodies the names Adam and Eve.
Facts
They are skeletons found in the caves discovered by Jack in the episode "White Rabbit". They have not been seen since.
The bodies themselves were not discovered until the episode "House of the Rising Sun", when Jack returned to the caves to collect water.
They have been dead at least 40-50 years according to the deterioation 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 main article: List of recurring themes: Black and white
Interview testimony
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?"
CUSE: The answer to that question goes to the nature of the timeline of the island. We don't want to say too much about it, but there are a couple Easter eggs embedded in "Not in Portland", one of which is an anagram that actually sheds some light on the skeletons and hints at a larger mythological mystery that will start to unfold later in the season.
LINDELOF: There were certain things we knew from the very beginning. Independent of ever knowing when the end was going to be, we knew what it was going to be, and we wanted to start setting it up as early as season 1, or else people would think that we were making it up as we were going along. So the skeletons are the living -- or, I guess, slowly decomposing -- proof of that. When all is said and done, people are going to point to the skeletons and say, "That is proof that from the very beginning, they always knew that they were going to do this."
One, the anagram, could be Mittelos -- "Lost Time" or "Time Slot" -- and there could also be something embedded within the Room 23 video.
The other may be the reversed audio from the Room 23 footage. When played backwards the words "Only fools are enslaved by time and space." is repeated extremely clearly. It is possible to make the names "Adam and Eve" from the letters of this phrase.
Two stones appear on a table in Desmond's flat in his flashback during "Flashes Before Your Eyes". (It is also possible that the "stones" on the table in Desmond/Penny's flat are in fact just the bottle caps that came from the empty beer bottles on the floor around Desmond. Higher resolution screen caps/set photos may be helpful)
