Conclusion

With all of the above evidence and what I'm about to suggest, I believe that we can all conclude that Sans and Papyrus are the same as Mario and Luigi. As seen within the story of Undertale, the game takes place underground in a realm that can be entered with ease but cannot be left without a great ordeal. I suggest that one day, the Mario Bros., went down a warp pipe that took them to this hidden world. I believe that they could not get back to their world because they were good enough people to not kill the monsters. They implemented themselves into their new society, finding jobs for the King and Queen and making a living. I also believe that Luigi, or Papyrus, was the one who invented spaghetti in this world, while Mario, or Sans, was going around and exploring and doing odd jobs for the King and Queen, hearing many jokes along the way. This is what I believe caused Sans to become a jokester. However, as time went on, the Mario Bros. were aging and aging, eventually dying. I believe that after they died, the community had grown depressed without their two friends, pushing the Royal Scientist into finding a way to reanimate them again. By the time this reanimation was eventually completed, the brothers bodies would have most likely decomposed into skeletons. This explains how they became their skeletal counterparts, but what about their names and souls you may ask? I believe that they would have lost their souls within this reanimation process, as they became neither monster nor man, causing them to also have altered memories and possibly different names. This is why I believe Mario and Luigi to be Sans and Papyrus respectively.