For many years, the X-Men had the backstory that the original team, minus Beast as he was with the Avengers at the time, went on a mission to the mysterious island of Krakoa and disappeared. Cyclops escaped and he and Professor Xavier put together a new team of mutants who went back to Krakoa and rescued the captured X-Men, making a new team that would go on to become Marvel’s biggest book.
More recently, a retcon came in. There had been for years talk of a third Summers brother after Cyclops and Havoc, and finally, that story was coming out. The third went by the codename of Vulcan, and he was part of a brief X-team that attempted a rescue of the original X-Men from Krakoa. That team appeared to die, and the distraught Cyclops was so upset that Professor X wiped his memory of this second X-Men team before sending out the familiar team that included Colossus, Wolverine, Storm, and Nightcrawler. Vulcan had managed to survive and came back much later very powerful and evil, but how?
Well, we can thank Darwin for that.