The X-Men watch a TV news report about a man in a loin-cloth in Antarctica, a sort of "latter day Tarzan", as Jean puts it, and conclude, not unreasonably, that he must be a mutant. Professor Xavier is certain otherwise, but sends them south anyway. Hidden there they find a strange area featuring an anachronous mix of species from previous eras, in a pretty clean lift from Burroughs' The Land That Time Forget. Apart from Ka-Zar, there are also native "swamp men", who capture Jean and Warren and of course intend to sacrifice them. It's jarring to read this type of thoughtless pulp-derived trope in a comic that will later try to become an allegory about racism. I mean, it's even called the "Savage Land". But then, the highest grossing film of 2006 did the same thing much more objectionably.
One could see this as an early mashup, as Burroughs and post-atomic science fiction are brought together in one text. This tendency will only grow, as Marvel continue to create links between series, eventually creating a shared world where cross-title continuity is essential.
Introduction of Ka-Zar, Zabu, and the Savage Land [not named]