John Carter is a Martian hybrid.
Could John Carter actually be one of the first experiments in Barsoomian hybridization? Perhaps his bloodline is White Martian and Yellow Martian, to give him his white skin and black hair. He could have been astrally-projected to Earth to improve his conditioning, then his former body on Barsoom was killed (presumably not purposely by the experimenters, but maybe a saboteur), and that could have caused his amnesia. He seems to have a Barsoomian lifespan and genetics, since we're not given any indication that he fathered children on Earth, but he can obviously father children with Barsoomian women.- Next you'll be suggesting he's some sort of Immortal warrior sent to Earth to fight others of his kind in the hope that he can someday return to his home planet. ...Hm.
John Carter is a former illusion created by the Lotharians.
Like Kar Komak, he's a construct who achieved flesh and blood life. Perhaps a Lotharian created him on Earth as a method of long range observation, and then cast him aside? Green Martian skeletons would show signs of vestigial limbs
The general body plan for Barsoom fauna seems to be octopodal; the Green Martians stand out for having only six limbs (the various Human Aliens on the planet stand out, too, but it's probably safe to say they have an entirely different evolutionary history from most of the Martian life we see). Presumably we would see evidence of the atrophied fourth set of limbs if we could examine Green Martian skeletons. On that note, Green Martians are the original native sentient race on Barsoom
The more humanoid races, probably all closely genetically related, came later on, possibly as refugees or an imperialist power. Green Martians are Ice Warriors
Or possibly the descendants of Ice Warriors, who took up a more nomadic "raider" lifestyle as the decaying Martian biosphere made city-states less sustainable. Presumably their urban ancestors trained under high simulated gravity, explaining their more compact builds and better agility in Earth-level gravity, and their secondary arms are compressed within the armor, possibly being used to work the complex machinery within. Barsoom's inhabitants are colonists from the Galaxy Far, Far Away who lost access to space travel a long time ago
Note that Barsoomian Basic uses "Jed" as a title of nobility, a status they tie to military command. This could be a corruption of the term "Jedi", given their role as generals in the days of the Old Republic.- More than that, their word for "lieutenant" is "Padwar", a probable corruption of "Padawan".
- The irony of this mass guess is particularly amusing, since George Lucas was heavily influenced by the books, and stole most of his terms, words, etc from the books. Including but not limited to Banths/Banthas, Jeddak/Jedi, Padwar/Padawan and even Sith (Which is a large predatory insect on Barsoom)
Green Martians are the Fallen
Although they go extinct sometime in the 19th or 20th century, they are cloned back to life with technology introduced by the Traveler.