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.
Green Martian skeletons would show signs of vestigial limbsThe 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.
Green Martians are Ice WarriorsOr 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 they train under high simulated gravity, explaining their more compact builds, and their secondary arms are compressed within the armor, possibly being used to work the complex machinery within.