H.G. Wells's novella The Time Machine is a classic in science fiction for its elaboration and codification of time travel tropes. When the time traveler reaches the far future, he meets two distinct species of human, the Eloi and the Morlocks.
Although be briefly supposes the Morlocks to be non-human, he quickly comes to the theory that Man has split along two evolutionary paths; the aristocratic intellectual elite remained on the surface, pursuing knowledge and art, before losing sight of their intellect and falling to sloth and simplicity -- these are the Eloi. Meanwhile, the workers who toiled in underground chambers, maintaining machinery and generally keeping things working, slowly lost their intelligence as well, but to a much greater degree; as the machines broke down, the underground men became more like animals, even turning to cannibalism -- these are the Morlocks. Both species are descended from 19th century Man, but the circumstances of their evolution has altered their biology and mental ability.
The Upper-world people might once have been the favoured aristocracy, and the Morlocks their mechanical servants: but that had long since passed away. The two species that had resulted from the evolution of man were sliding down towards, or had already arrived at, an altogether new relationship. The Eloi, like the Carlovingian kings, had decayed to a mere beautiful futility... and the Morlocks made their garments, I inferred, and maintained them in their habitual needs, perhaps through the survival of an old habit of service.
(Wells, The Time Machine, eNotes eText)
Additionally, as the quote shows, there is still some connection between the two species, even as the Morlocks move from unthinking servitude to predators.