They're clearly not human. They're apparently mammalian, but they have no apparent gender other than by appellation. Thus I assume they're aliens, also evidenced by them being surrounded by technology beyond current human capabilities, going as far as actually containing electronic screens in their thoracic cavity. From their behavior, they seem unskilled and are largely uninterested in this technology that surrounds them and nurtures them (the speaker that announces events, the "nu nu", the food machine).
From this I assume that they are aliens but they are not the same aliens that created the technology. They are either a sub-species of alien or perhaps mentally deficient members of an advanced alien society, and they have been collected together and are being nurtured by a biosphere that's been constructed to their liking (grass, bunnies, baby-sun). This biosphere appears to be self-sustaining, as no other living creatures are ever seen other than the bunnies, grass, and flowers. And the semi-sentient technology that surrounds them appears to have been created to maintain them, namely the food machine that makes Tubby Toast and Tubby Custard, and the "nu nu" which cleans up. I assume they're not a larval form of the more advanced aliens, as the technology makes no effort to educate them or advance them to a state where the bunny-filled biosphere would no longer be suitable.
I assume the Tubbies' intelligence and capacity for ethics are severely limited, which is why they always shout "NAUGHTY NU NU" at the robotic cleaning-up machine even when it's clearly done nothing wrong. Perhaps their environment is intended to keep them separate from the larger population, where their misplaced sense of "naughtiness" might cause trouble for society at large.
So from this I am drawing the conclusion that the Teletubbies are mentally defective children of a technologically advanced alien race. Since they enjoy each others' company and clearly don't mind performing the same rote tasks every day, they were collected into a self-maintaining biosphere that's designed to keep them together and happy to the extent that their limited capacity for learning and entertainment can allow.
Whether this biosphere exists on the alien planet, another planet, or is floating through space somewhere is not clear. The Tubbies lack the curiosity to explore the frontiers of their environment, so it was never shown on the show.
Next week: The Professor never fixed the boat because professors can't get hot chicks like Ginger and Mary Ann unless they're stuck on an island.
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