What is the real human realm, Is it Earth or are there many other places / Planets with humans? What are the words of Lord Buddha on this topic?
I am looking for the Theravada perspective.
The original home of human beings according to the Pali Canon is the Abhassara realm, lit. "from whose bodies are emitted rays like lightning." This is the top realm of three, next above the Brahma world, in the Form world of the devas (lit. "shining beings"), corresponding to the second jhana of meditative attainment. As a result of their increasing infatuation with sensuous pleasure, at least some of the residents of this realm "fell" into rebirth in the earthly realm, the manusyaloka. Since this degeneration was gradual, it implies that there are many "vibratory" or energetic levels of human beings in our universe, ranging from animal-like human beings with short life spans (the texts mention a minimum of 10 years) to glorious deva-like beings with long life spans and great wisdom and power (the texts say 80,000 or 84,000 years, which probably symbolize an unknown but large number; 84 is a Buddhist symbolic number representing totality). The latter correspond to extra-terrestrial civilizations and perhaps trans-dimensional beings as well. The shortest-lived non-human primate has a lifespan of 8 years (including the greater dwarf lemur, purple faced leaf monkey, bare faced tamarin, and western tarsier). This implies that primates are the degenerated remnants of human beings, rather than the reverse. Since this appears to contradict the evidence for evolution, one might speculate that human evolution was triggered by some sort of intervention, but this is going beyond my knowledge of the Pali Canon. The longest-lived human beings are genderless and do not have physical bodies, but appear rather as luminous beings who fly through the air including, presumably, interstellar space. Such "luminous bisexual spheres" are also mentioned by Plato. The comparison with UFOs, the physical existence of which cannot be denied (see Dr. Jacques Vallee, Dr. J. Allen Hynek, Dr. John Mack, Dr. Karla Turner, John Keel, and others), is unavoidable.
There is a view that there are multiple planes with humanoid like being which constitute the human realm.
"As far as the sun & moon revolve, illumining the directions with their light, there extends the thousand-fold cosmos. In that thousand-fold cosmos there are a thousand moons, a thousand suns, a thousand Sunerus — kings of mountains; a thousand Rose-apple continents, a thousand Deathless Ox-cart [continents], a thousand northern Kuru [continents], a thousand eastern Videha [continents]; four thousand great oceans, four thousand Great Kings, a thousand [heavens of the] Four Great Kings, a thousand [heavens of the] Thirty-three, a thousand [heavens of the] Yamas, a thousand [heavens of the] Tusitas, a thousand heavens of the Nimmanaratis, a thousand heavens of the Paranimmitavasavattis, and a thousand Brahma worlds. And in that thousand-fold cosmos, the Great Brahma is reckoned supreme. Yet even in the Great Brahma there is still aberration, there is change. Seeing this, the instructed disciple of the noble ones grows disenchanted with that. Being disenchanted with that, he becomes dispassionate toward what is supreme, and even more so toward what is inferior.
"There comes a time when this cosmos devolves. When the cosmos is devolving, most beings head to the [heaven of] the Radiant. There they remain for a long, long time — mind-made, feeding on rapture, self-radiant, faring through the sky, abiding in splendor. When the cosmos is devolving, the Radiant Devas are reckoned supreme. Yet even in the Radiant Devas, there is still aberration, there is change. Seeing this, the instructed disciple of the noble ones grows disenchanted with that. Being disenchanted with that, he becomes dispassionate toward what is supreme, and even more so toward what is inferior.