What is the real "Human Realm" 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.