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Extraterrestrial (or extra-terrestrial) may refer to any object or being beyond (extra-) the planet Earth (-terrestrial). Aliens are a general term for extraterrestrial lifeforms.
Contact with aliens
Legends and myths dating back many centuries have been written about superior beings that visit Earth from the heavens. In the 20th century these legends came to be interpreted by some as stories of alien visitors, possibly even as ancient ancestors to the human species. (Calling the Twilight Zone)
Astronomers and other scientists that study space have estimated that there are 100,000,000 worlds in space in which evolution has led to the development of higher forms of life. With this estimate in mind, some began to listen to the stars with large radio-telescopes for any signals that may be sent indicating that such life is present. (TZDELL: 02: "Calling the Twilight Zone (comic story)")
Some Earth anthropologists have stated that although man developed on Earth as a four-limbed creature, along with many other Earth creatures, other worlds may have had evolutionary changes that created much different physiologies. On other worlds, it may be more common for living organisms to perhaps have six or more limbs. (TZDELL: 02: "Calling the Twilight Zone (comic story)")
Such changes can be witnessed when encountering the Twilight Zone. For example, aliens from Venus have been observed with a third eye, while Martians have been seen with three arms. Like on Earth, living beings from the same planet did not all share the same physiology. For example, while some humanoid aliens from Mars did have three arms, others had only two. Some Martians had one head, while others had two heads. The same was true of Venusians, some being very small in stature while others were a height typical to humans and not all possessed three eyes. (TZ1: "Will the Real Martian Please Stand Up?", "Mr. Dingle, the Strong")