An extraterrestrial or alien is a lifeform that did not originate on Earth. (The word extraterrestrial means 'outside Earth'.) Extraterrestrials are a common theme in modern science-fiction, and also appeared in much earlier works such as the second-century parody True History by Lucian of Samosata.
History
Antiquity
The 2nd century writer of satires, Lucian, in his True History claims to have visited the Moon when his ship was sent up by a fountain, which was peopled and at war with the people of the Sun over colonisation of the Morning Star.
The way people have thought about extraterrestrials is tied to the development of actual sciences. One of the first steps in the history of astronomy was to realize that the objects seen in the night sky were not gods or lights, but physical objects like Earth. This notion was followed by the one that celestial objects should be inhabited as well. However, when people thought about such extraterrestrials, they thought of them simply as people, indistinguishable from humans. As people had never considered a scientific explanation for the origin of humankind or its relation with other lifeforms, any hypothetical rational lifeforms had by necessity to be humans. Even in mythology, all deities are mostly humanlike. Johannes Kepler's Somnium (1634), Francis Godwin's The Man in the Moone (1638), Cyrano de Bergerac's Les estats et empires de le lune (1657) and others all thought of selenites that differ from humanity only in culture or habits. Few writers ventured beyond anthropomorphic designs, some exceptions were Bergerac's ' and Miles Wilson's The History of Israel Jobson, the Wandering Jew (1757).
Early 20th century
thumb|A human encounter with an alien on the cover to [[Amazing Stories]]
Pulp magazines emerged as a new venue for science fiction. Many stories were set in worlds with quasi-human aliens, menaced by dangerous monsters and beautiful women serving as a love interest for the hero. This is the pattern of Ralph Milne Farley's The Radio Man (1924) and others. Pulps also featured monstrous alien invaders, in the style of The War of the Worlds. In the first space operas, such as those from Amazing Stories, good and evil aliens were clearly distinct: spider-like, octopoid, squidoid, and most reptilian aliens were villains, and humanoid, mammalian, and birdlike aliens were the good ones. It was also frequent for the classic trope of the alien invasion to be inverted, with humans conquering alien worlds instead; such stories were usually unapologetically genocidal.
The Barney and Betty Hill incident took place in 1961 when the couple claimed that they were abducted by aliens and subjected to invasive experiments. It was the first recorded claim of an alien abduction, soon followed by others. The description of the aliens made by the Hills, with oversized heads, big eyes, pale grey skin, and small noses captivated the public imagination and was later used by TV shows and films. This started the grey alien archetype. According to Wade Roush, a science and technology writer, "The standard depiction of aliens at that point became the little grey man. So, when Steven Spielberg came along and made probably what are the two most influential movies about aliens – Close Encounters of the Third Kind, and E.T. the Extra-Terrestrial – the aliens and those movies were both basically variations on the 1950s and 1960s little green or little grey man image".
The advent of TV and films, with extraterrestrials played by actors, toned down the fantasy. For budget reasons, humanlike aliens with just some specific non-human body features became the new standard. This is especially noticeable in the Star Trek franchise.
The way to depict aliens changed again since the 1990s with the advent of computer-generated imagery (CGI), and later on as CGI became more effective and less expensive, as it allows to generate bizarre lifeforms without being constrained to actors with costumes or mechanical effects. Their biological aspect may be humanoid, may be similar or include features of other Earth species, or have weird forms. In some cases, such weirdness may lead to the human characters to initially fail to recognize the aliens as such.