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The word Earthling, an alternate term to describe the Human race, was the favored term for that species during the Ur-Quan Slave War. Originally used to describe all life of Earth origin, it came to be used by the Humans themselves as an ironic diminutive, humbly naming themselves by their planet of origin rather than using the term "human", which for them carried connotations as a universal judgment of sentience or moral worth.
The term "Earthling" had been used with this purpose in mind among Humans even before first contact, often in their science fiction as a reference to Human immaturity compared to older, imaginary, alien civilizations. Later, it seems to have been primarily used by Humans in their dealings with other members of the Alliance to avoid pejorative connotations of "human" vs. "nonhuman", particularly from the more xenophobic members of their own species.
In all other contexts, including most Hierarchy communications, "Human" remained the most common term, and by the time of the Star Control II, Humans had become accustomed enough to their place among other alien civilizations that it appeared "human" had mostly lost its original connotations and become a specific term referring only to their species.
"Earthling" has now mostly passed out of use, though it remains part of the official designation of the primary Human combat vessel, the Earthling Cruiser.