Early works posited that aliens would be identical or similar to humans, as is true of Edgar Rice Burroughs's Martians (see Mars; "A Princess of Mars"), with variations in skin color, size, and number of arms. ... Later writers realized that such humanoid aliens would not arise through parallel evolution and hence either avoided them or introduced the explanation of ancient races that populated the cosmos with similar beings. The notion surfaces in Ursula K. Le Guin's Hainish novels (see "The Left Hand of Darkness"; "The Dispossessed") and was introduced to justify the humanoid aliens of "Star Trek" (who even intermarry and have children) in the ' episode ' (1993).Another common idea is aliens who closely resemble animals.