Earthling \URTH-ling\ , noun;
1. An inhabitant of the earth; mortal
This word is first attested in 1593, though there is a suspiciously similar word in Old English, yrþling, which meant "plowman." Earthling in the 1500's meant "inhabitant of the earth," but the sci-fi sense didn't come about until the 1940's. It replaced an earlier sci-fi word, earthite, which dates to the 1820's.