The noun earthling is composed of earth and the suffix -ling, meaning, in this case, a person belonging to—as in worldling.

In science fiction, it is used by aliens to refer to a person who lives on, or comes from, the earth.

But this noun originally denoted an inhabitant of the earth as opposed to heaven. It is first attested in Christs teares ouer Ierusalem (1593), by the English pamphleteer Thomas Nashe (1567-circa 1601):

(1613 edition)
Wilt thou ratifidely [= positively] affirme, that God is no God, because (like a Noune substantiue) thou canst not essentially see him, feele him, or heare him. Is a Monarch no Monarch, because hee reareth not his resiant [= resident] throne amongst his vtmost subiects? Wee (of all earthlings) are Gods vtmost subiects, the last (in a manner) that he brought to his obedience: shal we then forget that wee are any subiects of his, because (as amongst his Angels) he is not visibly conuersant amongst vs?

This noun also denoted a worldly or materialistic person. In Of the obseruation, and vse of things (1600-01), the English essayist William Cornwallis (circa 1579-1614) wrote:

I haue not beene ashamed to aduenture mine eares with a ballad-singer, and they haue come home loaden to my liking, doubly satisfied, with profit, & with recreatiō. The profit, to see earthlings satisfied with such course [= coarse] stuffe, to heare vice rebuked, and to see the power of Vertue that pierceth the head of such a base Historian, and vile Auditory.
The recreation to see how thoroughly the standers by are affected, what strange gestures come from them, what strained stuffe from their Poet, what shift they make to stand to heare, what extremities he is driuen to for Rime, how they aduenture their purses, he his wits, how well both their paines are recompenced, they with a silthy noise, hee with a base reward.

Before being used in science fiction, earthling has denoted a person who lives on the earth as opposed to another planet. For example, the American poet William McKendree Carleton (1845-1912) wrote the following in Flight of the Aged Balloonist, from A Festival of the Sky Club, published in City Festivals (New York, 1892):

You younger men, who loudly sing
Old marvels night and day,
Now, listen while to you I bring
Wise words from far away.

In yonder castle is my den:
I dwell a hermit there,
Amid a lot of crazy men,
O’er whom I watch and care.

These crazy men, some of them, too,
Deem they do watch o’er me;
I let them think that thus they do,
So quiet they may be.

I let them guard me through the day,
I do as they have said;
These doctor-maniacs have their way,
Till I be safe in bed.

And then—so soft and still I rise!
I spurn this planet’s ground;
My air-ship sails me to the skies,
Where flocks of stars abound.

[…]

In Neptune I a story gat
Few earthlings would indorse:
Men treat their bodies well as that
Of any blooded horse.

An article published in The Washington Post (Washington, District of Columbia) of 17^th May 1910 about the reappearance of Halley’s comet uses earthling in the sense of an inhabitant of the earth as opposed to another celestial body:

As Earth Passes Through Tail of Heavenly Wanderer Tomorrow Night Thousands of Eyes in Washington and Millions in United States Will Be Turned Upward—Scientists Predict No Harm to Earthlings—Comet Parties Formed.

 

But the word has been used in various other senses. For example, the author of an article published in The Manchester Courier (Lancashire) of 5^th August 1907 wrote, about the advantages of taking a trip in a balloon during the holiday season:

No casual acquaintance nor tripper can suddenly haunt you and force on you the platitudes that propriety and good breeding extract on shortest notice from the anchored earthling.

Similarly, a correspondent who had been the passenger of a bomber “equipped with the amazing automatic pilot” published the following in The Northern Whig (Antrim, Ireland) of 14^th August 1930:

I have flown scores of thousands of miles in the daylight, including a flight to Cape-town and back, but this first experience of night flying brought a new charm of complete isolation. In the night with only a mystery of grey skies and dimly seen ground it is easier to forget that you are an earthling.

And, in What Disobedience did for Bobbin, a fairy story published in The Lichfield Mercury (Staffordshire) of 24^th December 1926, earthling is used in contrast to elf.