As reported on Etymonline : Old English yrþling "plowman" (see earth + -ling ); the sense of "inhabitant of the earth" is from 1590s.
Earthman was originally (1860) "a demon who lives in the earth;" science fiction sense of "inhabitant of the planet Earth" first attested 1949 in writing of Robert Heinlein.
Earlier in this sense was earthite (1825).
However, I found this too: Ælfric
(955–1020) Teacher :hwæt sægest þu yrþlingc hu begæst
þu weorc þin
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The word earthling is formed from the word earth and the suffix -ling , meaning something from earth, or something related to earth.
Here earth can either refer to the planet, or to the soil.
It's now mostly used in science fiction to mean humans from earth, but it existed even before that.
One older use is in the meaning of farmer or ploughman , another is in the meaning of mortal as contrast to heavenly creatures or gods.
In science fiction the term Terran is also commonly used.
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NOAD gives for “-ling”: forming nouns from nouns, adjectives, and verbs (such as hireling, youngling).
forming nouns from adjectives and adverbs (such as darling, sibling, underling).
forming diminutive words : gosling | sapling.• often with depreciatory reference : princeling
This gives you some more examples...
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Origins are well covered.
Another synonym is Earther .
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'Earthling', before the sci-fi era, had somewhat the same connotations as 'troglodyte', an lesser, earthy person (see the etymonline provenance as in mplungjan's answer).
This is what probably inspired Heinlein to use it to have people not from Earth refer to people from Earth.
The sci-fi literature has created a number of alternatives, like 'earther' and 'terran'. '
Earthling' has too much of a 1950's scifi connotation and is not used in contemporary sci-fi, except for pejoratively.
As to other planets, generally recognized inhabitant names, though having only imaginary referents, are:
Venusian Martian Jovian Saturnian 'Mercurial','venereal', 'martial', jovial, and 'saturnine' are the respective metaphorical versions (that is, not referring to an inhabitant, but to 'slippery/fickle', 'sex-related', 'military-like', 'convivial', and 'gloomy', respectively). '
Earthy' would be the closest analog for Earth.
The other planets don't really have commonly recognized inhabitant names, but they can be easily created anew:
Mercutian, Uranian, Neptunian, Plutonian (yes, I know... then Erisian, Cerean, Haumean, Makemakean...).
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What is the origin of the word  earthling ?  Are there other words with a similar meaning ( marsling ,  venusling )?
What is the origin of "earthling"?
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<etymology><nouns><suffixes>
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It appears that the word "Earthling" has existed since about 1590, and before the sci-fi era, it had the same connotations as "troglodyte". Other synonyms include the words "Earther" or "Terran". The term dates back to the 1590s and means "inhabitant of the earth" and even before its sci-fi connotations in the 1950s it meant a lesser, "earthy" person.  In addition to "earthling", sci-fi literature also introduced other terms such as "earther" and "terran". Today "earthling" bears a sci-fi meaning "people from planet Earth". The word is formed by adding the suffix -"ling" to "earth",similar to words such as "hireling" and "youngling"
It appears that the word "Earthling" has existed since about 1590, and before the sci-fi era, it had the same connotations as "troglodyte". Other synonyms include the words "Earther" or "Terran". The suffix -ling is a common way to form nouns from nouns. In this case, the term earthling or earthman previously signified someone dwelling in caves, working with the earth, living under the earth (e.g. a demon) or somehow associated with the earth. Later it developed the meaning distinguishing earth dwellers rather than those living on other planets.
The word  earthling  is formed from the word  earth  and the suffix  -ling , meaning something from earth, something related to earth or inhabitant of the earth  The word "earthling" is formed from the word "earth" and the suffix "-ling" and means of the Earth.
Here earth can either refer to the planet, or to the soil. This is what probably inspired Heinlein to use it to have people not from Earth refer to people from Earth.
Here earth can either refer to the planet, or to the soil. This is what probably inspired Heinlein to use it to have people not from Earth refer to people from Earth.
Term dates back to 1590s How it is formed. Other synonyms: Earther
The suffix -ling is a common way to form nouns from nouns. In this case, the term earthling or earthman previously signified someone dwelling in caves, working with the earth, living under the earth (e.g. a demon) or somehow associated with the earth. Later it developed the meaning distinguishing earth dwellers rather than those living on other planets.
The word "earthling" is formed from the word "earth" and the suffix "-ling" and means of the Earth.