1650s as a stand-alone adjective; also used as an adverb and noun in 17c. (see extra-); modern usages -- including sense of "minor performer in a play" (1777) and "special edition of a newspaper" (1793) -- probably all are from shortenings of extraordinary, which in 18c. was used extensively as noun and adverb in places extra would serve today.

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Definitions of extra from WordNet
extra (n.)
a minor actor in crowd scenes;
Synonyms: supernumerary / spear carrier
extra (n.)
an additional edition of a newspaper (usually to report a crisis);
extra (n.)
something additional of the same kind;
he always carried extras in case of an emergency
Synonyms: duplicate
extra (adj.)
more than is needed, desired, or required;
extra ribs as well as other supernumerary internal parts
found some extra change lying on the dresser
extra (adj.)
added to a regular schedule;
Synonyms: special
extra (adj.)
further or added;
an extra pair of shoes
need extra help
Synonyms: additional
extra (adv.)
unusually or exceptionally;
an extra fast car