Is it farther or further?
late 15c., "one who acts," from Latin agentem (nominative agens) "effective, powerful," present participle of agere "to set in motion, drive, lead, conduct" (see act (n.)). Meaning "any natural force or substance which produces a phenomenon" is from 1550s. Meaning "deputy, representative" is from 1590s. Sense of "spy, secret agent" is attested by 1916.
1610s, from agent (n.).
agent a·gent (ā'jənt)
A force or substance, such as a chemical, that causes a change.
A substance that can bring about a chemical reaction or a biological effect. Compare reagent.