[Origin: bef. 900; ME, OE, uninflected s. of the demonstrative pronoun, the, thy, that]

late O.E. þe, nom. masc. form of the demonstrative pronoun and adj. After c.950, it replaced earlier se (masc.), seo (fem.), þæt (neut.), and probably represents se altered by the þ- form which was used in all the masc. oblique cases. O.E. se is from PIE base *so- “this, that” (cf. Skt. sa, Avestan ha, Gk. ho, he “the,” Ir., Gael. so “this”). The s- forms were entirely superseded in Eng. by c.1250, excepting dial. survival slightly longer in Kent. O.E. used 10 different words for “the”, but did not distinguish “the” from “that.” That survived for a time as a definite article before vowels (cf. that one or that other). Adv. use in the more the merrier, the sooner the better, etc. is a relic of O.E. þy, originally the instrumentive case of the neuter demonstrative þæt.

Scandinavian languages are also Germanic in origin, but attach their articles like “-et” and “-en” on the ends of words.

Slavic languages like Russian don’t use articles, but Russian and perhaps others have versions of “this,” “that,” “these,” and “those” (“это,” “эта,” “тот,” “эти”).

Romance languages like Spanish use a forms like “la,” “le,” “las,” “los,” etc.

These are all Indo-European languages. The below source traces it back to Old English, and then further to the Proto-Indo-European base ” *so- ‘this, that’.”