than (conj.) Old English þan, conjunctive particle used after a comparative adjective or adverb, from þanne, þænne, þonne "then" (see then). Developed from the adverb then, and not distinguished from it in spelling until c.1700.
The earliest use is in West Germanic comparative forms, i.e. bigger than (cf. Dutch dan, German denn), which suggests a semantic development from the demonstrative sense of then: A is bigger than B, evolving from A is bigger, then ("after that") B. Or the word may trace to Old English þonne "when, when as," such as "When as" B is big, A is more (so).
then (adverb of time), from Old English þanne, þænne, þonne, from Proto-Germanic thana- (cf. Old Frisian thenne, Old Saxon thanna, Dutch dan, Old High German danne, German dann), from PIE demonstrative pronoun root *to- (see the). For further sense development, see than. Similar evolutions in other Germanic languages; Dutch uses dan in both senses, but German has dann (adv.) "then," denn (conj.) "than." Now and then "at various times" is attested from 1550s; earlier then and then (c.1200).