thanyoudaoicibaDictYouDict[than 词源字典]
than: [OE] Than is ultimately the same word as then, and the two were used interchangeably until the end of the 17th century. It is not clear how the adverb came to be used as a conjunction denoting comparison, although it is possible that the comparison implicit in expressions like ‘This one is better; then there is that one’ may have led on to ‘This one is better than that one’.
=> then[than etymology, than origin, 英语词源]
than (conj.)youdaoicibaDictYouDict
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 by spelling until c. 1700.

The earliest use is in West Germanic comparative forms introducing the second member, i.e. bigger than (compare 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).