mid-15c., a parallel variant of human (adj.), with a form and stress that perhaps suggest a stronger association with Latin humanus than with Old French humain. Human and humane were used interchangeably in the senses "pertaining to a human being" and "having qualities befitting human beings" (c. 1500). The latter at first meant "courteous, friendly, civil, obliging," then "marked by tenderness, compassion, and a disposition to kindly treat others" (c. 1600). By early 18c. the words had differentiated in spelling and accent and humane took the "kind" sense.
Compare germane, urbane. Meaning "inflicting less pain than something else" is from 1904. Inhuman is its natural opposite. The Royal Humane Society (founded 1774) was originally to rescue drowning persons; such societies had turned to animal care by late 19c.
-ly 2
common adverbial suffix, forming from adjectives adverbs signifying "in a manner denoted by" the adjective, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cognates: Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). Cognate with lich, and identical with like (adj.).
Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.