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.