We’re not sure!

The word human can be traced back to the Proto-Indo-European root (dh)ghomon-, meaning “earthly being” from *dhghem- (earth), as opposed to “godly/heavenly beings”.

The Latin word humanus was used to refer to “humane” behavioural traits, associated with kindness and civility. This lead to Old French (12th Century) humain meaning “of or belonging to man”.

The same PIE root developed into Old English as guma and Old High German gomo.