humans vs mankind

humans and mankind both are nouns.

Word	Noun	Adjective	Verb	Adverb
humans	Yes	No	No	No
mankind	Yes	No	No	No

As nouns, humans and mankind are synonyms defined as:

Other synonyms of humans include human beings, human race, humanity, humankind, man, world.

humans (noun)	mankind (noun)
all of the living human inhabitants of the earth	all of the living human inhabitants of the earth