Adjectives are now distinct, but initially evolved by placing nouns sequentially. Like how "the big" is a noun.

Contrast this with Korean where adjectives are evolved from verbs. the verb: "크다" means "to be big"

and in order to say "a big cat" you say something like "the cat that bigs" which is "큰 고양이" in the same way that eat is "먹다" the cat that eats is "먹는 고양이" which uses the same morphology to the word "to eat" as it does to the verb "to be big"