It's the closest that humans come to being superheroes.
-New Yorker-
If I rephrase it, it becomes
- It's the closest.
- Humans come the closest to being superheroes.
What I wonder is are the part of speech of "closest" in 1 and the part of speech of "closest" in 2 different?
I think "closest" in 1 is a noun and "closest" in 2 is an adverb. Or are they both adjectives?