It is a dependent clause: a group of words containing a subject and a verb and that does not express a complete thought on its own.
It acts as a noun in a sentence; thus, it can function as a subject, a direct or indirect object, a predicate nominative, or an object of a preposition.
It tends to begin with the words how, that, what, whatever, when, where, whether, which, whichever, who, whoever, whom, whomever, or why.
Therefore,<u> "Whoever is interested in the past" is a noun clause</u> because it does not express a complete thought on its own, meaning that it needs another clause to depend on, it acts as a noun and the subject of the sentence: it is what is dealt with in the sentence, and it begins with the word "whoever."
