GMAT Sentence Correction (SC) | Re: According to scholars, the earliest writing was probably not

Hi experts, I answered B correctly and understand why it is correct.

What I want to explore here is the structure of the sentence. What is independent and what is dependent clause? The idiom NOT ... BUT confuses me here.

Is ',and only later merged with spoken language' an independent clause nested under but (is such thing possible, I wouldn't be surprised if it isn't), or should we look at the sentence as a combination of two parallel independent clauses?

According to scholars, the earliest writing
...

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