Sentence Correction

Following is a correct answer according from OG

The first trenches cut into a 500-acre site at Tell Hamoukar, Syria, yields strong evidence of centrally administered complex societies in northern regions of the Middle east arising simultaneously but also independently of the more celebrated city-states of southern Mesopotamia, in what is now southern Iraq.

How can we justify the usage of the word "but also"? I always assumed that "Not only" follows "but also" but in this case there is no presence of "Not Only".

Kindly Clarify
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venjam wrote:Following is a correct answer according from OG

The first trenches cut into a 500-acre site at Tell Hamoukar, Syria, yields strong evidence of centrally administered complex societies in northern regions of the Middle east arising simultaneously but also independently of the more celebrated city-states of southern Mesopotamia, in what is now southern Iraq.

How can we justify the usage of the word "but also"? I always assumed that "Not only" follows "but also" but in this case there is no presence of "Not Only".

Kindly Clarify
Thanks

The actual option was cut into a 500-acre site at Tell Hamoukar, Syria, have yielded strong evidence that centrally administered complex societies in northern regions of the Middle East arose simultaneously with but

Now notice the parallelism simultaneously with......but also independently of