I am writing an essay and don't know if you can start a sentence with "seeing as". Here is my sentence:
Seeing as I’ve chosen to examine the Wari culture a common theme between these articles is Peru.
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Yes, you can use it to begin a sentence, grammatically.
It is too informal for an essay, in the opinion of this US English speaker.
It doesn't make sense in this context. "Seeing as how..." means something like "for the reason of" or "because". So you see how it doesn't quite make sense here - because of the fact that you've chosen to examine the Wari culture, a common theme between these articles is Peru? You would use the phrase like "Seeing as how my theme was the Wari culture, I didn't examine the Nazca artifacts very closely."