Original sentence: One provocative theory espoused by Sand and presented with extensive supporting evidence in the work, but challenged by other historians as "a myth with no factual basis," is the claim that Ashkenazi Jews are descended from Khazars, who purportedly converted in the early Middle Ages
Edited sentence: One provocative theory espoused by Sand, but challenged by other historians as "a myth with no factual basis," is the claim that Ashkenazi Jews are descended from Khazars, who purportedly converted in the early Middle Ages