Complex sentence: His English ancestor progenitor, from where all the United States Livermores derive (assumed), was John Livermore who came from Ipswich in 1634 and settled in Watertown, Massachusetts.
Split into simpler sentences: His English ancestor progenitor, believed to be the immigrant ancestor for all the United States Livermores (assumed), was John Livermore. He came from Ipswich in 1634 and settled in Watertown, Massachusetts.