Complex sentence: Historian Plamen Pavlov theorizes that Jacob Svetoslav was a descendant of the princes ("knyaze") of Kievan Rus', and estimates his birth date as being in the 1210s or 1220s.
Split into simpler sentences: Historian Plamen Pavlov theorizes that Jacob Svetoslav was a descendant of the princes ("knyaze") of Kievan Rus'. He estimates Jacob Svetoslav's birth date as being in the 1210s or 1220s.