Nahum was Exilarch of the Jews in Babylon about 140-170. He is the first Exilarch of whom there is any historical record. He was Exilarch during the reign of Hadrian as Roman emperor, giving him a birthdate of about 100 to 120 CE. Contemporary Jewish leaders believed that the Exilarchs were descended in the male line of King David through the King Jehoachin, who was carried off to Babylon after the destruction of Jerusalem in 607 BCE. As Jehoachin was a young man at that time a birthdate about 620-610 BCE is likely for him. The return of some of the Jews to Judah occurred about 538 BCE when the tribal leader was Zerubbabel, Jehoachin's grandson, who can be assigned a birthdate of about 550 BCE. This leaves a gap of about 660 to 680 years between Zerubbabel and Nahum for which there are virtually no historic markers for the Jews of Babylon. One of the lists of supposed earlier Exilarchs gives Shechaniah as the leader during the destruction of the temple in Jerusalem, which took place in 70-72 CE. Shechaniah was third on this list back from Nahum. If we add these three leaders [a reasonable possibility as at some scholars suggests the Babylonian Jewish hierarchy may have arisen during later Parthian rule], the gap narrows to a little less than 600 years. This gap would contain about 17 to 20 generations. None of the known lists give a sufficient number of people to fill all of these years. The list from the Jewish Enclycopedia has been used in this compilation with a break between Meshullam noted by Chronicles as son of Zerubbabel and Hananiah. Within this break are an appropriate number of generations.
Sources:
1. Dills Gedcom-William L. "Toby" Dills
2. Stuart, R.W. "Royalty for Commoners" line 329.
3. "Encyclopedia Judiaca", Vol.6, pp.1024-1025.
4. "The Jewish Enclyclopedia" Vol.V, pp.288-290.