akuperma, the person who wrote “The Unbroken Chain” is Dr. Neil Rosenstein, who I mentioned above. I’ve spoken with him and corresponded with him (he sent me some sources for my family tree that I didn’t have) and he is, from a few lines, a distant relative of mine.
The assertion you make certainly wasn’t the focus of his book, and I don’t remember reading that when I read the very long and very technical book. ( He stopped making new editions and started putting together CDs with the information as he updated it. Much easier than publishing an incredibly long and very expensive book.)
It is clear that at some point during the crusades, the Jewish families of Western and Central Europe were reduced to a few thousands or tens of thousands. But this does not account for the ongoing marriages and movements of families from Iberia, North Africa, the Italian states, and parts of the Byzantine Empire to and with these drastically reduced numbers of families. The argument goes both ways – yes, extended kinship, but no, less of a genetic bottleneck than usually perceived.
My own ancestors at the time were as widely placed as Gerona, Lunel, Narbonne, elsewhere in the Languedoc, Paris, Trier, Troyes, Worms, Treves, Rome, and they and others were beginning to migrate north and east. See R’ Naftali yaakov Hakohen’s Otzar Hagedolim (out of print since the sixties, but I have 7 of the 11 volumes) for some more details about that time period and where Jewish communities and their leaders lived.
So while I agree that there is extensive kinship among perhaps a plurality of Jewish families, I do not agree that it is ALL of them.