Much of european eastern jewry can be traced in the main back to the mystery of Khazaria, a Jewish state that appeared in the east of Russia from around 652-1016+ ad.
Hybridization the other way around to the Sephardic:
Caught between the Holy Roman Catholic Empire and Khilafah-Caliphate of Islam, the mongol khan converted to Judaism and made it the official state religion (though other jews, christians and muslims and pagans still lived there). Spanish rabbis were imported and students sent for spainish jewish eduction. Khazaria became a place of refuge for jews from around the world, and it saw itself as the champion and protector of world jewry. The Torah was for a time made law...
none was more extraordinary than that concerning the Khazars."
- Nathan Ausubel, in Pictorial History of the Jewish People (1953)
"The Khazar people were an unusual phenomenon for Medieval times. Surrounded by savage and nomadic tribes, they had all the advantages of the developed countries: structured government, vast and prosperous trading, and a permanent army. At the time, when great fanatism and deep ignorance contested their dominion over Western Europe, the Khazar state was famous for its justice and tolerance. People persecuted for their faiths flocked into Khazaria from everywhere. As a glistening star it shone brightly on the gloomy horizon of Europe, and faded away without leaving any traces of existence."
- Vasilii V. Grigoriev, in his essay "O dvoystvennosti verkhovnoy vlasti u khazarov" (1835), reprinted in his 1876 compilation book Rossiya i Aziya on page 66
"Though the Jews were everywhere a subject people, and in much of the world persecuted as well, Khazaria was the one place in the medieval world where the Jews actually were their own masters.... To the oppressed Jews of the world, the Khazars were a source of pride and hope, for their existence seemed to prove that God had not completely abandoned His people."
- Raymond Scheindlin, in The Chronicles of the Jewish People (1996)
Khazaria saw itself as the sucessor of ancient Judah and Israel, but not all jews though it was and instead longed for a return to zion. This was a topic of debate between the spanish sephardic jews and the Khazars.
"At the present time, it is known that Eastern European Jews have a significant Eastern Mediterranean element which manifests itself in a close relationship with Kurdish, Armenian, Palestinian Arab, Lebanese, Syrian, and Anatolian Turkish peoples. This is why the Y-DNA haplogroups J and E, which are typical of the Middle East, are so common among them."