I've seen this brought up a lot in my internet travels: that the Jews are not really the Jews, so that makes it okay to hate them. So, let's get into it. The Khazarian Theory postulates that a significant portion of European Jews, particularly Ashkenazi Jews, are descended from the Khazars. The Khazars were a semi-nomadic Turkic people who originated from the Central Asian region and migrated to the area north of the Caucasus Mountains, in present-day southern Russia, western Kazakhstan, eastern Ukraine, and parts of the Caucasus.
As the story goes, the Khazars converted to Judaism in the 8th or 9th century, possibly for political reasons. The ruling class may have wanted to maintain neutrality between the Muslim Caliphate to the south and the Byzantine Empire to the west. However, other religions were also practiced. including Islam, Christianity, and Tengrism. This continued until the Khazar state declined and then collapsed in the 10th century. No one knows for sure what became of them, but they likely assimilated into neighboring populations.The Khazarian theory suggested they migrated westward and became the Jews we know today. The theory originated in the 19th and 20th centuries by some scholars and historians. However, it was popularized by writer Arthur Koestler, himself a Hungarian Jew, in his 1976 book "The Thirteenth Tribe." Koestler was critical of nationalist ideologies and some aspects of Zionism. By providing an alternate explanation that removed the biological connection with Biblical Jews, he thought it might remove the anti-Semitism that was so prevalent in Europe. Ironically, his ideas have fueled it.
And what might have compelled Koestler to write such a book? Koestler was raised Jewish but became disillusioned with religion in general and Judaism specifically. He later rejected all religious beliefs, becoming an atheist and a communist. As an atheist, he would have rejected the biblical origins of the Jewish people. As a communist, he would have been opposed to Jewish nationalism. As both an atheist and a communist, he would have viewed the Jewish people strictly through a secular lens, seeking to divest them from their religious and cultural identity.
Koestler was neither a historian nor a geneticist. DNA research was still in its infancy at the time. It wasn’t until the late 1980s that comprehensive DNA mapping of the human genome began. And it wasn’t until the 1990s that systematic research on Jewish DNA was conducted. The study of Jewish DNA is perhaps the greatest blow against the Khazarian theory. I've actually looked at some of those studies. Ashkenazi Jews definitely have some European admixture, but they also have Middle Eastern DNA as well. Here are a few studies:
*Researchers at the Technion - Israel Institute of Technology compared DNA sequences from nearly 2000 Jews with those of 11,500 non–Jewish people in 67 different populations around the world. They found both Ashkenazi and non–Ashkenazi Jews are descended from four founding mothers of middle-eastern descent. Source
*Geneticists from The Hebrew University of Jerusalem and Harvard Medical School studied the DNA of 33 German Ashkenazi Jews excavated from a 14th century cemetery. Analysis revealed these German Jews were descended from two distinct subgroups, one with greater Middle Eastern ancestry, and another with greater Eastern and Central European ancestry. Further evidence found they were all descended from a single ancestral woman. Source
*Several studies were analyzed to understand the genetic relationships, origins, and historical migrations of Jewish populations. Patterns are consistent with a shared Near East ancestry along with variable degrees of admixture (blending) and introgression (genetic mixing) from local populations. A high degree of European admixture (30–60 %) was observed among Ashkenazi, Sephardic, Italian and Syrian Jews. The North African Jewish groups demonstrated North African and Middle Eastern admixture with varying European admixture. Source
Marrying women from the local populations was not uncommon even in Biblical times. Moses married an Ethiopian (Cushite, Num 12:1) and a Midianite woman (Exo 2:21). Joseph married an Egyptian (Gen 41:45). Judah and Simeon married Canaanite women. Boaz married Ruth (a Moabite). Salmon likely married Rahab, a Canaanite woman (Jos 6:25). Solomon had many foreign wives, and so on. Regardless of the addition of local admixture, that still makes them descendants of Abraham. I would add, not all the Jews left after the destruction of the Second Temple (70 CE) and the Bar Kokhba Revolt (132–135 CE). There has always been a continuous Jewish presence in that region.
There is one study that supported the Khazarian theory, but even those whose data set they used later refuted them. Quote:
"One recent study (Elhaik, 2013), making use of part of our data set (Behar and others, 2010), focused specifically on the Khazar hypothesis, arguing that it has strong genetic support. This claim was built on a series of analyses similar to those performed in our original study that initially reported the data. However, the reanalysis relied on the provocative assumption that the Armenians and Georgians of the South Caucasus region could serve as appropriate proxies for Khazar descendants (Elhaik, 2013).
This assumption is problematic for a number of reasons. First, because of the great variety of populations in the Caucasus region and the fact that no specific population in the region is known to represent Khazar descendants, evidence for ancestry among Caucasus populations need not reflect Khazar ancestry. Second, even if it were allowed that Caucasus affinities could represent Khazar ancestry, the use of the Armenians and Georgians as Khazar proxies is particularly poor, as they represent the southern part of the Caucasus region, while the Khazar Khaganate was centered in the North Caucasus and further to the north. Furthermore, among populations of the Caucasus, Armenians and Georgians are geographically the closest to the Middle East, and are therefore expected a priori to show the greatest genetic similarity to Middle Eastern populations.
Indeed, a rather high similarity of South Caucasus populations to Middle Eastern groups was observed at the level of the whole genome in a recent study (Yunusbayev and others, 2012). Thus, any genetic similarity between Ashkenazi Jews and Armenians and Georgians might merely reflect a common shared Middle Eastern ancestry component, actually providing further support to a Middle Eastern origin of Ashkenazi Jews, rather than a hint for a Khazar origin." Source