I have read the Thirteenth Tribe and you completely misrepresent Koestler's thesis. He argues that a part of Ashkenazi Jewish heritage comes from the collapsed kingdom of Khazaria...something I have written about quite often, actually. He never argues that Jews in general (including Sabras, the Jews who ALWAYS have live in Israel, or Sephardim) are descendents of the Khazars or even that all Ashkenazim are descendents of the Khazars. Koestler himself was a Zionist, for that matter.

I have read many genetic studies in the scientific literature and they ALL indicate that Jews from all over the world are closely related not only to eachother, but also to the Palestinians. Palestinians are more related to the Jews than they are to the Arabs, who are also closely related to both. THAT makes all those groups Semites. If Jews don't belong there then neither do the Palestinians and visa versa. Truth is, genetic, archaeological and historical evidence all point to both Palestinians AND Jews being descendents of the native Canaanites, though both have mixed with other groups throughout their history.

Don't base your conclusions based on only one or two sources. I base my conlcusions on genetics (Palestinians and Jews, including Ashkenazim EXCEPT Askenazi Levites, are closely related), archaeology (both cultures evolved out of the native Canaanite culture), history (the earliest records of Israel and the Philistines indicate a contemporary evolution) and international law (regardless of origins both nations were created out of the ashes of the Ottoman Empire by the same legal mechanism and have equal legal basis). I suggest broadening your mind and your reading.