[Inyokern]

So are Jews today with the surname Kagan descended from the Khazars, and the rest from the ancient Israelites?

No. Kagan is the Russian pronunciation of the Hebrew word Cohen (priest). Russians always turn an "h" into a "g." It is also the Russian pronunciation of the Turkic word Khan. The Khazar word Kagan is the same as Khan, as in Ghengis Khan.

The Hebrew word Cohen, the Turkic word Khan and the Hawaiian word Kahuna probably all come from the same prehistoric root, along with the English word King.

[Jedi Master Pikachu]

"The Hebrew word Cohen, the Turkic word Khan and the Hawaiian word Kahuna probably all come from the same prehistoric root, along with the English word King."

That's from a wide swath of language family groups (Semitic, Turkic (Mongolian?), Austronesian, and Indo-European). Has there been a conclusive study suggesting a common root language for humanity as a whole who's descendants are the modern languages?

[Verginius Rufus]

I didn’t know that about Russian changing an H into a G. Some Slavic languages turn G’s into H’s (Czech, Slovak, Upper Sorbian, and maybe Ukrainian)—that is, if you compare their wordstock with the equivalent words in the other Slavic languages. Of course Kagan would be a loan-word.