Yiddish began as a secret language invented by Iranian Jewish traders who travelled the Silk Road, a group of researchers has claimed.
According to their analysis of the genetic ancestry of Yiddish-speakers, Ashkenazi Jewry originated in a group of north-eastern Turkish villages rather than in Germany as often thought.
"The search for ancient Ashkenaz has been going on for maybe 1,000 years," said Dr Eran Elhaik, one of a team of researchers from Sheffield, Tel Aviv and Johns Hopkins Universities who have published their research in the journal Genome Biology and Evolution. "Now we believe we have found it," he said.
The prevailing view is that Yiddish was a German dialect while the alternative theory is that it is Slavic in origin, said Dr Elhaik.
Analysis of the DNA of Yiddish speakers pointed back to an area of north-eastern Turkey, he explained in a video posted on the Sheffield website.
When they consulted old books, the team found a group of ancient villages whose names bear a close resemblance to the word "Ashkenaz" - Aschuz, Iskenaz, Eskenez and Ashanas.
"This is where silk roads and other trade roads converged," he said.
"We speculate that Iranian Jewish traders on the Silk Road made this region their centre. They converted the locals and, to maintain their monopoly on trade, invented a secret language."
The existence of more than 250 terms in Yiddish for the words "buy" and "sell" suggests its origins among traders.
The authors also argue that the term "kosher", applied to eating, derives from an Iranian term "to slaughter an animal", rather than the biblical term which has no connection to food.
The Ashkenazi centre in Turkey moved to the Kingdom of Khazaria, between the Black and Caspian Seas, whose leaders adopted Judaism, they believe.
Five hundred years later, after the collapse of the Khazars, Jews shifted to Germany and Yiddish began accumulating German words while retaining its Slavic grammar.
But Dr Khayke Beruriah Wiegand, lector in Yiddish at the Oxford Centre for Hebrew and Jewish Studies, described the "Irano-Turko-Slavic" theory of the language's origins as "utterly ridiculous". And Professor Dovid Katz, who taught Yiddish studies at Oxford for 18 years, said: "There is not a single word or sound in Yiddish that comes from Iranian or Turkish, and older Western Yiddish thrived before there was a single Slavic-derived word in the language."