Despite the fragmented and incomplete historical record, experts pretty much agree that some popular beliefs about Jewish history simply don’t hold up: there was no sudden expulsion of all Jews from Jerusalem in A.D. 70, for instance. What’s more, modern Jews owe their ancestry as much to converts from the first millennium and early Middle Ages as to the Jews of antiquity.

Other theories, like the notion that many of today’s Palestinians can legitimately claim to be descended from the ancient Jews, are familiar and serious subjects of study, even if no definitive answer yet exists.

But while these ideas are commonplace among historians, they still manage to provoke controversy each time they surface in public, beyond the scholarly world. The latest example is the book “The Invention of the Jewish People,” which spent months on the best-seller list in Israel and is now available in English. Mixing respected scholarship with dubious theories, the author, Shlomo Sand, a professor at Tel Aviv University, frames the narrative as a startling exposure of suppressed historical facts. The translated version of his polemic has sparked a new wave of coverage in Britain and has provoked spirited debates online and in seminar rooms.

Professor Sand, a scholar of modern France, not Jewish history, candidly states his aim is to undercut the Jews’ claims to the land of Israel by demonstrating that they do not constitute “a people,” with a shared racial or biological past. The book has been extravagantly denounced and praised, often on the basis of whether or not the reader agrees with his politics.

The vehement response to these familiar arguments — both the reasonable and the outrageous — highlights the challenge of disentangling historical fact from the sticky web of religious and political myth and memory.

Consider, for instance, Professor Sand’s assertion that Palestinian Arab villagers are descended from the original Jewish farmers. Nearly a century ago, early Zionists and Arab nationalists touted the blood relationship as the basis of a potential alliance in their respective struggles for independence. Israel’s first prime minister, David Ben-Gurion, and Yitzhak Ben Zvi, Israel’s longest-serving president, made this very argument in a book they wrote together in 1918. The next year, Emir Feisal, who organized the Arab revolt against the Ottoman empire and tried to create a united Arab nation, signed a cooperation agreement with the Zionist leader Chaim Weizmann that declared the two were “mindful of the racial kinship and ancient bonds existing between the Arabs and the Jewish people.”

Both sides later dropped the subject when they realized it was not furthering their political goals.

(Though no final consensus has emerged on the ancestral link between Palestinians and Jews, Harry Ostrer, director of the Human Genetics Program at New York University Langone Medical Center, who has been studying the genetic organization of Jews, said, “The assumption of lineal descent seems reasonable.”)

Books challenging biblical and conventional history continually pop up, but what distinguishes the dispute over origins from debates about, say, the reality of the exodus from Egypt or the historical Jesus, is that it is so enmeshed in geopolitics. The Israeli Declaration of Independence states: “After being forcibly exiled from their Land, the People kept faith with it throughout their Dispersion and never ceased to pray and hope for their return to it.” The idea of unjust exile and rightful return undergirds both the Jews’ and the Palestinians’ conviction that each is entitled to the land.

Since Professor Sand’s mission is to discredit Jews’ historical claims to the territory, he is keen to show that their ancestry lines do not lead back to ancient Palestine. He resurrects a theory first raised by 19th-century historians, that the Jews of Central and Eastern Europe, to whom 90 percent of American Jews trace their roots, are descended from the Khazars, a Turkic people who apparently converted to Judaism and created an empire in the Caucasus in the eighth century. This idea has long intrigued writers and historians. In 1976, Arthur Koestler wrote “The Thirteenth Tribe” in the hopes it would combat anti-Semitism; if contemporary Jews were descended from the Khazars, he argued, they could not be held responsible for Jesus’ Crucifixion.

By now, experts who specialize in the subject have repeatedly rejected the theory, concluding that the shards of evidence are inconclusive or misleading, said Michael Terry, the chief librarian of the Jewish division of the New York Public Library. Dr. Ostrer said the genetics also did not support the Khazar theory.

That does not negate that conversion played a critical role in Jewish history — a proposition that many find surprising given that today’s Jews tend to discourage conversion and make it a difficult process. Lawrence H. Schiffman, chairman of the Skirball department of Hebrew and Judaic Studies at New York University, said most historians agree that over a period of centuries, Middle Eastern Jews — merchants, slaves and captives, religious and economic refugees — spread around the world. Many intermarried with people from local populations, who then converted.

There is also evidence that in antiquity and the first millennium Judaism was a proselytizing religion that even used force on occasion. From the genetic research so far, Dr. Ostrer said, “It’s pretty clear that most Jewish groups have Semitic ancestry, that they originated in the Middle East, and that they’re more closely related to each other than to non-Jewish groups.” But he added that it was also clear that many Jews are of mixed descent.

“The ancient admixed ancestry explains the blond hair and blue eyes of Ashkenazi Jews whose grandparents and great-grandparents all lived in shtetls two and three generations ago,” Dr. Ostrer said. They brought the genes for coloration with them to Eastern Europe. These genes were probably not contributed by their Cossack neighbors.”

What accounts for the grasp that some misconceptions maintain on popular consciousness, or the inability of historical truths to gain acceptance? Sometimes myths persist despite clear contradictory evidence because people feel the story embodies a deeper truth than the facts. Marie Antoinette never said “Let them eat cake,” but the fictional statement captured the sense of a regime that showed disdain for the public’s welfare.

A mingling of myth, memory, truth and aspiration similarly envelopes Jewish history, which is, to begin with, based on scarce and confusing archaeological and archival records.

Experts dismiss the popular notion that the Jews were expelled from Palestine in one fell swoop in A.D. 70. Yet while the destruction of Jerusalem and Second Temple by the Romans did not create the Diaspora, it caused a momentous change in the Jews’ sense of themselves and their position in the world. For later generations it encapsulates the essential truth about the Jews being an exiled and persecuted people for much of their history.

Professor Sand accuses Zionist historians from the 19th century onward — the very same scholars on whose work he bases his case — of hiding the truth and creating a myth of shared roots to strengthen their nationalist agenda. He explains that he has uncovered no new information, but has “organized the knowledge differently.” In other words, he is doing precisely what he accuses the Zionists of — shaping the material to fit a narrative.

In that sense, Professor Sand is operating within a long established tradition. As “The Illustrated History of the Jewish People,” edited by Nicholas Lange (Harcourt, 1997), notes, “Every generation of Jewish historians has faced the same task: to retell and adapt the story to meet the needs of its own situation.” The same could be said of all nations and religions.

Perhaps that is why — on both sides of the argument — some myths stubbornly persist no matter how often they are debunked while other indubitable facts continually fail to gain traction.