IN 2008, Shlomo Sand wrote an incendiary best seller called The Invention of the Jewish People. For Sand, invention meant faking: the Jews were never truly a people, he argued. Instead, Sand insisted, being Jewish simply meant adhering to Judaism the religion — that is, until a handful of scholars contrived the idea of Jewish nationhood in the late 19th century (the first culprit, Sand said, was Heinrich Graetz, who wrote a History of the Jews that appeared in the 1850s and later). When Sand followed up his shocker a few years later with a book called How I Stopped Being a Jew, his suggestion was even clearer: unless you are religiously observant, Jewishness is a mere fiction, and you might as well stop pretending it exists.

One of the many negative reviewers of The Invention of the Jewish People, Michael Berkowitz, remarked that if Sand is to be believed, Islam, Christianity, and Rome must have suffered from a collective delusion, for they all agreed that the Jews were a people and not merely a religious group. Most disturbingly, Sand’s biggest weapon against Jewish nationhood was reminiscent of long-discarded ideas about “race” and culture. For him, the varied genetic origins of Jews implied they weren’t a true people, as if nationality ought to be based in racial homogeneity. Apparently, Jews needed to share a biological essence to be judged a legitimate people.

Sand’s case is troubling because it shows how, even in our supposedly postmodern age, we want to find ironclad historical origins; if we can’t have them, we feel free to debunk claims of nationhood — at least when it comes to the Jews and a few other groups. Palestinian nationality was invented less than two centuries ago, but it certainly exists. Yet Golda Meir famously asserted that there was no such thing as a Palestinian, and some fiercely anti-Arab Israelis echo her even today.

Sand’s weak book to the contrary, the invention of the Jewish people appears to have happened several thousand years ago. Jews have been around since antiquity, and so their origins are not so easy to lay hold of. As of now, no one can say how and why being a Jew got started. But in his new volume The Origin of the Jews, Steven Weitzman makes a valiant effort to survey some partial answers. He addresses faulty ideas like Sand’s, but also bizarre, brilliant ones, like Freud’s thought that Moses derived monotheism from Egypt, as well as the slow, painstaking work of archaeologists and geneticists.

The Bible insists that God and Moses invented the Jewish people: together they imposed the law on the unruly Israelites and made them into Torah-observant Jews. The Roman historian Tacitus, who had probably heard hints of Sinai’s official story, gives the Torah’s tale of origins a derogatory spin. Tacitus says the Jews were a colony of lepers who fled Egypt, and that Moses taught them outlandish customs like circumcision in order to mold them into a proudly separate people. But whether you believe Torah or Tacitus (or, for that matter, Freud, who saw Moses as a follower of the early Egyptian monotheist Akhenaten), you are stuck with an implausible top-down version of Jewish identity. Instead of Moses laying down the law for all time, Jewishness was most likely a slow development. In the beginning, the people of Israel were barely distinguishable from the surrounding Canaanites.

Since 1962, when the Biblical scholar George Mendenhall first proposed the idea, many archaeologists have argued that the Israelites were actually a group of Canaanites who broke away from the dominant culture. Suddenly, in the 12th century BCE, great numbers of Canaanites began moving to small hilltop settlements and stopped eating pork. Disappointingly, that’s nearly all we have to go on when it comes to the birth of Israel. The Egyptian Merneptah stele from the late 13th century BCE suggests that a people or region called Israel existed, but it reports only that “Israel is laid waste and his seed is no more,” destroyed by the Egyptian king’s armies — fake news, quite possibly. Other traces of ancient Israel are sparse. There seems to be no real evidence outside the Bible for an exodus of Israelite slaves from Egypt. And so absence of pork bones is probably our most meaningful clue to the origin of the Israelites.

But why did the rebellious group of Canaanites head for the hills? Were they simply the ancient equivalent of hipster vegans, the hilltops their Portland or Brooklyn? Weitzman reports that two archaeologists, Zvi Lederman and Shlomo Bunimovitz, have a fascinating theory: they say the catalyst for the birth of Israel was the Philistines’ arrival in Canaan. The Philistines, one of the Sea Peoples who came from the eastern Mediterranean to Canaan around 1150 BCE, built the cities of Ashdod, Ashkelon, and Gaza, and after a few generations they began to mingle with the Canaanite inhabitants. But some Canaanites had mixed feelings about allying themselves to the Philistines. The Bible might echo a real historical friction between Philistines and pre-Israelites when it depicts Samson falling for Delilah and the woman of Timna, those Philistine prototypes of the dangerous shiksa. Lederman and Bunimovitz have been digging in the ruins of Beth-Shemesh, a Canaanite city near Philistine territory that seems to have resisted the newcomers. Maybe Israel began when the Canaanites of Beth-Shemesh marked themselves off from the alien Philistines. At Beth-Shemesh, there are no pork bones, in stark contrast to neighboring Philistine cities.

We might have some faint inkling about when Israel began, but another question remains: when did the Israelites become Jews? Usually scholars say that the shift happened in the Persian period, after the return from Babylon to Jerusalem propelled by Cyrus the Great’s conquest in the sixth century BCE (though most of the Babylonian exiles stayed in Babylon). The books of Ezra and Nehemiah report on the rebuilding of the temple and Jewish law in Jerusalem, and they use the word Yehudim, Jews or Judeans. So does the Book of Esther, probably from about the same time. But in his 1999 book The Beginnings of Jewishness, Shaye J. D. Cohen argued that the Hasmoneans, who ruled in the second and first centuries BCE, first made it possible to become Jewish. The Maccabeean revolt brought the Hasmoneans to power: Simon Maccabaeus, Judah Maccabee’s brother, founded the dynasty.

Until the Hasmoneans, Cohen argues, a Yehud was simply a resident of Judea, in the same way that an American is a citizen of the United States. But the Hasmonean period imported into Judea the Greek idea that one can become a member of a nation. You could be educated into Greekness, even if you weren’t born Greek. Similarly, Jewishness became an achieved identity, rather than one assigned at birth. Study and ritual practice made you a Jew. The mikvah and the wearing of tefillin, according to Cohen, began with the Hasmoneans, and both were modeled on Greek practices. (Greeks would sometimes affix a text from Homer to their foreheads to cure illness.)

We tend to think of Jewish Hellenism as craven assimilation, and occasionally it was. Some Jews tried to undo their circumcisions, painfully making a new foreskin so they could compete naked in the gymnasium without shame. But Cohen makes the case that Greek influence enabled Judaism to come into its own. It’s not quite right to say with St. Paul and James Joyce that “Jewgreek is Greekjew”: Hasmonean Jews drew on Greekness as a model while still fiercely insisting on their difference. Conversions to Judaism abounded.

Weitzman is a fluent and learned guide, but when he discusses Cohen’s book he falls into a bad, modish habit: using the origin of a theory to raise doubts about its persuasiveness. Weitzman points out that Cohen was taught by Elias Bickerman, who was educated in Berlin, and who stressed the importance of Greek ideas for Jewish tradition. He then adds that Greek texts were central to the German notion of Bildung, which Bickerman embraced, as if this somehow makes Cohen, or Bickerman, less persuasive. Bickerman paved the way for Cohen when he showed that the Maccabees and their heirs drew on Greek ideas to strengthen Judaism. Something of an eccentric, Bickerman never learned Hebrew: Greek was his beat. He went to synagogue once a year, on Purim, when he would annoy his fellow Jews by wearing a tallis, in defiance of Jewish custom. He took his own way, rather then merely suffering from German influence.

I find Cohen’s and Bickerman’s ideas convincing, and I’m confident that the yekkes among my ancestors play no role in my inclination to accept it. Theories can, and should, be separated from the people who devise them: they stand or fall by the evidence. To say otherwise is to flirt with an easy relativism — to suggest that every idea is tainted by ideological bias, and therefore unreliable.

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Often we want to find origins because we imagine that they might show us something about our current selves, but this is a shallow fantasy. Pop genetics at times resembles past-life regression, encouraging you to ornament your identity with an exotic, prestigious touch: maybe you can trace your ancestry all the way back to King David, for instance. The actress and mystic Susan Roth fancies herself a descendant of Israel’s most famous king, and she claims that David’s family tree also includes Jerry Seinfeld, among many others. There’s one big problem with such genetic detective work: a Davidic ancestry was highly prized in the ancient and medieval worlds, especially for rulers, who invoked David the way American pols cite Lincoln. As a result, phony line-of-David genealogies proliferated.

Genetics tempts us to think of Jewish history as a family story writ large. But the analogy between nation and family leads us down the wrong path. Genetic inheritance is not the same thing as the cultural traditions that make a people. As of now, it appears that a slight majority of Jewish men have genetic roots in the ancient Middle East. Whether or not this is true, the fact of Jewish peoplehood remains the same.

We seek out the origin of Jewishness because we want to know what makes a Jew a Jew, or, as Shaye J. D. Cohen puts it, “What is it that makes us us and them them?” — since the only way to define a Jew is to contrast her with a gentile. But what makes a Jew feel Jewish has changed over time, and is radically different from person to person and from place to place. If you’re an Israeli Jew, you don’t have to work at being Jewish. For the rest of us, things are more confusing. At one extreme is the utterly secular Jew (or half- or quarter-Jew) who bridles at an anti-Semitic remark because she realizes that she is the target, and that the anger she suddenly discovers in herself unites her in a tangential but unbreakable way with every other Jew on earth. At the other extreme is the person who constantly reminds himself of his Jewishness through study or observance. Most of us live somewhere in the middle, and Vive la différence. How the Jews got started is less important than the continuing pressure every Jew feels to stay Jewish, even if only a little.

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David Mikics is a columnist for Tablet magazine and author of the author of Bellow’s People (Norton) and Slow Reading in a Hurried Age (Harvard/Belknap). He is John and Rebecca Moores Professor of English at the University of Houston.