There’s a perennial temptation to go looking for a golden age: some point in the long dark history of the human species when we reached the acme of our development and since which we’ve been in decline. For Bruno Lacombe, the French philosopher at the heart of Rachel Kushner’s fantastic new novel, “Creation Lake,” that point was the Neanderthal.

“Thals,” Bruno’s affectionate shorthand for Neanderthals, had larger heads than Homo sapiens, and therefore more brains. The Thal did not like crowds, was prone to depression and loved smoking — a sort of prehistoric French philosopher. Thals lived in caves; Bruno follows their lead, moving from his farmhouse in southwestern France to the expansive network of caves running just below its soil. For nine-tenths of human history, Bruno claims, humans lived underground. When he goes into the cave system for the first time, he experiences visions made possible by the total darkness and hears voices from every stage of human history pooling around him. He experiences “the human community in the earth, the deep cistern of voices, the lake of our creation.”

Has Bruno lost his grip on reality or plumbed a deeper one? This question comes to preoccupy the novel’s protagonist, who goes by the alias Sadie Smith. “Sadie” is a disgraced American intelligence agent tasked by shady forces with infiltrating Les Moulinards, a group of French agri-terrorists. What she knows about Bruno — who serves as an éminence grise for the younger activists — has been gleaned from hacked emails sent by Bruno to the leader of the Moulinards.

Sadie is the perfect foil for Bruno: young, female, American, profoundly cynical. But the weird magic of “Creation Lake” lies in the gradual rapprochement Kushner orchestrates between the flinty American spy-for-hire and the ideas of a cave-dwelling French philosopher, considered past his prime by his own protégés. The plot is as zany as it is heady, as if an episode of “Emily in Paris” had been co-directed by Don DeLillo and Michel Houellebecq and had somehow turned out excellent.

In fact, Kushner slyly acknowledges that she’s venturing into Houellebecq territory by turning the bad boy of French literature into a minor character in “Creation Lake”: a decrepit but very famous novelist named Michel Thomas, who shows up, uncomfortable, at a small-town agricultural fair to research “an agronomy novel.” It’s tempting to see the novel in question as Houellebecq’s “Serotonin” (2019), which uses French agricultural politics as a window into the decadence of European civilization.

Like Houellebecq’s novels, “Creation Lake” casts a gimlet eye on modern Europe. “The real Europe is not a posh café on the rue de Rivoli with gilded frescoes and little pots of famous hot chocolate, baby macaroons colored pale pink and mint green,” Sadie muses as she squats on the side of a French highway, staring at a pair of Day-Glo orange underwear snagged on a bush. “The real Europe is a borderless network of supply and transport. It is shrink-wrapped palettes of superpasteurized milk or powdered Nesquik or semiconductors. The real Europe is … windowless distribution warehouses, where unseen men, Polish, Moldovan, Macedonian, back up their empty trucks and load goods that they will move through a giant grid called ‘Europe,’ a Texas-sized parcel of which is called France.”

But Kushner’s preoccupations are almost wholly different from Houellebecq’s — she has said that she’s unlike the “despicable stylists” in that she loves people rather than hates them — and Sadie otherwise has very little in common with Houellebecq’s protagonists, who tailspin into various forms of nihilism while lamenting France’s spiritual decay.

Sadie is utterly unintimidated by European civilization, past or present. She notes with contempt that Italians seem to raise their children on Nutella, which she describes as “wartime goo” and declares English a “wildly superior language” to French, though she speaks the latter fluently, with an American accent for which she makes no apologies. “Having a good accent is nothing,” she declares. “It’s a consolation prize for people who aren’t fluent.”

Sadie is prone to boasting, whether about her breasts or her ability to drive better after a few drinks. She’s caustic, hubristic and unabashed, but she’s ultimately more sensitive — to social relations, philosophical nuance, the natural world — than the intellectuals-turned-activists with whom she’s professionally obliged to cosy up.

“Creation Lake” is Kushner’s fourth in a series of highly acclaimed novels. She’s been shortlisted for the National Book Award three times and for the Man Booker Prize twice. Her second novel, “The Flamethrowers” — which also revolves around a leftist social movement — was recently named one of the “100 Best Books of the 21st Century” by the New York Times. Her third novel, “The Mars Room,” takes readers on a searing tour of life in a California prison and won Kushner the Prix Médicis Étranger. Before “Creation Lake,” her admirers already included an older guard of American writers like Don DeLillo and George Saunders, as well as younger writers like Ben Lerner.

Yet “Creation Lake” feels like Kushner shifting into a new gear. The novel is as erudite as its predecessors, but it’s less self-conscious — funnier and freer. The novel feels pared back to the essentials and yet filled with life: as simple and effective as the cave paintings at Lascaux.

Late in the novel, Bruno comes across a new piece of research suggesting that these paintings — which he had previously thought were mere greedy catalogues of conquered peoples and beasts — were actually attempts to map the stars. This possibility makes him wonder if Homo sapiens might not have been such an evolutionary wrong turn. Their shared preoccupation with the movements of the heavens leads Bruno to conclude that Homo sapiens and Neanderthals were simply two iterations of the human being, “both beset, and deeply, by a need to know, the same need that plagues me, now.”

It was this need to know that had sent Bruno in search of a prehistoric golden age in the first place, and of a subsequent fall that could show us where we went so wrong. Kushner’s novels, too, seem animated by this need to know. In previous novels, that need had sometimes felt desperate. “Creation Lake” makes an uneasy peace with it. We need to know more than a withholding cosmos will share and yet here we are, and here we have been for tens of thousands of years. “Creation Lake” grapples joyfully with these perennial paradoxes, even as it shines an unsparing light on our present age.