A popular song lyric from 1927 ran: “I’ve danced with a man, who’s danced with a girl, who’s danced with the Prince of Wales.” The poet Allen Ginsberg liked to boast that a line of three intermediate lovers connected him back to his idol, Walt Whitman. Well before the development of social media, the 1993 film Six Degrees of Separation made many of us aware of just how few steps it takes to connect you, or me, to millions of others around the globe.

Webs of human connection are an important subject for historians. Tracing out who people in the past interacted with—and how, and where, and when—can provide rich insights into the dynamics of their societies. How rigid were the social structures? To what extent was preindustrial life really “undignified, stagnatory, and vegetative,” to quote Karl Marx about India before the arrival of the British? Just as medical scientists can learn how the body functions by following radioactive isotopes as they move through the blood or tissues, so too can historians learn about societies by exploring the contacts and connections of ordinary individuals as they pass through life.

The study of human connections has been central to the rich current of scholarship called microhistory, especially as practiced by Italian historians such as Carlo Ginzburg and Carlo Poni. In a key essay from 1979, they made the case for a “science of the lived” (scienza del vissuto) in which biographies written from below would yield “a history that is full of individuals and stories and is not of necessity a history of the great and the celebrated.” This approach is at the heart of Harvard historian Emma Rothschild’s captivating new book, An Infinite History.

As a starting point for this study of modern France, Rothschild chose, essentially at random, a woman who led a seemingly unremarkable life in a seemingly unremarkable town in the west-central part of the country in the 18th century: Marie Aymard, the illiterate daughter of a shopkeeper, who was born in Angoulême in 1713 and died there 77 years later. She married a furniture maker named Louis Ferrand in 1735 and had eight children, two of whom did not survive infancy. Louis left to work in the French West Indies in 1753 and died in Martinique several years later. In 1764, the couple’s surviving daughter, Françoise, married a tailor’s son named Etienne Allemand. Eighty-three people, mostly from the worlds of the trades and minor officialdom in Angoulême, signed the marriage contract.

Beginning with Marie Aymard and this marriage contract, Rothschild traces out webs of connection in both space and time. She first provides what amounts to a social MRI of Angoulême in 1764, concentrating on the economic strata of the contract’s signatories and illustrating their connections to the wider world, including especially France’s Caribbean colonies, which then formed the most important (and profitable) part of the country’s overseas empire. Rothschild then follows the family and their connections forward in time, through the French Revolution, the political and economic transformations of the 1800s, and into the early 20th century. One of Marie’s descendants was a woman who ran a wine shop and drinking establishment in Paris that counted Baudelaire, Manet, Courbet, and Nadar among its customers. There was also a banking heiress who married into the family of Baron Georges-Eugène Haussmann, the man who rebuilt Paris under Napoleon III. The best-known member of the clan was Charles Martial Allemand Lavigerie (1825–92): a Roman Catholic cardinal, primate of Africa, prolific author, antislavery campaigner, founder of the missionary order of the White Fathers, and fierce enemy of what he called, prefiguring the hostile modern usage, “Islamism.”