Kyle Harper (photo: Kate Joyce)

Roman historian Kyle Harper is using technological leaps in the natural sciences to revitalize the study of human history. A tidy example of his approach comes from a 2018 project. Harper, who calls himself a “heretic of the humanities,” had for years been studying the Nazareth Stone, a two-foot tall, one-foot wide gray marble slab inscribed around the first century AD with a brief Greek decree that can be summed up as “Leave This Tomb Alone!”  Historians had long thought the Nazareth Stone might be the earliest physical trace of Christianity.

That fall, Harper, a former Guggenheim Fellow, wrote in a piece for the LA Review of Books that, “The original circumstances behind the Nazareth inscription may remain forever beyond our grasp.” But that winter, Harper called a geology colleague and asked if isotope analysis might decipher the stone’s origin. A few months later, their cross-disciplinary collaboration had “put the nail in the coffin” of a century-old historical debate. The Nazareth Stone came from the Greek island of Kos — not Nazareth — and didn’t reference Christ but the tomb of a fallen tyrant. “It’s a seductive story,” says Harper. And worthwhile research, but it’s also far narrower in scope than the big questions he believes a new approach to history can solve. 

Harper, a 42-year-old historian at the University of Oklahoma, joined the Santa Fe Institute's Fractal Faculty this year. He is a part of a group of historians trying to move their field beyond the study of “the narrow slice of time when humans produced documents.” E.O. Wilson called this approach “consilience,” a conversation between disciplines. “Why do physicists get the Big Bang? Geologists get Earth history? Biologists: evolutionary history? The sacred model of history confines historians to the study of writing and states,” Harper says. “That's a very arbitrary way of divvying up the past.” Nor is it one he feels is well-suited to answering the Big Questions: questions of the cosmos, climate change, humanity’s future.

Plagues upon the Earth, by Kyle Harper (Princeton University Press, 2021)
Cover of "Plagues Upon the Earth," by Kyle Harper (Princeton University Press, 2021)

Harper is the author of four books; his past two — “The Fate of Rome: Climate, Disease, and the End of the Empire” (Princeton; 2017) and “Plagues Upon the Earth: Disease and the Course of Human History” (Princeton; 2021) — demonstrate how the hard sciences can forge novel perspectives on historical questions. “Sometimes new perspectives come from new evidence like DNA, tree rings, and isotopes,” he says. “And sometimes from new or better models of complex systems that help us understand really hard problems.” 

Harper was first exposed to how technology can reveal historical secrets while getting his Ph.D. at Harvard. It was the early 2000s, and Harper, who had a long interest in physics and biology but chose to go to graduate school in history, had just met Michael McCormick, a medieval historian that Harper calls another “maverick against the forces compartmentalizing knowledge.” McCormick was working on the Plague of Justinian, a medieval pandemic that erupted in 541 AD and is often seen as the Black Death’s predecessor. But was it the plague, the same disease caused by the bacterium Yersinia pestis?

On a hunch, McCormick asked doctoral candidate Harper and a microbiologist to go into the Harvard medical school to scrape the dental cavities of a possible plague victim buried sixteen hundred years earlier. They ran flecks of the ancient remains through a PCR machine and the results solved exactly no mysteries. The technology needed another decade to mature enough to confirm that the Plague of Justinian and the Black Death were in fact caused by the same germ. But for Harper, the damage was done. He’d been infected with a passion for diseases (“Plague is near and dear to my heart,” says Harper, unironically) and an awareness that the natural sciences held the keys for answering questions once thought lost to history. “Genetic data is a historical archive for historians who are paying attention,” Harper says, adding to that idea every other scientific discipline that looks at change through time.

His latest book builds on those themes. “Plagues Upon Earth” explores why the human disease pool is far richer and nastier than any other species’. From humanity’s chimpanzee-like ancestors to present, the book spans thousands of years of pandemics. Harper knew that conventional historical research wasn’t equipped to answer the big questions. Could dendrochronology help explain whether climate change helped trigger the Plague of Justinian? Could DNA analysis of Yersinia pestis’s different genetic strains help clarify how and why this disease spread, mutated, and haunted humanity for centuries? What did economics say about how globalization made our species more vulnerable to pandemics like COVID-19? 

Over the three years that COVID raged, Harper thought about parasites deeper than most of us. Viruses and bacteria are parasites that need energy and host cells to thrive. What better source than humans? There are cities of 20 million people; 8 billion of us on Earth — each individual an irresistible target for microparasites. Harper began to see each pandemic in relation to contemporaneous technological advances that “let us extract more energy from the environment and make more of us.” Domesticating the horse. Trans-oceanic sailing. Germ Theory. Fertilizer. Antibiotics. Six million people on 100,000 flights every day. Technology detonated Paul Erlich’s population bomb and then tied every person on Earth together in one big interconnected knot. We’re a parasitic feast. “Humanity is the architect of its own misery. It’s the paradox of progress,'' Harper says. “Only it isn’t an ecological paradox at all. Our story is also the story of our parasites.” 

With “Plagues Upon Earth” published last fall (Forbes called it a “sweeping masterpiece”), Harper has turned to a related question: How has human success depended on and shaped biodiversity? And what does the historical relationship between humans and biodiversity say about our species’ chances of surviving Earth’s ongoing sixth mass extinction event? Harper started his research for his new book in the University of Oklahoma library. At the Santa Fe Institute, where he’ll spend time with scholars transcending the conventional boundaries between fields of knowledge, he’ll continue it through conversation.