The same could be said about Pace’s other passion—life on Earth. Most of it is microscopic and, until recently, most of it was completely unknown. To understand life on Earth, we need to first answer this simple question: What lives where? This is the essence of biogeography. It’s the question that sent naturalist-explorers like Charles Darwin and Alfred Russel Wallace sailing around the world, cataloguing its species, and eventually gaining the inspiration for the theory of evolution. It’s also a question that microbiologists had long ignored. The feeling was that “we already knew what was out there,” Pace once told me, when I interviewed him for my recent book. “People went out, overturned a rock, found a bacterium and thought it exemplary of what’s out there. It was stupid.”

When Pace started his career, scientists who studied bacteria and other microbes did so by growing them in flasks and beakers, but only a tiny fraction of these organisms will thrive in a laboratory. “When I started as a microbiologist, [we] just accepted that we had no idea of what 99.9 percent of microbes in the environment did, and focused our energies on working out what the 0.1 percent did,” says Hazel Barton, a microbiologist at the University of Akron and one of Pace’s protégés.

But in the 1980s, Pace pioneered ways of identifying microbes without having to grow them at all. Instead, he just pulled their genes out of the environment and sequenced them. That changed everything, allowing scientists to fully grasp just how many microbes were out there—in the soil, in water, in our very bodies. The science of the human microbiome—the community of microbes that live in and on us—is one of the hottest fields of biology right now. And in the words of the microbiologist and former postdoctoral researcher of Pace’s, Ruth Ley, “There wouldn’t be the microbiome without Norm Pace.”

“I don’t expect an epitaph,” Pace says, “but if I had one, I’d want it to be: ‘He blew the door off the natural microbial world.’ What more can one say?”

“I was always fascinated with the hidden world,” Pace once said. Growing up in rural Indiana, he spent a lot of his childhood gazing at objects through a toy microscope, while playing around in the fairly sophisticated lab that he had set up in his basement. (“Those dabblings lead one into rocketry and other things that aren’t good for you,” he tells me, in a cryptic hint at what sorts of experiments he might have gotten up to down in that basement.)

A few decades later, in the early 1970s, Pace had a proper lab of his own at the National Jewish Hospital and Research Center. There, he focused on understanding molecules called rRNA, which form the protein-making machinery that all living things depend upon. Pace’s friend Carl Woese had recently shown that one could work out the relationships between living things by comparing the subtly different versions of rRNA in their cells, rather than just by looking at their anatomy or other physical traits. This revolutionary approach set the course for much of modern biology. It also revealed the existence of the archaea—a group of microbes that had been wrongly mistaken for bacteria, but are actually a separate domain of life.