Jeff Leach is an anthropologist who’s less interested in studying humans than what lives inside humans. He’s embarked on an unlikely quest to understand an overlooked aspect of our primeval selves. What did the community of microbes that inhabited the human body before antibiotics, processed food, clean water—before settled living and farming—look like?

Here’s the context: Imagine that you live at a time when scientists discover a new organ in the human body. It’s nearly as important as your liver or lungs; without it, they say, you’ll turn into half-digested goo. And the organ isn’t small; its mass is roughly equivalent to that of your brain—about 2 percent of body weight. Nor is it hidden. In fact, it’s smeared all over you, especially in the warm, wet recesses of your gut.

That’s essentially the situation we find ourselves in the early 21st century. The organ in question is the human microbiome—the collection of microbes that inhabit the body. Microbes in fact outnumber “human” cells by a factor of 10 to 1. The collective genome of this “microbial organ” is at least 150 times that of your own. The term often used to describe the totality of You—10 percent human and 90 percent microbe—is "superorganism."

This “forgotten” organ is turning out to be immensely important for our health. Shifts in the microbiota may contribute to autoimmune and allergic diseases, obesity, cancer, heart disease and even possibly dementia. In other words, our microbes may have a hand in the top killers of the developed world.

All of which explains why the field has become white hot.

At its heart, the study of the microbiota is about symbiotic relationships—how humans transmit “good” microbial communities, how we inherit microbes from soil, animals and even the food we eat. It endeavors to understand how, exactly, a piece of the environment ends up living inside us, and how we’ve inadvertently disturbed this community.

Enter Leach, cofounder of the American Gut Project. To better understand what the ancestral human microbiota might have looked like, Leach is studying one of Earth’s last-remaining hunter-gatherer tribes—the Hadza of Tanzania. They call him Dr. Mavi, which in Swahili means “Dr. Shit,” on account of his frequent requests for fecal samples.

Some speculate that the modern microbiota has lost dynamism—become relatively static and boring— and that this loss of vigor may itself contribute to disease. That remains to be proven, but one thing is certain: the Western microbiota is far less diverse than microbial communities from people living in more traditional, less sanitized environments. And reams of research suggest that a diverse set of microbes protects against chronic disease.

Soon, Leach plans to adopt a Hadza-like lifestyle himself—to “go native,” as it were, and to monitor how his own microbes respond. The experiment is meant to illuminate an important unresolved issue: Can modern westernized adults meaningfully shift their microbial communities, or are we essentially lost causes?

His focus on the Hadza rests on the empirical observation that autoimmune, allergic and inflammatory diseases rise in direct proportion to westernization and affluence. The less westernized you are, the less likely you are to wheeze or develop multiple sclerosis. You are, of course, more likely to die before your first birthday from infectious disease. So the question is, can we identify and safely "export" what’s protective about those "dirtier" environments?

Leach has a personal stake in addressing this question. At age two, Leach’s daughter was diagnosed with type-1 diabetes. Those with type-1 diabetes require daily injections of insulin, or they may pass into a coma and die. Evidence is mounting that shifts in the microbiota may trigger this and other autoimmune diseases. So Leach’s quest is in some respects also a search for a cure for his daughter.

I propose a story on Jeff Leach’s quest to understand the “ancestral microbiota.” I’ve written a book that lays out the background science, entitled An Epidemic of Absence: A New Way of Understanding Allergies and Autoimmune Diseases. But the story I’m proposing here isn’t a proof—it’s an adventure story. It’s Heart of Darkness with microbes—and without colonialist overtones. It’s an adventure that involves cutting-edge 21st century science, an ongoing paradigm shift in our understanding of our own biology, and a visit to one of the last remaining groups of people who live as we all did before the advent of agriculture.

My plan is to visit Leach in Tanzania—to observe, describe, and document his quest, and to visit the Hadza, whose way of life probably won’t persist much longer.