Living in your gut are trillions of microscopic squatters, mostly bacteria. There are likely thousands of different kinds of them, and they started moving in soon after you were born.

The bacteria may not be a part of your body like your brain or heart, but they nevertheless play a major role in shaping who you are. These microbes, known as the gut microbiome, help digest your food, set your metabolism rate, regulate your weight and moderate your immune system.

But in recent years, scientists have wondered whether the wrong balance of microbial populations — brought about by an unhealthy, Western-style diet — might be responsible, at least in part, for the rise in some modern chronic diseases, such as obesity and irritable bowel syndrome. [Body Bugs: 5 Surprising Facts About Your Microbiome]

So, a group of scientists led by microbiologist Justin Sonnenburg of Stanford University decided to study the gut microbiome in its most natural, preindustrial setting: by examining the Hadza people of Tanzania.

The Hadza comprise one of the last groups of humans living a traditional, nomadic, hunter-gatherer lifestyle, as all humans once did just a few tens of thousands of years ago. Despite having no access to modern health care, the Hadza are largely free from the chronic diseases that plague Americans.

What the scientists found is that the Hadza have a far more diverse gut microbiome compared to Americans. What's more, the types of gut bacteria vary greatly in number as the Hadza alter their diet from season to season.

The new study, published Aug. 25 in the journal Science, suggests that a person's diet strongly dictates the diversity of the gut microbiome, and that those people living in the industrialized world have a far less vibrant gut microbiome that may adversely affect their health.

An unchanged, ancestral lifestyle

The Hadza are an indigenous population living in the central Rift Valley of north-central Tanzania. There are fewer than 1,000 Hadza people left. Among them, only about 200 live a completely traditional lifestyle unchanged from that of their ancestors, gathering food daily and moving from region to region seasonally.

The Hadza's homeland has two main seasons: wet and dry. During the wet season, the Hadza forage for berries and honey; during the dry season, they hunt game such as antelope. Their diets differ dramatically during these two seasons, and the only common elements are fiber-rich tubers and the fruit of the baobab tree, both available year-round. But the Hadza eat no processed food, nor do they eat farmed food. [11 Ways Processed Food is Different from Real Food]

To study the gut microbiome of the Hadza people, the researchers took stool samples from nearly 190 Hadza men and women over a period of four seasonal changes, or about 18 months — seven collection dates in all.

Sonnenburg's group then analyzed the samples and found that the gut microbiome varied along with the change in diet from season to season, the first such evidence of a cyclical change in humans. Bacteria species present in stool samples collected in the dry season all but disappeared in the wet season, only to return in the next dry season.

Aside from having a greater diversity of gut bacteria compared with Americans, the Hadza have many bacteria species that other traditional groups in South America and Papua New Guinea also have — and that Americans lack, the researchers found. It's as if something about modern society is causing the disappearance of microbial gut species, the researchers said.

"The challenge is to understand the importance of the ecological role and functional contributions of [microbial] species with which humans coevolved but that are now apparently underrepresented or missing in industrialized populations," the researchers concluded.

One factor influencing gut microbiome diversity may be dietary fiber, the researchers said. The Hadza consume upwards of 150 grams (5.3 ounces) of fiber daily, 10 times more than what Americans consume, on average, according to the researchers.

Sonnenburg said that his group cannot determine whether the Hadza microbiome is protective against chronic diseases, but there are "many arrows pointing in this direction right now."

He added that the relatively low life expectancy for the Hadza, 46 years, is due primarily to a high infant mortality rate and dying from accidents, such as falling out of a tree while collecting honey. But chronic diseases are rare, even among the older Hadza, he said.

Care for some raw honey comb and bee larvae? They're in season.

Follow Christopher Wanjek @wanjekfor daily tweets on health and science with a humorous edge. Wanjek is the author of "Food at Work" and "Bad Medicine." His column, Bad Medicine, appears regularly on Live Science.