Looks like many of us don't have the right stomach for a paleodiet. Literally.

Two studies give us a glimpse into our ancestors' microbiome — you know, those trillions of bacteria that live in the human gut.

And the take-home message of the studies is clear: Western diets and modern-day hygiene have wiped a few dozen species right out of our digestive tracts. One missing microbe helps metabolize carbohydrates. Other bygone bacteria act as prebiotics. And

In other words, Americans' digestive tracts look like barren deserts compared with the lush, tropical rain forest found inside indigenous people.

"The concern is that we're losing keystone species," says microbiologist M. Gloria Dominguez-Bello, at the New York University School of Medicine. "That's a hypothesis, but we haven't proved it."

Dominguez-Bello and her colleagues are the first to characterize the gut bacteria of people completely isolated from modern medicine, food and culture. As cultures around the world become more "Western," they lose bacteria species in their guts, Dominguez-Bello says. At the same time, they start having higher incidences of chronic illnesses connected to the immune system, such as allergies, Crohn's disease, autoimmune disorders and multiple sclerosis.