The microbes living in people's guts are much less diverse than those in humans' closest relatives, the African apes, an apparently long evolutionary trend that appears to be speeding up in more modern societies, with possible implications for human health, according to a new study. Based on an analysis of how humans and three lineages of ape diverged from common ancestors, researchers determined that within the lineage that gave rise to modern humans, microbial diversity changed slowly and steadily for millions of years, but that rate of change has accelerated lately in humans from some parts of the world.

People in nonindustrialized societies have gut microbiomes that are 60 percent different from those of chimpanzees. Meanwhile, those living in the U.S. have gut microbiomes that are 70 percent different from those of chimps.

"It took millions of years, since humans and chimpanzees split from a common ancestor, to become 60 percent different in these colonies living in our digestive systems," said Howard Ochman, professor of integrative biology at The University of Texas at Austin and co-author of the study. "On the other hand, in apparently only hundreds of years -- and possibly a lot fewer -- people in the United States lost a great deal of diversity in the bacteria living in their gut."

That rapid change might translate into negative health effects for Americans. Previous research has shown that compared with several populations, people living in the U.S. have the lowest diversity of gut microbes. Still other research has linked a lack of microbial diversity in human guts to various diseases such as asthma, colon cancer and autoimmune diseases.

The results of this latest study, carried out by researchers from The University of Texas at Austin, Yale University, the University of Pennsylvania and elsewhere, appear this week in the journal Proceedings of the National Academy of Sciences. The lead author is Andrew Moeller, a visiting scholar at The University of Texas at Austin and a graduate student at Yale University.