Newly discovered cellulose degrading human gut bacteria are going extinct in industrialized societies

Humans, like all mammals, depend on the gut microbiome for digestion of cellulose, the main component of plant fiber, but evidence for cellulose fermentation in the human gut is scarce. We have identified new ruminococcal species that assemble functional cellulosomes and degrade plant cell wall polysaccharides as gut microbiome inhabitants of some human populations. These species likely originated from either the primate or ruminant gut and evolved to adapt to the human gut through gene acquisition from other microbes. They are abundant and widespread among human hunter-gatherer and rural populations, but are extremely rare in populations from industrialized societies, indicating extinction in response to the westernized lifestyle. The disappearance of the identified species accounts for the scarcity of cellulose fermentation in the human gut microbiota, suggesting that dietary change and reintroduction may restore fiber digestion where the trait is lacking.