Gut-dwelling microbes may have played a major role in shaping animal evolution, an analysis of faeces from dozens of mammals, including humans,has indicated.

According to a report in New Scientist, trillions of mostly harmless bacteria and other microbes inhabit the guts of all mammals, outnumbering the number of mammalian cells by 10 to one.

Though scientists have long known that our stomachs team up with harmless bugs, it is only now that the breadth of bacterial diversity in our bellies is being appreciated.

Now, a census of the microbes living in 60 mammals, from cows to kangaroos to capybaras, has found that closely related animals and those with similar diets tend to have the same species of gut microbes.

By reading the chemical letters of a molecule called 16s ribosomal RNA - a cousin to DNA - researchers can catalogue bacterial species and conduct a rough census of their numbers.

Jeff Gordonand Ruth Ley, microbiologists at Washington University in St. Louis, tried this approach with stool samples collected from zoos and from animals in the wild.

They found more than 20,000 different kinds of bacteria. But the closer two species were on the tree of life, the more bacterial species they had in common living in their guts.

Diet also shaped an animal's gut content.

Meat eaters, such as bears and cheetahs, tended to share many of the same microbes. The same was true for omnivores, which included humans, as well as plant eaters.