Ancient split

C. jejuni and C. coli are thought to have shared a common ancestor, or parent, in the ancient past. When the microbial descendent split up and evolutionary pressures stepped in, two new species began to take shape and fill different niches within the guts of wild chickens, pigs and other animals.

Although the definition of a species is one of the most hotly debated topics among biologists, Sheppard said the two microbes are strikingly different, despite sharing about 85 percent of their genetic code.

"Chimpanzees and humans are known to be about 98 percent genetically similar, so the bacteria's converging toward becoming a single species, as we think, is pretty impressive," Sheppard told LiveScience. "That's a big genetic gulf to leap. Maybe like a lobster mating with a fly."

Sheppard said the bacteria likely began reversing their growing divergence, or genetic separation, when human agriculture came along.