Raw red alga of the genus Porphyra is part of a traditional Japanese diet. This alga is rich in porphyran, a polysaccharide that human digestive enzymes cannot break down. Marine bacteria that live on the alga produce a porphyran-digesting enzyme. In Japan, a homologous enzyme is common in one species of intestinal bacteria. Outside of Japan, people have the same species of intestinal bacteria, but they do not have the gene for the porphyran-digesting enzyme. The marine bacteria and the intestinal bacteria are not close relatives, so they most likely did not inherit the enzyme from a shared ancestor. Propose a scenario to explain the observed distribution of the enzyme.