Rat are notorious pests, with populations throughout the world living off the scraps and garbage left behind by humans. Rats, it seems, are just born to mooch off humans, since they've gone down this same evolutionary path four different times.
Smithsonian Institution researcher Ken Aplin, along with an international team of collaborators, recently undertook the first comprehensive study of rat genetic diversity throughout the world. Examining rat genes from 32 countries, Aplin determined that there are six distinct groups that diverged about a million years ago. That's long before modern humans emerged, and yet four of these six groups all developed the exact same parasitic relationship with humans.
That logically has to mean that these different rats underwent the same parallel evolution four times over. All sorts of animals can fill many different types of evolutionary niches, but it would seem that rats are about as perfectly suited to living off of humans as any species could be.
Aplin and his team explain how this research can help explain why some pest control methods will work on one rat population but not on others, which is very useful information to those struggling with a pest problem:
Commensalism clearly arose multiple times in R. rattus and in widely separated geographic regions, and this may account for apparent regionalism in their associated pathogens. Our findings represent an important step towards deeper understanding the complex and influential relationship that has developed between Black Rats and humans, and invite a thorough re-examination of host-pathogen associations among Black Rats.