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Ken Aplin of Australia’s national science agency, the Commonwealth Scientific and Industrial Research Organisation, has been analyzing the mtDNA of 170 black rats from over 76 regions in 32 countries. He’s been doing so to investigate the animals’ genetic links and origins.

As you may know, the humans and rats share precarious symbioses. Actually, come to think of it, it is a misnomer to describe the relationships of rats and humans as symbiotic. It is anything but symbiotic. A more accurate term to define the relationships of humans and rats is as a parasitic relationship, where rats’ association with humans benefit them more than what we get in return.

World Distribution of Plague, 1998The close association of rats to human societies have brought about many diseases. One of the more major ones is the black plague. Rats are ultimately a carrier of the primary vector (fleas) that carry the infectious pathogen of the plague, the bacterium Yersinia pestis. Millions of people in Europe died from plague in the Middle Ages, when human homes and places of work were inhabited by flea-infested rats. Since then, outbreaks of the plague still occur, the last urban plague epidemic occurred in Los Angeles in 1924-25. The image to your right documents the most recent distribution of reported plague outbreaks. Unfortunately, it’s from ten years ago.

The plague has been an extremely virulent pathogen. Even though we have strong antibiotics to treat the plague, if untreated people can die within a day or so. You can read more about the molecular biology and virulence of Yersina pestis, in this outstanding freely accessible research paper, Yersinia pestis–etiologic agent of plague.

I’m clearly digressing. Back to Aplin’s results, he and his team identified six lineages that all originated in Asia – in India and the Himalayan region, Thailand, the Mekong Delta of Vietnam, Indonesia…. These six lineages can be traced to four major episodes of global rat expansion, all associated with human migration or trade. See, about 20,000 years ago, one lineage moved from western India to the Middle East and from there to Europe. Apin hypothesizes that this expansion was probably associated with the growth of trade networks during the Neolithic and Bronze Ages . But I think Apin has his dates all mixed up, the Neolithic is thought to have sprung up about 11-8,000 years ago and Bronze Age around 5-4,000 years ago. His work then shows how the European established rat lineage then spread to Africa and the Americas with European explorers. The East Asian lineage, meanwhile, arrived in Micronesia, from Taiwan via Japan, the Philippines and Indonesia, about 3,500 years ago.

I don’t have any fancy diagrams to link you up with, nor do I have an actual paper where Apin describes these conclusions and his methodologies. He’s actually presenting his results at the Archaeological Science Conference at the Australian National University, Canberra this week. His work seems like a basic phylogenetic reconstruction of rat populations based upon mtDNA. Nothing all too novel, but it does give us a unique genetic insight to migrations of humans some 20,000 years ago. Prior to this line of evidence, one of the only ways we could trace human migratory patterns was using the archaeological record — which often doesn’t give as much resolution as the genetic evidence.

I’ve tried to do some sleuthing to see if current literature explains the migrations Aplin’s suggesting. I can’t come up with anything conclusive at this time. If other’s know of papers that document people moving from India to Europe some 20,000 years ago, I’d be interested to know because I’m doing a tangiential study on human skin genetics.

One last thing, this research reminds me of these two papers, “Traces of Human Migrations in Helicobacter pylori Populations,” and “An African origin for the intimate association between humans and Helicobacter pylori,” the latter of which I covered around this time last year. I like how these two studies investigate the genetics of organisms associated with humanity. Often times, human genetics gets muddled up in the complexities of culture and human behavior. These organisms have the potential to provide much more simplified genetics that give a better representation of large scale migrations.

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