A genetic analysis announced this week reveals exactly when the brown rat, one of the most common animals today, conquered the world.
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The rat originated in southeast Asia, it turns out, spread to northeast Asia around 200,000 years ago, rested and spread to the Middle East around 3,600 years ago.
Then, the rat evolutionary tree shows, about 2,600 years ago rats reached Africa. Another 800 years would pass until they arrived in Europe, wrote the researchers this week in Molecular Biology and Evolution.
With that, the scientists put an end to the misconception that the brown rat invaded Europe in the 18th century and also showed that some zoologist was dead wrong to name the versatile rodent Rattus norvegicus. The brown rat didn’t hail from Norway and why anybody thought it might have is lost to history.
Do not confuse the brown rat with the rodent associated with the Black Plague, which is, fittingly enough, the black rat, or Rattus rattus. It’s a different species.
The discovery of the brown rat’s Southeast Asian origin was made by a vast team involving more than 20 research institutions, which carried out whole-genome DNA sequencing of 110 wild brown rats from around the world. They built the rat evolutionary tree based on the information from the 25 million samples of ratty DNA they collected from these 110 individual animals.
Riding the briny
How exactly the rat spread is unknown, but chances are, it coincided with trading by land and sea — caravans and ships both have convenient nooks for the odd rat to hide in. Archaeological evidence shows brisk trading going back thousands of years, and if there were rats on board, they made merry.
“Maritime trade has been in existence in the Indian Ocean and southern East Asia region for over 4,000 years. These early human activities could have facilitated the migration and dispersal of brown rat from southern Asia to other regions,” said co-author Dong-Dong Wu.
Genetic comparisons of rats from different geographical areas helped identify numerous genes involved in the immune response that adaptively evolved, through natural selection, in the wild rats, the scientists write.
Meanwhile, the rats became associated with many a disease in human beings, and it remains a complete mystery how they could harbor pathogens without becoming sick themselves.
“An ‘arms-race’ that drives the rapid evolution of the immune system in a host might have endowed rats with this potential,” Wu said.
Maybe this resistance to pestilences and immune robustness explain why, with the exception of humans, rats are the most successful species in terms of their geographical distribution. They thrive in every continent today, except Antarctica.
Israelis call the brown rat the “coast rat” or the “beach rat.” We also have a rat species called the “common rat.” They are not popular. Nor do they disseminate plague — there's a positive thought.
Cats, by the way, originated in the Middle East, according to separate genetic studies — specifically, in the Fertile Crescent.
When and how domestication took place remains a matter of yowling debate, but it may have been as much as 14,000 years ago that they domesticated us. Now we can say with certainty that whatever these proto-house cats were eating in the grain silos of the ancient Middle East, it wasn’t brown rats.
With writing by Ruth Schuster