How Cats Conquered the World

When your cat leaves a mangled mouse on your pillow, he wants you to know that he’s a conqueror. In fact, he is part of a race of conquerors, the successful descendants of a winding journey in which cats made use of humans to conquer the world. Now researchers have used genetics to create the most extensive map ever made of cats’ path to worldwide domination, published this week in the journal Nature.

Modern domestic cats all descend from a single type of wild cat: Felis silvestris lybica. From archaeological studies, researchers believe that F. s. lybica’s reign begins in the Near East, in a region stretching from modern-day Turkey down to Lebanon. Around 10,000 years ago, farmers began storing grain, which attracted pesky mice. Cats, it turned out, could help out with that.

But F. s. lybica also ruled in Ancient Egypt, where they left their traces in cultural artifacts from cat mummies to statues and paintings. Researchers wanted to know: How did these two separate cat-doms lead to today's global feline success? 

That wasn’t a question that could be answered with modern cat genetics alone. Around the world, the gene pools of modern cats are surprisingly similar, thanks to millennia of tagging along with human travelers and interbreeding wherever they went. “The modern domestic cats in Australia are the same as in Europe and as in America,” says Eva-Maria Geigl, paleogeneticist at the Institut Jacques Monod, CNRS and University Paris Diderot, and an author on the study.

So for this latest study, the team turned to the genetics of ancient cats around the globe to untangle their collective rise to power. By sifting through 9,000 years of genetic data, the researchers found that there were two separate waves of human-cat coexistence, with cats befriending both farmers and Vikings in their quest to spread around the globe. It also seems that over the course of this relationship, domestication happened fairly late in the game—if at all.

To collect enough samples, the researchers reached out to other scientists around the world for feline bones or teeth, whose toughness and stability make them most likely to harbor useable DNA. They ultimately analyzed over 200 ancient cat skeletons that spanned roughly 9,000 years. They also collected samples from modern cats for comparison. For each of these samples they looked at mitochondrial DNA, genetic material found in every cell that is passed on from mother to child, making it a useful tracer of evolution.

Combining the genetic information with the archeological and human historical records, the researchers teased out the basic pathways for kitty success. After cats befriended Near East farmers, and the farmers recognized their use, they began to crop up along the path of the farming movement. One striking example is a 9,500 year-old cat that was buried in a human grave on the island of Cyprus, where cats are not native. Some 6,000 years ago, after Neolithic farming practices began spreading, it seems that these people-friendly felines foraged northward and westward with humans into Bulgaria and Romania.

Thousands of years after cats in the Near East caught on, a second wave of cats began cohabitating with humans in Egypt. As we know from archeological evidence, cats began living with Ancient Egyptians from at least the 4th century B.C. But DNA shows that during Roman times, these Egyptian felines also began expanding through the Mediterranean, mixing with the Near East cats, and then heading up through the Baltics. Around the fifth and 13th centuries, they ventured through Europe and into Southwest Asia.

It seems cats had hit upon a winning strategy: Stick with humans. When the Viking era began, the expansion of Egyptian felines exploded, likely due to the popularity of ship cats that traveled along the trade routes keeping pests in check. “Rodents on ships not only eat and spoil the food, they also destroy the ropes, so rodents could be a disaster for sailors,” says Thierry Grange, a molecular biologist the Institut Jacques Monod, CNRS and University Paris Diderot and an author on the study. “Cats prevent these types of disasters.”

The researchers even found evidence of these human-loving cats at the Viking port of Ralswiek on the Baltic Sea, says Geigl, and the Iranian port of Siraf, confirming that the faithful mousers commonly joined sailing crews. And the cats’ venture didn’t end there: For thousands of years, these furry globetrotters have followed humans wherever they went, conquering every continent except for Antarctica.

Cat domestication: From farms to sofas

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This genetic tour de force was made possible not only thanks to the cheapness and efficiency of modern DNA sequencing, but also new methods in obtaining ancient DNA. The new research “adds to an array of studies coming out now with increasing success of obtaining ancient DNA,” says Melinda Zeder, curator of Old World archaeology at Smithsonian’s National Museum of Natural History. “(It) is clarifying the picture of initial domestication of animals ... and their dispersal … It’s a real technical accomplishment."

Yet while the new study may clarify how and when cats traveled with humans, it also raises new questions. Namely: Were these cats actually domesticated? And if so, when?

These queries are more challenging than they may first appear. What constitutes domestication, like what constitutes a species, is still a matter of fierce scientific debate. Many researchers, Zeder included, define it in terms of a relationship: “For me, domestication is a two-way relationship in which the animal ... is actually benefiting from its relationship from humans,” she says. But that kind of relationship isn’t something that is easy to pinpoint using DNA alone.

Another marker of domestication that researchers often use is distinct changes in the animal's physical looks, like the floppy ears in dogs—a trait that humans likely didn't specifically select for, but seems to be associated with desirable qualities like a less aggressive personality, and can be identified in the genome. Yet modern house cats, besides being slightly smaller and stubbier, don’t look much different from their wildcat cousins, says Giegl. “It's basically still the same shape,” he says. “It has still the same behavior. It has still the same food habits.”

Genetics can’t tell the entire story of domestication, but it can offer clues. In this case, researchers traced a genetic marker for the splotchy tabby fur color. A similar increase in color variation crops up in other animals when selective breeding began, and could be linked with a range of desirable behavioral traits, explains Zeder. It’s also possible that ancient humans could be selecting for these marks, since it may have helped them spot their animals in a crowd. Either way, identifying when this coloring started in cats could help them pin down when selective breeding (rather than just cohabitation) began.

Researchers found the tabby marker in roughly 80 percent of the modern cats tested. However, it didn’t appear in the ancient kitties until around 1300 A.D. This means that efforts to breed cats to look or act a specific way likely didn’t happen until very late in the game. Some scientists even suggest that modern house cats still aren’t fully domesticated—something that will come as little surprise to cat owners.

While the genetic picture is growing clearer, much is still fuzzy when it comes to our cat conquerors, says Wim Van Neer, bioarchaeologist at the University of Leuven who came up with the idea for the study after finding several cats buried in a human cemetery in Egypt dating back 6,000 years ago, the oldest human-cat relations found in the region so far.

Van Neer still wants to know: Where did the first cats—those worshipped in ancient Egyptian—come from? To answer this, researchers need to find still-older Egyptian cats with intact ancient DNA, not an easy proposition in the hot and humid tombs. In the future, researchers could also use isotopes, variations of an element that weigh different amounts, to learn more about kitty diet, as well as study ancient cat jaws to learn more about how their delicate physique has changed through the ages.

What’s certain is that, while cats have changed little as they followed humans around the world, both have grown and benefited from the relationship. The rest, of course, is hiss-tory.

Maya Wei-Haas | | READ MORE

Maya Wei-Haas is a freelance science writer who specializes in geology of Earth and beyond. Her work has been featured in National Geographic, News from Science, and AGU’s EOS.