Cats are among the most popular pets on Earth, rivaling even man's best friend. Yet while we know a lot about our history with dogs, which may date back 40,000 years, the origins of domestic cats — like cats themselves — are more mysterious.

Long before they became mascots of internet mirth, cats spent thousands of years working their way into human cultures. And thanks to new research on feline DNA, our ancient relationship with these clever predators is finally coming into focus.

Scientists still disagree about how domesticated cats really are, since they look and behave so much like their wild relatives, and some experts consider them only "semi-domesticated." Cats typically retain more of their natural instincts and hunting skills than dogs do, making them less dependent on human support, and while many cats are affectionate with people, they have earned a reputation for being aloof.

Genomic research has also been relatively aloof about cats, dedicating far more attention to dog DNA. This has obscured key facts about our feline friends, says Eva-Maria Geigl, an evolutionary geneticist at Paris' Institut Jacques Monod who led the new study. "We don't know the history of ancient cats," Geigl tells Nature News. "We do not know their origin, we don't know how their dispersal occurred."

But Geigl and her co-authors are helping change that. Their study, which they presented recently at the International Symposium on Biomolecular Archaeology in Oxford, U.K., analyzed mitochondrial DNA from 209 ancient cats. These cats were found at more than 30 archaeological sites across Europe, the Middle East and Africa, and they lived between 15,000 and 300 years ago — a time frame that roughly spans from the dawn of agriculture to the Industrial Revolution.

Reading between the felines

cat mummy in Egypt Domestic cats were sacred in ancient Egypt, where people mummified millions of them. (Photo: Andrea Izzotti/Shutterstock)

As Geigl and her co-authors discovered, what's good for humanity has historically been good for cats, too. Some of our species' biggest breakthroughs — namely farming and seafaring — seem to have catapulted cats onto the global stage.

"We found for the first time that in prehistoric times cats from the Near East and, in classical times, from Egypt accompanied people on their journeys, thereby conquering the ancient world," Geigl tells the Australian Broadcasting Company. "They were the ancestors or our present-day domestic cats all over the world."

Based on previous research, we already have a vague idea about when people began taming cats. In 2004, scientists reported on a 9,500-year-old human burial from Cyprus that also held the remains of a cat, suggesting humans kept domestic cats as far back as the advent of agriculture. Farming began in the Fertile Crescent about 12,000 years ago, and would have provided a practical reason for people to ally with cats, given the threat rodents can pose to grain supplies.

We also know cats held special status in ancient Egypt, where they were apparently domesticated by about 6,000 years ago, and later widely mummified. But there are still big gaps in our history of human-cat relations, and that's what inspired Geigl and her colleagues, Claudio Ottoni and Thierry Grange, to dig deeper.

Letting the cat out of the bag

cat in front of a yellow background People may have initially tamed cats for rodent control, but their roles have shifted more toward companionship in modern times. (Photo: Mary Fedotovska/Flickr)

After studying the mitochondrial DNA of those 209 ancient cats, the study's authors say cat populations seem to have expanded in two waves. The first occurred in early Middle Eastern farming villages, where wild cats with a distinct mitochondrial lineage grew along with the human communities, eventually reaching the Mediterranean. As rodents congregated to steal food, wild cats were probably just capitalizing on the easy prey at first, then were adopted as farmers realized their benefits.

The second wave came millennia later, as the descendants of Egyptian domestic cats spread around Africa and Eurasia, Nature News reports. Many of those Egyptian cat mummies had a particular mitochondrial lineage, and the researchers found that same lineage in contemporary cats from Bulgaria, Turkey and sub-Saharan Africa.

This rapid expansion of cats was most likely linked to ship travel, the researchers say. Like farmers, mariners were often plagued by rodents seeking their food stores — and thus naturally predisposed to welcome rat-killing carnivores onboard. Geigl and her co-authors even found this same DNA lineage in cat remains at a Viking site in northern Germany, which they dated between the eight and 11th century.

"There are so many interesting observations," Pontus Skoglund, a population geneticist at Harvard Medical School who wasn't involved in the study, tells Nature News. "I didn't even know there were Viking cats."

What the cat dragged in

Freyja with carriage pulled by cats The Norse goddess Freyja, riding in a carriage pulled by cats. (Photo: Ludwig Pietsch/Wikimedia Commons)

There is other evidence that Vikings fancied feline friends. Cats were a popular theme in Norse mythology, according to Jes Martens of the Cultural History Museum in Oslo, Norway, who tells ScienceNordic that cats likely joined Vikings on long journeys.

"Freja, the goddess of love, had two cats that pulled her carriage," Martens says. "And when Thor visited Utgard, he tried to lift the giant, Utgard-Loki's cat. It turned out to be a serpent, the Midgard Serpent, which not even Thor could lift."

People often wore cat skins by the late Viking age, adds conservator Kristian Gregersen from the Natural History Museum of Denmark, and most likely also kept the animals as pets. "We are sure that there were domestic cats then, because of their size," Gregersen tells ScienceNordic. "Small cats accompany people, and they are nowhere near the size of wild cats." There's even archaeological evidence of cats in Greenland, where they were almost certainly introduced via Viking ships.

Given their penchant for raids, Vikings could have played a key role in spreading cats around Europe. Yet while countless human lives are now enriched by feline companionship, cats have more in common with Vikings than it might seem. They've continued to invade new habitats with human explorers in recent centuries, often with disastrous results. Cats from Western ships have decimated native bird populations on variety of remote islands, and a recent study found they've contributed to more than 60 extinctions, and still threaten at least 430 species.

Of course, that says more about humans than cats, since they're just one of many invasive species we've unleashed around the world (including rats and dogs). It may seem like cats don't need us, but homeless cats pose a greater threat to birds and other wildlife than pets do, not to mention the health risks they face from a feral life.

Cats have been with us since the early days of civilization, and they wouldn't be where they are today without our help. It's the least we can do to give them a home, where they can work on phase two of world domination: taking over the internet.

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.