From Wolves to Chihuahuas: The Baby-ization of Dogs

Dogs evolved from wolves sometime before 12,000 to 15,000 years ago, when they were domesticated by humans. Because the small-dog genetic material is found in small breeds that are distantly related and found in distant regions, the researchers concluded the genetic instructions to make dogs small must be at least 12,000 years old.

"It's as ancient as all small dogs," Lark says. "Dogs are derived from wolves. Since this is found in all small dogs, it either got into dogs when they were first domesticated, or it was a small wolf that dogs descended from. The small dog haplotype is not found in wolves today."

The genetic instructions to make dogs small arose either because "a small wolf couldn't survive in nature, but it could survive in company with humans" or because an early human "wanted to domesticate a wolf and they didn't want to adopt a big sucker" due to confined quarters in early cities, he adds.

Lark says this "unnatural selection" led to global proliferation of small dogs.

"Everybody treats their dogs like their babies, so it's not surprising they would select for tiny dogs," says Chase, who owns a pair of 4-pound, toy poodle-Maltese mixes. "Tiny dogs are not particularly functional. They don't hunt with you. They don't protect your house. They don't pull carts. They're just small and sweet."

Some also are yappy, but Chase says: "Yappy we didn't study."

The researchers wrote that the genetic code to make small dogs "was readily spread over a large geographic area by trade and human migration."

Or, as Lark puts it: "A local sailor came in with a small dog and everyone said, 'Ooh, I want a dog like that! Can I use him to father my next dog?'"

Meanwhile, other people bred dogs to be large for use as guard dogs, hunting dogs, and war dogs (like mastiffs) and even to pull carts, Lark says.