You don't have to walk by a dog park to know that domestic dogs come in all shapes and sizes. From two-pound Chihuahuas to 150-pound Newfoundlands, chunky Labradors to slender Vizlas, our canine companions are some of the most physically diverse mammals on the planet.
It's commonly believed that this vast range in physical attributes is a product of the Victorian era, when kennel clubs started selectively breeding dogs to produce certain characteristics roughly 200 years ago.
A new analysis of hundreds of prehistoric canine skulls, spanning the last 50,000 years, shows it emerged much earlier.
"By about 10,000 years ago, half of the amount of diversity present in modern dogs was already present in the Neolithic," said Carly Ameen, a bioarchaeologist at the University of Exeter and one of the lead authors on the new study. "So very early on in our relationship with dogs, we not only change them from wolves but they begin to change amongst themselves and generate a lot of diversity."
To determine when those changes happened, Ameen and a team of international researchers created 3D models of 643 skulls of ancient and modern dogs and wolves. The models allowed them to discern subtle changes in the skulls' shape over time.
The results, published in the journal Science, show that by nearly 11,000 years ago, just after the last ice age, dog skulls were already different from those of wolves. They were shorter and wider. But perhaps more surprising, Ameen said, is that the dog skulls were already different from each other, meaning that the switch from wolf to dog had to have happened much earlier.
"The relationship between wolves and dogs had to already have been ongoing," she said. "It's not an instantaneous change — the dog comes from the woods into your house and changes the shape of its skull."
Those kinds of changes typically accumulate slowly, over many generations.
Scientists have long wondered when the domestication of dogs first started. Dogs are believed to be the first domesticated species — before cows, pigs, sheep, or plants like wheat.
The new study doesn't answer the question but "it narrows the window," Ameen said, and gives us insights into how humanity's mutually beneficial relationship with dogs physically changed them over time.
That relationship was the focus of another new study, published in Science, that used ancient DNA from dogs to find that humans were traveling with — and even trading — domestic dogs in Eurasia for at least the last 10,000 years.
The study's lead author, Minmin Ma, a researcher at Lanzhou University in China, said it makes sense that prehistoric hunter-gatherers were bringing dogs with them during migrations because they could assist with hunting.
But for prehistoric farming and pastoral societies that raised animals like cattle, sheep and horses, "dogs weren't particularly essential in that economic sense," she said. And yet, their study found that those groups made the effort to bring dogs with them during migrations too.
"Although the roles [dogs have] played varied across different periods, they have consistently been close companions to humans," Ma said. "We should cherish this bond even more."
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