Scientists were able to identify the bone—about the size of an adult's pinkie nail—as a piece of the right occipital condyle of a canine. Occiptal condyles are parts of vertebrate skulls where the skull meets the spine.
The bone is the earliest known evidence of dog domestication in the Americas, predating other claims by nearly 8,000 years, said study co-author Samuel Belknap III, a graduate student at the University of Maine's Climate Change Institute.
"As a genetically identified domestic dog, it's significantly older than any other ones that we have from the New World," Belknap said. (Related: "Where Did Dogs Become Our 'Best Friends'?")
What's more, the bone fragment was found inside ancient human feces, which suggests dogs were being domesticated in the Americas for more than chores and companionship.
"It's small enough to pass through the gut, but it's still larger than you would expect to find," Belknap said of the bone piece. "It's surprising the sizes of some of the bones that people were swallowing. They didn't chew their food quite as well as people do today."
Dog Eaten for Ancient Rite?
The dog bone sample was found in 2009 in Hinds Cave in southwestern Texas. Previous archaeological evidence suggests a group of unidentified hunter-gatherers occupied the cave more than 9,000 years ago.
Working under Kristin Sobolik at the University of Maine, Belknap sent the newfound bone to the University of Oklahoma, where molecular anthropologist Cecil Lewis and his team conducted the genetic tests.
Based on previous DNA evidence, scientists think humans began breeding dogs from gray wolves sometime between 40,000 and 15,000 years ago.
From the size of the bone, Belknap and fellow grad student Robert Ingraham think the dog from the Texas cave weighed roughly 25 to 30 pounds (11.3 to 13.6 kilograms) and may have resembled some breeds of Mexican and Peruvian dogs.
Archaeological evidence suggests some Native Americans, such as the Sioux of the Great Plains, once used dogs to transport goods in much the same way that Inuit in Alaska use sled dogs.
Historical accounts from Spanish missionaries and early Europeans explorers starting in the 15th century also document some cultures across the Americas eating dogs in times of famine or for ceremonial purposes.
(Also see "Dogs First Tamed in China—To Be Food?")
Finding Ancient Dog Bones a Dirty Job
Although the dog bone was embedded in dried fecal remains, finding the sample was messy work: The bone was revealed only when Belknap rehydrated the feces and ran the resulting mixture through a sieve.
Despite its age, the rehydrated poop gave off an unmistakable odor, Belknap said.
"I tend to scare off people in the anthropology department when I'm decanting my samples," he said.
Still, for Belknap, the results of these efforts highlight the possibility that more ancient-canine bone samples may turn out to belong to domesticated dogs.
"I think scientists should be open to utilizing the latest techniques to determine whether canid bones are dog, coyote, or wolf and not just rely on morphological identification alone as means of distinguishing the difference," he said.
The oldest-dog research will be detailed in an upcoming issue of the American Journal of Physical Anthropology.