Duke University dog researchers Brian Hart and Vanessa Wood discovered a stunning, yet disgusting, understanding of canine’s evolution – and it involves some feces-eating.

“Poop is central to the story of how dogs came into our lives,” wrote the two researchers in their new book, Survival of the Friendliest: Understanding Our Origins and Rediscovering Our Common Humanity.

Hart and his colleagues at the University of California at Davis School of Veterinary Medicine surveyed nearly 3,000 dog owners on their companion animals’ penchant for poop. 23 percent had observed their dogs eating feces, while 16 percent said their dogs did so frequently. More than 900 of them decided to try out at least one of the commercial products to help reduce canine coprophagia, but to no avail.

The researchers hypothesize that feces-eating is an obsolete trait that modern dogs inherited from their ancestors, the grey wolves.

Back then, the wolves consumed feces to eliminate a major source of intestinal parasites near their dens. But Hart and Woods suggested feces-eating could have also played an additional role in the evolution of the animal.

According to them, the anti-parasite hypothesis makes sense, but it does not explain why sometimes, dogs eat human feces.

Hart discussed about his study on wolves and dogs in the mountains of Ethiopia, and how dogs, but not the wolves, regularly consumed human feces.

Ray and Lorna Coppinger, another researchers on dog evolution, proposed the dog evolution is a scavenger hypothesis. They explained the evolution of dogs from wolves began when Paleolithic humans shifted away from hunters and began to form stable settlements. The wolves would have been more tolerant of being around humans, which gave them a leg up on the Darwinian competition for resources and spread the genes associated with tameness. That self-domestication strategy proved successful as there are now a billion dogs on Earth compared to 300,000 wolves.

Anthrozoologist John Bradshaw, however, argued the scavenger hypothesis cannot be the whole story. He said the emergence of dogs began between 15,000 and 25,000 years ago, which meant humans could produce enough garbage to make food scavenging worthwhile. He then said, but also quickly dismisses the idea, that human feces could have been the source of energy for dogs.

The Department of Biology at the University of Addis Ababa investigated whether the free-ranging dogs competed for food with endangered dogs in Ethiopia’s Bale Mountains National Park.

They discovered that dogs rarely go into the wolves’ territory, and they had different dietary preferences. Wolves ate rodents almost exclusively, while the dogs varied from barley husks, and human poop.

A study on dogs in Zimbabwe by James Butler and his colleagues also showed 56 percent of 1,000 dogs scats sampled included human feces. Butler noted that human poop contained twice as much protein as the dogs’ most common food. “[Human feces] was comparable to the upper range of energy content for mammal tissue, vegetables, and fruit.”

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