How did wolves evolve into dogs? Ancient fossils provide intriguing clues

The evolution of the domestic dog took place over a long period of time, during which wolves and dogs continued to interbreed—muddying the waters when it comes to analyzing canid fossils and even DNA.

In considering canine fossils, researchers examine their morphological features, such as size and arrangement of the teeth, size and length of the snout and mandibles, and the shape of the skull. The researchers then compare them to modern dogs, modern wolves, confirmed early dog fossils, and prehistoric wolf fossils.

Some features of ancient dog fossils include short skulls and snouts, crowded and smaller teeth (due to the shortened snouts), and wide palates and craniums. Additionally, scientists can use an advanced bone-measuring technique called geometric morphometrics to analyze the curves of a skull so individual specimens can be more easily compared to each other.

It’s not always easy to determine exactly what these bones are. Some Ice Age wolf-dog fossils are classified as “incipient dogs,” meaning they are in the early, transitional stages of development—not quite wolf, not quite domesticated dog, but somewhere in between. 

These incipient dog fossils are more like wolf-dog hybrids, the earliest ancestors of domestic dogs. The oldest of these on record, a large skull, was unearthed in a cave in Goyet, Belgium, in the 1860s. According to radiocarbon dating, the ancient fossil is nearly 36,000 years old.

Belonging to what is considered to be a Paleolithic dog, the Goyet dog skull more closely resembles prehistoric dogs than modern wolves. Radiocarbon dating and an anatomical analysis of another fossil skull, one discovered in a cave in the Altai Mountains of Siberia in 1975, places it at approximately 33,000 years old.

Researchers concluded that this doglike skull found in Siberia came from an incipient dog in the early, transitional stages of development.

Part of the family

We can learn a lot about the relationship between ancient humans and dogs from analyzing dog fossils. For instance, the oldest confirmed dog fossil, known as the Bonn-Oberkassel dog, is believed to be a little over 14,000 years old. The dog’s remains, along with those of a man and a woman, were found in 1914 in an ancient grave in Oberkassel, Germany.

The Bonn-Oberkassel dog was, in fact, a puppy, about seven months old. A recent examination of this fossil concluded that the dog suffered from distemper, and that humans provided care and nursed it through bouts of illness before it died.

This fossil is also the oldest confirmed evidence of a domestic dog burial with humans. Whether buried alone, with other dogs, or with humans, dog burials indicate a closeness between dogs and humans that goes beyond keeping an animal for its functional uses. It signifies a high level of regard and hints at the dog’s eventual transition from wild animal to pampered pet.

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In the 1970s, the skeletal remains of three domestic dogs were unearthed at an archaeological site called Koster, in the Illinois River Valley near the Illinois– Missouri border. The bones were discovered in shallow pits, suggesting they were buried deliberately. Because no tool marks were found on the bones (which would indicate they were killed by humans), the dogs are believed to have died from natural causes. Subsequent radiocarbon dating revealed the Koster dog bones were 10,000 years old.

Beating these out for the title of oldest domestic dog fossil in North America is a 10,150-year-old bone fragment found in Alaska. Though the bone was initially thought to be from an ancient bear, DNA proved it was from a domestic dog. Further analysis of this fossil revealed it to be closely related to a canine ancestor that lived in Siberia 23,000 years ago. All this suggests that Siberian hunters from the Ice Age may have domesticated dogs, and that humans—and their canine companions—migrated to North America from Siberia some 4,000 years earlier than previously believed, before the glaciers melted. By tracing how dogs moved, we understand more how humans moved. 

Looking even further back, researchers analyzed previously sequenced mitochondrial dog genomes and found that all ancient American dogs may have origins traced back to a common canine ancestor that lived in Siberia about 23,000 years ago. Ancient dogs living in North America all but vanished after a few thousand years; this was likely due to the Europeans arriving to the Americas with their own breeds, which quickly took over.

Where did dogs come from?

Various studies have focused on three main geographic regions—Asia, the Middle East, and Europe—as places of origin for domesticated dogs. Some scientists believe dogs may have been domesticated twice, in different geographic locations, while others think domestication was a singular event.

Science has not yet conclusively identified exactly where dogs originated, but every new study brings us one step closer to solving the mystery. Ancient doglike fossils found in Belgium and Siberia, as well the Czech Republic—all estimated between 36,000 and 33,000 years old—could imply more than one instance of attempted domestication of wolves, in multiple geographic locations. 

A few DNA-based studies have also suggested dual lineage, including a large 2022 study that analyzed ancient wolf DNA and found evidence that two domestication events may have taken place in East Asia and the Middle East. Other research, including two studies published in 2021, has shown evidence of one place of origin for the domesticated dog, one that traced dog origins back to Siberia 23,000 years ago, and one that identified the extinct Japanese wolf as the subspecies most closely related to domestic dogs, suggesting that the ancestor of domestic dogs may have lived in East Asia.

Digging into dog DNA

Though scientists have made great strides investigating the evolution of the domestic dog, much of the research is contradictory. We still don’t know exactly when wolves became dogs, nor is there a consensus on where domesticated dogs originated.

Mitochondrial DNA analysis, which uses a highly sensitive technique to look at a specific type of DNA found in ancient fossils, has opened a new world of information for researchers trying to pinpoint a time frame for the origin of the modern dog. Because dogs and gray wolves share 99.9 percent of their DNA, researchers can analyze genetic variations. However, DNA analysis is not always clear-cut, making it hard to reach definitive conclusions. It’s also tough to use observable traits (such as body size, hair length and color, head and leg shape) within individuals of a species—characteristics called phenotypes—to compare today’s dogs with their ancestors, a still unknown subspecies of the gray wolf. 

Though fossil evidence points to dog domestication happening around 14,000 years ago, DNA-based research often puts the split between wolves and dogs much earlier. The 2022 DNA study, which analyzed 72 ancient wolf genomes spanning 100,000 years, concluded that dogs likely appeared as long ago as 40,000 years, which roughly lines up with time frames some earlier studies pinpointed. In 2017, for example, researchers analyzed genomes from three ancient dog fossils from Germany and Ireland. After comparing those ancient genomes with genetic data from more than 5,000 modern dogs and wolves, the team estimated that dogs and wolves parted ways between 37,000 and 41,000 years ago. That study also determined that dogs split into two populations between 17,000 and 24,000 years ago: eastern (the progenitors of East Asian breeds) and western (which would go on to become modern European, South Asian, Central Asian, and African breeds). Based on these time frames, they estimate dog domestication occurring sometime between 20,000 to 40,000 years ago.

Science and technology continue to provide new and improved resources for researchers to employ in their search for answers. The more we investigate, the more we will discover about the origins of the dog.