How long have we been raising dogs? According to new research, long enough that we ought to call them 'caveman's best friend.'

A 33,000 year-old dog skull was unearthed in a cave in the Altai Mountains of Siberia, according to a recent study in the online journal PloS One. This skull represents some of the oldest known evidence of dog domestication, along with equally ancient dog remains found in a cave in Belgium. Together, the two samples indicate that domestic dogs may have come from more than one ancestor and more than one area, contrary to what some scientists thought.

Researchers judge by the structure of the remains that both the Belgian find and the Siberian find are domesticated species.

“Essentially, wolves have long thin snouts and their teeth are not crowded, and domestication results in this shortening of the snout and widening of the jaws and crowding of the teeth,” Greg Hodgins, a researcher at the University of Arizona’s Accelerator Mass Spectrometry Laboratory and co-author of the study, said in a written statement. "What's interesting is that it doesn't appear to be an ancestor of modern dogs."

This is likely because the Siberian skull predates the last great ice age, which occurred between 26,000 and 19,000 years ago. Because the ice sheets severely disrupted life for humans and animals during this time, Hodgins believes neither the Belgian nor the Siberian lineages survived the severe conditions.

"Typically we think of domestication as being cows, sheep, and goats, things that produce food through meat or secondary agricultural products such as milk, cheese, and wool. Things like that," said Hodgins.

"Those are different relationships than humans may have with dogs," he continued. "The dogs are not necessarily providing products or meat. They are probably providing protection, companionship and perhaps helping on the hunt. And it's really interesting that this appears to have happened first out of all human relationships with animals."