This diseased spine may hold clues to early dog-human relationship

Scientists are still debating when and where dogs were domesticated, but there’s one thing most of them agree on: Early canines were working animals. Dogs evolved from gray wolves earlier than 15,000 years ago—before humans settled down in permanent villages—and they likely helped us hunt small game like deer and rabbits and pulled sleds or other transport equipment across vast plains. To buttress the idea that early dogs helped us carry supplies, archaeologists have often pointed to an aberration in the spines of many ancient canines: an overgrowth of bone known as spondylosis deformans, which researchers thought was caused by hauling heavy loads.

But a new paper debunks that idea. Reporting in PLOS ONE , Katherine Latham, a graduate student at the University of Alberta in Edmonton, Canada, finds that heavy lifting cannot be definitively linked to spondylosis deformans in dogs. The condition, however, may tell us something equally fascinating about our ancestors’ bond with canines. Latham discussed her new work with Science .

This interview has been edited for clarity and length.

Q: What does this spine condition look like?

A: If you look at the spine of a dog with spondylosis deformans, you can see these bony growths on the vertebrae, from small spurs to large scoop-shaped growths. In some cases, they grow over the joints that separate vertebrae. Spondylosis deformans is very common in mammals; if you’re over 30, you probably have it. But most people—and dogs—don’t have symptoms unless the growths are very large, in which case they can sometimes lead to back stiffness.

Q: Why would load pulling lead to this condition in ancient dogs?

A: The idea is that the stress of pulling or carrying loads might contribute to the disease in dogs, as it seems to in other draft animals like cattle. Since at least the 1970s, many archaeologists have assumed the condition is a telltale sign that early dogs pulled heavy loads. But there was no empirical evidence. It’s an idea that has become perpetuated in literature without anyone going back and testing it.

Q: How did you test it?

A: I spent about 4 or 5 months traveling to museums and universities in the U.S. and Europe that housed wolf and dog remains. I looked at the bones of 136 dogs, the vast majority of which were pets and had not been used as working dogs. I also looked at 19 sled dogs and 241 modern wolves; most were wild, but a few had lived in zoos.

Spondylosis deformans was very common in dogs, regardless of whether or not they pulled sleds. It was also common in wolves. The biggest correlation had to do with age: By 3 to 5 years of age, half of the dogs had some sort of spondylosis deformans, and the older they got, the more of them had it. By 9 years old, almost everyone had it. There’s no evidence that spondylosis deformans should be used as an indicator of dogs pulling loads. It’s just a product of the normal wear and tear of aging, as it is in people.

Q: So what can the presence of the condition in ancient dogs tell us?

A: Ancient dogs with a lot of spondylosis deformans are probably older dogs. And in order for them to have reached that age, someone must have been taking care of them. Humans were likely giving them food and sharing the warmth of their fires and the protection of their shelters. Also, if these dogs got injured when helping humans hunt, people probably tended to their wounds. Wild wolves usually don’t live past age 5, often because they’re injured while hunting. And indeed we found that wolves living in captivity were far more likely to suffer from spondylosis deformans than wild wolves, probably because they were living a lot longer.

All of this suggests that humans viewed early dogs differently than other animals. It could be that they valued them as important hunting assistants, or it could have gone deeper to something like companionship.

I have two dogs—we spend thousands of dollars on their food and care. We spoil them rotten. I don’t think it was too much different in the past.