Here’s what you’ll learn when you read this story:

• Scientists have long known that the brains of modern wolves are larger than dogs, but it’s unclear when the impact of domestication began shrinking the size of a dog’s brain.
• A new study examined the endocranial volume of both prehistoric and modern dogs and wolves to piece together when this decrease began to take place.
• By the end of the Pleistocene (roughly 12,000 years ago), protodogs and wolves had the same-sized brains, but by 4,500 to 5,000 years ago, dogs showed a roughly 46 percent comparative decrease.

For 20,000 years—or possibly even longer—humans slowly domesticated wolves into more than 200 breeds of dogs, including everything from the tiny chihuahua to the absolutely massive English mastiff. However, this genetic transformation hasn’t come free of trade-offs, as a new study published in the journal Royal Society Open Science details how the brains of dogs began shrinking at least 5,000 years ago.

Of course, scientists have long understood that the modern dog sports a smaller brain than its lupine relatives, but exactly when this cranial transformation occurred isn’t clear. To answer that question, an international team of scientists compared the endocranial volumes of 185 modern and 22 prehistoric wolves (dating from 35,000 to 5,000 years ago) to trace when brain sizes began to shift.

Brain volume alone isn’t a perfect measurement for intelligence—humans, for example, don’t even come close to having the largest brains in the animal kingdom—as it doesn’t account for changes to the cerebral cortex that occurred as dogs learned specific behaviors and became attuned to human social cues. But a smaller brain size does tend to correlate with an overall reduction in cognitive challenges as the pressures of survival and the demands of mating lessen. The authors used an absolute volume measurement to ensure that smaller dogs didn’t register as having smaller brains, for example.

According to the results, early “protodogs” of the Upper Pleistocene age, which ended around 12,000 years ago, showed no dramatic difference in brain size compared to Pleistocene wolves. However, around 4,500 to 5,000 years ago, during the Late Neolithic, things began to change. The researchers noted that for the earliest domesticated dogs, the rising interaction with humans may actually have produced a slight increase in brain size, but there was “a dramatic 46% brain size reduction by 5000–4500 yr BP in Late Neolithic dogs compared with contemporaneous wolves, with brain volume close to that of recent Toy breeds, providing potential evidence for very early behavioural selection.”

One theory is that these smaller brains might’ve worked to a neolithic human’s advantage. Because smaller dogs would be less trainable and more wary of changes in their environment, they could serve as efficient alarm systems. If you’ve ever rung the doorbell of someone with dogs, you get the idea. It’s also possible that smaller dogs were able to survive on fewer resources when food was less readily available.

“The way our dogs live nowadays doesn’t give them the opportunity to always express most of their intelligence,” French National Centre for Scientific Research’s Thomas Cucchi, lead author of the study, told The Guardian. “But they are extremely clever and domestication didn’t make them stupid, but made them really capable of reading us and communicating with us.”

We shouldn’t be too quick to look down on our canine friends’ shrunken brains, though. According to some studies, the human brain has been slowly shrinking for thousands of years as well, possibly because it’s just too metabolically expensive to have a larger brain.

Humans and dogs—we truly do everything together.