We lived alongside mammoths, had not yet discovered bronze or writing and spent our lives moving constantly from place to place. But even 10,000 years ago, human beings loved their pet dogs.

This fact is borne out in an ancient gravesite in Illinois, where a trio of dogs were found buried alongside humans. The discovery is the earliest example of individual dog burials in the world.

New research from zooarchaeologist Angela Perri, presented in April 2018 at the annual meeting of the Society for American Archaeology and reported in Science News, presents new questions about man’s oldest friendship. Perri describes how these three long-forgotten dogs were given their own graves in the plot. Their intentional burial indicates they were likely grieved when they died.

Early Americans are believed to have had dogs for thousands of years, but the precise relationship has been hard to pinpoint. The new research uses radiocarbon dating of the dogs’ bones to show they are 1,500 years older than previously believed. This makes them the oldest known dog remains found in the United States. (The next oldest dog bones come from a Texas site dated 9,300 years ago: Those grisly remains seem to have been food, rather than friends.)

Koster Dog 3D Model by Jason Herrmann on Sketchfab, courtesy of the Center for American Archeology

So, who were these Illinois dogs, and how did they get to the United States? Skeletal analysis of the jaws and teeth suggests that they bore more than a passing resemblance to modern wolves, and may have interbred with present-day coyotes. There’s no damage to their bones, either—these dogs died naturally.

Exactly how their ancestors arrived in the Americas is much more mysterious. Man’s best friend may have joined humans on their migration from Asia to the northern Great Plains about 11,500 years ago, says Perri. Still, that doesn’t explain how these dogs made it all the way down to present-day Illinois, nor why we’re yet to find evidence of earlier canine companionship further north.

“The appearance of the earliest domesticated dogs in the Midwest around 10,000 years ago presents a conundrum both temporally and spatially,” she wrote in an abstract to the research. “If dogs arrived with the first migrating human groups, the earliest dog remains should appear in northern and western North America during the Paleoindian period.”

Sniffing out the truth is likely to require further research, and a bit more digging.