Today, roughly 48 percent of U.S. homes have a dog; some of these pups – nearly 90 million in total – are so beloved they even have their own Instagram accounts. Where and when dogs came to share space, and later beds, with humans remain uncertain, but one thing is clear: Dogs are man's oldest animal friend.

Researchers agree that all dogs descended from the wild ancestors of gray wolves, but when, where and even how many times this domestication occurred has been the subject of ongoing debate. In 2016, an international team of archaeologists and geneticists sequenced DNA from both modern and ancient dogs, and concluded that two different wolf populations – one in Europe, the other in Asia – gave rise to our modern mutts some 14,000 years ago.

But a new theory, published in "Nature Communications" in 2017, contradicts their dual-origins hypothesis, suggesting instead that dogs were domesticated just once and much earlier, some 20,000 to 40,000 years ago. They didn’t split into the genetically distinct eastern and western groups until later, upwards of 17,000 to 24,000 years.

Archaeologist Maria Guagnin and a team of researchers from the Max Planck Institute in Germany, spent three years cataloging more than 1,400 rock art panels at sites in northwestern Saudi Arabia. Nearly half of these panels, as described in the "Journal of Anthropological Archaeology," depict humans with animals, including more than 300 instances of domesticated dogs. Dogs appear to be helping in hunts: In some cases, they’re shown biting the necks of ibexes and gazelles; in others, dogs are tethered to the waist of a hunter who's holding a bow and arrow. The medium-sized dogs have pricked ears, short snouts and up-turned curled tails, resembling a bushy-tailed Basenji or Pharaoh Hound – or as the authors suggest, the modern Canaan dog.

If the researchers’ estimates are correct, the engravings are possibly 8,000 to 9,000 years old, making them the oldest depictions of domesticated dogs, and the best evidence of humans using early dogs to hunt. And the use of leashes are by far the earliest known in the archaeological record.

Outside Bonn, Germany, on the eve of the first World War, workmen quarrying basalt rock uncovered a grave containing two complete human skeletons – an adult man and woman – along with what was then believed to be wolf and other animal bones. The animal bones were stored and untouched for more than 50 years, before they were finally identified as that of not one, but two domesticated Paleolithic dogs. The site, known as Bonn-Oberkassel, is the earliest strong evidence for canine domestication to date, and it’s also the oldest-known grave where humans and dogs were buried together.

In 2017, veterinarian and archaeologist Luc Janssens revisited these canine bones. He determined the younger of the two dogs was just six to seven months old, and based on dental evidence, probably had been gravely ill with canine distemper. Damage to teeth suggests the dog contracted the often-fatal disease as a puppy and endured three bouts of serious illness between the ages of 19 and 23 weeks. According to Janssens in a university press release, “Without adequate care, a dog with a serious case of distemper will die within three weeks,” leading him to believe humans intensively cared for the animal for at least eight weeks, a period in which the animal would have had no utilitarian value. This, coupled with the dogs’ burial alongside humans, suggest the unique emotional ties between man and man’s best friend may stretch back for millennia.