People still argue about when, where and how dogs were domesticated. Scientists have worked out the time, place and ancestry of nearly every other domesticated animal, from sheep to chickens, but not dogs. Some archaeologists say there were domestic dogs by at least 15,000 years ago. How much earlier is argued, with some saying 30,000 years. Where dogs first appeared is hard to say: it could have been in the Middle East, China, Siberia or several places at once. Most genetic evidence says dogs emerged in the farming era, but some dog fossils are much older, dating as far back as about 33,000 years ago.

Dogs came from wolves, a very long time ago. They belong to a family of animals called canids, along with wolves, coyotes, foxes and jackals. One idea about how dogs began is that calmer wolves hung around people's camps to eat leftover food. Scientists chose Siberian silver foxes for a farm-fox experiment because they are close genetic cousins of wolves. Russian scientists bred the tamest foxes with each other, and after many generations the foxes acted like friendly dogs. A puppy buried with two people about 14,000 years ago had been sick for weeks, and people took care of it.

Dogs have been helping people since ancient times. People have used dogs for hunting, guarding and herding. Working dogs often use their excellent sense of smell to help where people fall short. Guide dogs help people who are blind get around. As of November 2026, the Seeing Eye in New Jersey is the oldest existing guide dog school in the world. It works with three main breeds, German shepherds, Labrador retrievers and golden retrievers. Search and rescue dogs use their noses to find missing people, including after disasters. Breeds that often do well in search and rescue include border collies, bloodhounds, German shepherds, golden retrievers and Labrador retrievers.

A dog's sense of smell is far, far stronger than a person's. Scientists have estimated that dogs smell thousands of times better than people, though the estimates vary a lot. A dog's nose has more than 100 million smell receptors, while a person's has about 6 million. The part of a dog's brain that works on smells is about 40 times bigger than the same part of a person's brain. A wet nose helps a dog smell because scent particles stick better to damp surfaces.

For many everyday sounds dog and human hearing are more alike than people think. The big gap is for high-frequency sounds. Dogs hear sounds as high as about 45,000 hertz, while people top out near 20,000 hertz. Their sharp hearing for high sounds probably goes back to before dogs were domesticated, when their wolf ancestors hunted small animals like mice. Puppies have more rods in their eyes than people do, so they see much better in dim light. Dogs have two kinds of color cones where people have three, so dogs see blues and yellows well and have trouble telling red from green.

Dogs cool off mostly by panting, not by sweating the way people do. A dog has some sweat glands in its paw pads, but they do little to cool it down. Dogs also widen blood vessels in their ears and face to cool down. A dog's fur coat helps keep it warm in winter and can help it stay cooler in summer. A puppy is born with its eyes closed and cannot see at first; its eyes open when it is about one to two weeks old.

Dogs with flat faces are brachycephalic breeds with shorter skulls, like pugs and bulldogs. People bred them to have these features and found the look attractive. Dogs' flat faces can make it hard for them to breathe. Flat-faced dogs also have a harder time cooling off than other dogs. A nearly 900-dog study found flat-faced dog breeds are at very different risks of breathing trouble: boxers and Staffordshire bull terriers rarely had it, but Pekingese and Japanese chins did.

A wagging tail or a crouching body does not always mean a dog is friendly. Before you pet a dog, ask its owner if it is okay. If a grown-up says you can, let the dog sniff your closed hand first. If a dog you do not know comes up to you, stay still and quiet instead of running. Even a friendly family dog can snap if it is startled, so give dogs their space.

Words to know

• domesticated: tamed over time
• brachycephalic: short-skulled
• receptors: smell sensors
• ancestry: family line
• herding: moving sheep