A species
Dog
Canis familiaris
The ancient wolf that moved in and learned to read our faces.
You and the dog last shared an ancestor about 95 million years ago (on 23 December, if life's history were one year).
Your dog's wild ancestor was a now-extinct kind of wolf that came to live with people tens of thousands of years ago, when all people lived by hunting and gathering food. Over many generations, living with us changed dogs' minds as well as their looks. Even young pups can read us and follow a human point to find hidden food with little training.
All dogs belong to one species, from a tiny Chihuahua to a huge Great Dane. Little ones stand only about 15 centimetres tall and weigh about as much as a large bag of sugar. Big ones stand about as tall as a kitchen bench and weigh about as much as a grown person.
Coats vary hugely. Fur can be short, long, wiry or curly, in black, white, brown, golden, grey, red or cream, often with patches or spots. Dogs now live with people in homes, farms and city streets around the world.
Puppy-dog eyes
Most dogs have a small muscle above the eyes that wolves barely have. It lifts the inner eyebrow to make the eyes look large and baby-like, which makes people want to care for them.
The world's favourite pet
A 2016 study found about one home in three had a dog. That made the dog the most popular pet in the study.
A blue and yellow world
Dogs do not see only in black and white. They mainly see blue and yellow colours, so blue and yellow toys are easiest for them to spot.