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The dog is a mammal in the order Carnivora. Dogs were first domesticated from wolves at least 17,000 years ago, but perhaps as early as 150,000 years ago based upon recent genetic fossil and DNA evidence though there is doubt about the accuracy of this mitochondrial DNA evidence.
In this time, the dog has developed into hundreds of breeds with a great degree of variation. For example, heights at the withers range from just a few inches (such as the Chihuahua) to roughly three feet (such as the Irish Wolfhound), and colors range from white to black, with reds, grays (usually called blue), and browns occurring in a tremendous variation of patterns.
Dogs, like humans, are highly social animals and this similarity in their overall behavioral pattern accounts for their trainability, playfulness, and ability to fit into human households and social situations. Dogs fill a variety of roles in human society and are often trained as working dogs.
For dogs that do not have traditional jobs, a wide range of dog sports provide the opportunity to exhibit their natural skills. In many countries, the most common and perhaps most important role of dogs is as companions. Dogs have lived with and worked with humans in so many roles that their loyalty has earned them the unique sobriquet "man's best friend." Conversely, some cultures consider dogs to be unclean. In other cultures, some dogs are used as food.
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