The domestic dog (canis lupus familiaris) has been a part of human life for an estimated 15,000 years and is a domesticated descendant of the gray wolf. Dogs have become one of the most successful species on the planet; shortly after domestication, domestic dogs became ubiquitous worldwide. In the early days dogs were used as pack animals, herders, for hunting and pest control, and as warning systems signaling the presence of predators. As the adage holds, dogs may be man's best friend, but they were also part of the survival and success of early man.
Dog training began as early as the 18^th century, and is certainly part of today's culture, as evidenced by the success of The Dog Whisperer. This widespread focus on dog training may have sprung from necessity: according to a 2011 survey by the American Pet Products Association, there are approximately 78.2 million pet dogs in the U.S., and more than a third of U.S. households own at least one pet dog. The keeping of pet dogs became more popular after WWII and coincided with the rise of the suburban lifestyle.
Over the millennia, and often by selective breeding done by humans, dogs have evolved into hundreds of different breeds, over 150 of which are recognized by the American Kennel Club. According to the American Kennel Club, the ten most popular dog breeds in 2010 in the U.S. were the Labrador Retriever, German Shepherd, Yorkshire Terrier, Beagle, Golden Retriever, Bulldog, Boxer, Dachshund, Poodle and Shih Tzu. The Labrador Retriever has been one of the most popular pets in the U.S. for over 20 years.
Today dogs are still used in hunting and herding, and in addition to pets function as companions and helpers for the elderly and physically handicapped. Dogs are one of the most intelligent mammals, and inherited several characteristics from its progenitor the gray wolf which have made it particularly suitable for life among humans, including "pack behavior," expressive body language and a social disposition.