The domestic dog is a subspecies of the gray wolf, a member of the Canidae family of the mammalian order Carnivora. The term "domestic dog" is generally used for both domesticated and feral varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and pet animal in human history. The word "dog" may also mean the male of a canine species, as opposed to the word "bitch" for the female of the species.
DNA testing suggests an evolutionary split between dogs and wolves around 100,000 years ago, but no fossil specimens prior to 33,000 years ago are clearly morphologically domesticated dog. Dogs' value to early human hunter-gatherers led to them quickly becoming ubiquitous across world cultures. Dogs perform many roles for people, such as hunting, herding, pulling loads, protection, assisting police and military, companionship, and, more recently, aiding handicapped individuals. This impact on human society has given them the nickname "Man's Best Friend" in the Western world. In some cultures, dogs are also a source of meat. In 2001, there were estimated to be 400 million dogs in the world.