There are varying opinions on when the first domesticated dog entered the world. The most recent studies have shown approximately 15,000 years ago. What most scientists agree about is that the closest ancestor to the domesticated dog is the Eurasian Grey Wolf. Prior to this, there were wolves, coyotes and yackals and most likely wild canines (which were not domesticated and probably were very different from what we think of a dog today).
The domesticated dog is part of the Canidae family, a diverse group of carnivores which contains 36 extant species. This first known family of carnivores came on the scene about 40 million years ago and includes wolves, jackals, and foxes, as well as the coyote, the maned wolf, the bush dog, the African wild dog, the Dhole and the racoon dog. Within this family, the dog is most closely related to wolves, jackals and the coyote, as these canines all have the same number of chromosomes and are all capable of interbreeding to produce fertile offspring. Although all of the above are potential ancestors of the domestic dog, after years of scientific studies and most recently the study of DNA sequences, it is believed that the wolf is the most probably relative. There is a high probability that dogs and yackals or dogs and coyotes interbred over time. Research has also shown that the domesticated dogs most likely originated from Asia.
Keeping in mind that domesticated dogs are only one species in the Canidae family, it is interesting to note that 35 species of wild canines can be found from the tropics to the tundra. Some include the maned wolf who lives in the grasslands, the fennec fox who lives in the desert, the arctic fox who lives in the arctic, the grey fox who lives in the forest and the coyote who is a jack of all trades. With the exception of the arctic the coyote can survive anywhere.
As natural selection governs life in the wild, domesticated dogs are governed by artificial selection. Beginning thousands of years ago dogs of many sizes and shapes appear in archaeological and artistic records. Dogs were bred for hunting, working, herding, protection, etc. Today there are over 400 breeds of domestic dogs and most were establish after 1850. The desire for pure bred dogs began around this time.
There is so much more I could write on this subject but just not enough room! The bottom line is whether you have a 2lb Chihuahua or a 200lb Mastiff they all are descendents of the wolf. They are pack animals who need a strong leader (YOU). It is important that you are the boss over your dog. This means you should always be able to take food away from your dog, put him in a down position, and handle every part of his body without his objection. This will develop a stable and happy pack.
If you are interested in the history of the dog I encourage you to visit the following websites: