R. I. Pocockin 1935 suggested that four type of wolf contained the genetic information required to breed today’s breeds of dogs. His theory was also supported by the fact that the dogs are able to produce fertile offspring when mated to wolves. The four type of wolves; Northern Grey wolf, pale- footed Asian wolf, the desert wolf of Arabia, and the woolly coated wolf of Tibet and northern India. The more recent research by Alice Fiennes has tended to confirm pocock's view. (Source 'Encyclopedia of Dog' By Alison Goff)

The history of dogs goes back about 40 million years back, but we’ll make it brief here. A small carnivorous mammal called Miacis which flourished then gave rise to Cynodictis. The Cynodictis then gave rise to Cynodesmus and Tomoritus over 10 million years ago, which in turn gave rise to present day canids. The evolution then continued culminating in appearance of wolves foxes, jackals and coyotes from which the domestic dogs evolved.

The evolution of domestic dogs is very recent in fossil terms, but the seeds of association between humans and dogs may lead us back to the evolution of human kind, a million years ago. 

During these many years the dogs evolved adopting in many ways to meet the changing needs. But the real change in dogs kicked in about 10,000 years back when humans learnt to exploit the power of selective breeding. The selective breeding is the reason to make the dogs the most diverse mammal compared to any animal family in the world.

The dog family contains about 35 species grouped into fourteen genera.  They include the domestic dogs, the wolves, 14 species of fox and 4 species of jackal. They are Native to almost every part of the world and clearly belong to an adaptable family, living in across a diversity of habitat, from tropical rainforests to frozen northern woods, from deserts to icy tundra.

Today we can see about 400 different types of dogs which is an absolute result of selective breeding by humans in last 10,000 years. How ever the oldest fossil of domestic dog was recovered from a cave in Iraq which is estimated to be about 12,000 years old and there are other discoveries in Idaho, Turkey, Yorkshire, Denmark and Switzerland ranging from 8,000 to 10,500 years.

Regardless of growing archeological evidences, it will be still difficult to say with certainty from which wild ancestor domestic dogs evolved. Many scientists have made several attempts but none have come up with convincing theory with real scientific evidence. All thought experts agree that the domestic dogs are probably descendents of wolves; their bone structure, especially that of the skull and teeth, is nearly identical to small wolves and their behavioral patterns reinforce this evidence.

For more details explanation read ‘Encyclopedia of Dogs’ Publisher Alison Goff and ‘Complete Dog’ by Maria Costantino & Helen Digby.