The domestic dog, aka Canis lupus familiaris, originated in the domestication of the gray wolf (Canis lupus). The domestication process continues even today, if we consider the fact that the cross-breeding of dogs in order to create "designer dogs" still goes on.
There are several theories explaining how the domestication process actually took place. Thus, according to certain studies, wolf puppies that are taken at a very early age away from their parents to be raised and tamed by humans, are easily trained and socialized. There is at least one such study. Nonetheless, some other researchers state that in the case of wolf pups that are 21 years old, it's very hard to achieve any socialization or taming, and these attempts are rarely successful.
Many scientists consider that orphaned wolf cubs were adopted by humans, who took care of them together with their human babies, and thus appeared a new type of wolf-like domesticated animals which, in time, will turn into dogs. According to Dr. Raymond Coppinger of Hampshire College (Massachusetts), those types of wolves that were more interactive towards humans passed these traits to their following generation, and thus creating a kind of wolf that could be more easily domesticated by humans.
Coppinger speaks of a special behavior trait that was essential for the wolf's transformation into dog, and this trait was called "flight distance". It referred to the distance kept by an animal towards a human before running away from what they perceived as danger. In this respect, animals that live at a shorter distance from humans are likely to linger and feed even when these humans are present. Dr. Coppinger says that his argument is that tame or domesticated means being able to eat even in the presence of otherwise perceived as threatening humans. This is precisely what wild wolves cannot do. The side effects of domesticity-purposed selection were the selection of related physical features that were actually genetically caused, plus behavior such as barking.
There is a hypothesis according to which wolves got separated into two kinds of populations: the pack of hunters and village-directed kind of scavengers. The following selection steps were not actually defined, but perhaps there was a certain tension between these two groups. From the archeological viewpoint, there isn't certainty about the earliest known domestication until 7,000 BC. According to other evidence, dogs' domestication began in East Asia. There is a certain difficulty in deciphering the bones' structural differences, and because of that the culturally-based domestic dog identification is quite valuable. The first dog found buried together with a human dates back with about 12,000 years ago, in the country of Palestine.
Due to the domestication of wolves, a lot of changes occurred throughout time, changes that all domesticated mammals actually go through. Among these changes we could mention a reduction in the general size, changes in the coat's color and marks; the jaw got shorter, and the teeth got more crow in the beginning, and later on the teeth got shrunk. Another change was the fact that in front of the forehead there was developed a pronounced vertical drop. Wolf-like behavior such as regurgitating partially digested food for their young cubs also got lost in the process of domestication. Before the use of DNA, there were two research schools of thought. Most researchers considered that the early dogs descended from wolves through the process of taming. According to another category of scientists, although they still considered wolves to be the main ancestors of dogs, they also shared the suspicion that coyotes or jackals had also been involved in the domestication process.
The most extensive studies in this respect was conducted by Carles Vila. He has proved that according to DNA evidence, the only dog ancestors were actually wolves. Vila and his team studied 162 different samples of wolf DNA from no less than 27 European, Asian and North American populations. However, when we look at all the dog breeds we have today, it is really hard to imagine how the smallest dog to the tallest came from wolves, as well as all the other breeds with their specific looks and behavior. Could it be that they were created their own species in the first place?