Based on DNA evidence, the wolf ancestors of modern dogs diverged from other wolves about 100,000 years ago, and dogs were domesticated from those wolf ancestors about 15,000 years ago. This date would make dogs the first species to be domesticated by humans.

Evidence suggests this happened first in East Asia (possibly China); some of the peoples who migrated from the east to North America brought dogs with them from Asia. Continued human migration led to ongoing dog migration as well.

As man's life changed, from the nomadic existence of hunters and gatherers to agriculturally based permanent settlements, the need for dogs increased, along with a demand for specialization; that demand led to selective breeding and the creation of specialized working dogs and house pets.

Domestication: history and ancestry

The relationship between humans and canines is long and devoted. Archaeological and genetic evidence both indicate a time of first domestication between 17,000 and 14,000 years ago. That places man creating the first dogs in the late Upper Paleolithic Era, close to the Pleistocene/Holocene Era's beginning.

Genetic analysis has not yet conclusively determined whether dogs began in a single domestication event, or if they began in more than one location at close to the same time. Introgression — the process of repeated backcrossing, genetically — may have continued for quite some time, both by design and by accident. The earliest dog fossils date from 13,000 to 17,000 years ago; their likely ancestor is the large northern Holarctic wolf, Canis lupus lupus. Remains of smaller dogs from Mesolithic (Natufian) cave deposits in the Middle East have been interpreted as descendants of a lighter Southwest Asian wolf, Canis lupus Arabs; these date to around 12,000 years ago. Dogs were present from North Africa across Eurasia into North America 14,000 years ago according to rock art and skeletal remains. Dog burials at the Mesolithic cemetery of Svaerdborg in Denmark suggest that in ancient Europe, dogs were valued companions whose passing was observed with care.

Genetic research continues and we will certainly learn more as better technology aids scientific investigation. At present, results are not consistent. Vilà, Savolainen, and colleagues (1997) concluded that the ancestors of dogs split off from other wolves between 75,000 and 135,000 years ago. A subsequent analysis by the same group of scientists (Savolainen et al {2002}) indicated a common origin for all dog populations from a single gene pool in East Asia between 40,000 and 15,000 years ago. However, Verginelli et al. (2005) suggest both sets of dates must be reevaluated in light of recent findings showing that poorly calibrated molecular clocks have systematically overestimated the age of geologically recent events. Taking that fact into account and adjusting timelines accordingly, the archaeological evidence seems to indicate that 15,000 years ago is the most likely time for the wolf-dog divergence to have occurred.

In a well-documented attempt the Soviets conducted to domesticate the fox, ("Tame Silver Fox" Early Canid Domestication: The Farm-Fox Experiment, Lyudmila Trut) they were able to do so in just nine generations — far less than a human lifetime. Surprisingly, domestication also resulted in other changes: fox coat color became black, white, or black and white; these foxes developed year-round breeding ability and evolved to having curled-up tails and droopy ears. In short, they were remarkably like dogs in less than a decade of focused breeding for domestication.

The rapidity of this timeline has changed earlier ideas about how long it took human beings to create the domestic dog. If we agree that primitive people had no surplus of food — that in fact, they can be assumed to have lived on the edge of survival, perhaps with occasional food shortages, then we can guess those hungry humans would not have taken wolf pups and made pets of them. Conversely, logic supports that wolves would raid garbage dumps near human habitations. In this hypothesis, wolves keep a flight distance between themselves and a threat. If a group of wolves was scavenging at a dump when humans approached, some wolves would run a greater distance away from the dump than others. Conversely, those that ran the shortest distance would be the first to return to their scavenging when the humans moved away, so they would obtain the choicest orts and the greatest amount of food.

This scenario, so the reasoning goes, was repeated and repeated, until it created a selective breeding situation that resulted in a strain of wolves having shorter and shorter flight distances, until they were eventually comfortable near humans. In other words, they domesticated themselves. These wolves were tolerated by humans, so long as they were also useful, in such ways as catching rats or driving away other predators. In time, other uses, such as hunting, were found for them.

Breed development

Different breeds of dogs abound in the world and thanks to interest in creating hybrids and reintroducing bygone breeds, there are more all the time. Best guess is that there are about 800 breeds currently being recognized by kennel clubs around the world. No one organization recognizes them all.

For all that, most of the world's dogs belong to no recognized breed at all. A visit to your local pound will quickly illustrate that random breeding has created a diverse population with no particular pedigree, but they share the domestic traits we value in our canine companions.

Only a few basic breed types evolved during the 10,000 years while we and they became family members by choice. These are referred to as natural breeds, or landraces, meaning they arose with little help or interference by humans. That they evolved with similar characteristics in a particular region was about the climate and the work they performed.

All modern breeds are of relatively recent derivation; many are the product of a deliberate process of artificial selection. This has led to the creation of some highly specialized breeds and there is extraordinary morphological diversity across different breeds. Despite it all, dogs are able to distinguish dogs from other kinds of animals — and usually, so can we.

What is the definition of a dog breed? That's a controversial question. The simple answer is that it is a group of closely related (in terms of family tree, a.k.a. genetics) and visibly similar dogs. Today, it is reasonable to add that it is a group of dogs whose characteristic traits are selected and maintained by humans, since the modern breeds did not evolve naturally. A breed of dog is not a scientific term or classification. In fact, it might be fair to say that dog breeds are what clubs of dog owners call the groups of similar dogs they raise. Kennel clubs ultimately determine what is, and is not, a legitimate breed.

First and foremost, a breed must be of sufficient number to stably transfer its specific traits and characteristics over many generations. Obviously, inbreeding can lead to health problems as double recessive traits come to the forefront in later generations. This is known as the founder effect, and dog breeders use DNA tests, genetic records and health testing to maintain healthy populations that breed true, which means they produce puppies similar to themselves without passing on dangerous diseases or conditions. In the purebred world, kennel clubs and breeder associations maintain ancestry records that enable breeders to develop healthy dogs while enhancing breed characteristics.

There are dog clubs and kennel associations that use a 75 percent pure standard for determining the lineage of any particular animal; this also changes the definition of a breed to a less-stringent standard of genetic singularity. Even with that standard, a breed is also defined a dogs with specific traits and characteristics that can reproduce themselves.

We've established what a breed is, but how is an individual dog identified as belonging to a specific breed? It is those same records that will enable kennel clubs to confirm the ancestry of a particular animal, confirming its bloodline and its breed.

Crossbreeds are created when two different breeds are bred to create a new animal hopefully displaying the best traits of both parents' breeds. But that may not be the result, any more than it is with people. While you may hope your baby has your eyes and your husband's height, it may instead have your height and his eyes!

Outside the purebreds are all the other dogs — numbering in the millions across the globe at any moment.

Breeds ranked by popularity

Dog breeds go in and out of style over time, and choices vary tremendously in different regions and cultures. Since 1991, the American Kennel Club reports the Labrador Retriever is the most commonly registered breed in the U.S. Yet in New York City, Poodles were the top registered breed in 2005 with the Yorkshire terrier population in Houston, Texas a close second. In the 1980s, Cocker Spaniels were the most popular breed according to the AKC. The Labrador retriever took top honors in the U.K. as well from 1999 to 2005, according to The Kennel Club.

Of the mongrel population, the most commonly available dog in U.S. shelters is the American Pit Bull Terrier and Pit Bull mixes; these dogs make up nearly one quarter of the adoptable population overall, a number which may be higher in big cities. In the U.K. national shelters the most common adoptable dogs are Greyhounds and Staffordshire Bull Terriers.