Dogs as we know them today are descended from wolves. They take on so many different forms today because of thousands of years of deliberate breeding by humans, who selected for traits that would be useful, such as bodies capable of performing specific jobs, temperaments for war or herding or companionship, or for unique looks that could be shown off. There are around 370 different recognized dog breeds today, but there were many more in the past.
Before breeds were standardized for the purpose of dog shows, there were many more varieties for many more purposes. The "turnspit dog" for example, was a small but strong dog kept around the house to turn the meat as it cooked over a fire by running inside what looks like a hamster wheel (shown above). This freed up household labor for other things. When motorized spits came along, the breed was no longer needed. There was also the Hawaiian poi dog that only ate vegetables (shown at top), the big, strong Laconian hound that was bred to hunt deer, and the woolly Salish grown for its long hair that was harvested to make clothing.
What happened to these dog breeds was a confluence of events that spelled doom for any breed that wasn't currently stylish. While this signals a great loss of genetic diversity, there are some who have considered trying to bring lost breeds back. Read about those forgotten dog breeds and why they disappeared at BBC Future. -via Nag on the Lake