The dog or domestic dog is a domesticated descendant of the grey wolf. It has many notable characteristics, the most prominent of which is the upturning tail. The dog derived from an ancient, extinct wolf. Today, the modern grey wolf is the dog's nearest living relative. Experts assert that the dog was the first species to be domesticated. It is believed that domestication took place over 15,000 years ago and was performed by hunter–gatherers before the development of agriculture. Due to their long association with humans, dogs have expanded to a large number of domestic individuals and gained the ability to thrive on a starch-rich diet that would be inadequate for other canids.Over the millennia, dogs became uniquely adapted to human behavior, and the human-canine bond has been a topic of frequent study. The dog has been selectively bred over millennia for various behaviors, sensory capabilities, and physical attributes. Dog breeds vary widely in shape, size, and color.