The domestic dog (Canis lupus familiaris or Canis familiaris) is a domesticated canine which has been selectively bred over millennia for various behaviours, sensory capabilities, and physical attributes. Although initially thought to have originated as an artificial variant of an extant canid species (variously supposed as being the dhole, golden jackal, or gray wolf), extensive genetic studies undertaken during the 2010s indicate that dogs diverged from an extinct wolf-like canid in Eurasia 40,000 years ago. Their long association with humans has led to dogs being uniquely attuned to human behavior and are able to thrive on a starch-rich diet which would be inadequate for other canid species. Dogs are also the oldest domesticated animal. Dogs vary widely in shape, size and colours. Dogs perform many roles for people, such as hunting, herding, pulling loads, protection, assisting police and military, companionship and, more recently, aiding handicapped individuals. This influence on human society has given them the sobriquet, "man's best friend".