Dogs are descendents of wolves that roamed wild over much of the world in ancient times. People have associated with dogs for thousands of years, originally domesticating them as beasts of burden and guard animals. Today, there are still many working breeds, and the service roles of dogs have greatly expanded to include assisting disabled individuals (for example, those with visual or hearing impairments), working with human law enforcement by sniffing out illegal drugs and other contraband, locating survivors in search-and-rescue missions, and even using their incredible sense of smell for early detection of cancer cells in human patients. Of course, the primary role of most dogs is that of loyal companion.

Last full review/revision July 2011 by John A. Bukowski, DVM, MPH, PhD; Susan Aiello