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The dog is the ultimate human creation. With 400 breeds and counting, the dog is more varied in size and behavior than any other species on the planet. National Geographic looks at man's evolutionary manipulation of dogs' appearance, talents and temperament - and the accelerating efforts to create breeds to suit our needs (more than 80 percent of today's breeds did not exist 150 years ago). From desirable traits like supersensitive hearing and smell to the unintended consequences of genetic diseases, learn how nature and man have partnered to deliberately create new and specialized versions of man's best friend.