Descended from the gray wolf (Canis lupus), the domestic dog was the first domesticated animal, with origins 15,000–40,000 years ago. Over 400 recognized breeds exhibit the most extreme morphological diversity of any mammal—from <1 kg (Chihuahua) to >70 kg (Great Dane/English Mastiff). Coats, skull shapes, limb proportions, and behaviors vary enormously. Dogs are facultative carnivores that have evolved starch-digestion genes (AMY2B) through domestication. They have an extraordinary sense of smell (~300 million olfactory receptors vs. ~6 million in humans) and can be trained for detection, herding, guarding, assistance, and companionship. They are highly social and have a unique capacity for reading human emotional cues. Gestation: ~63 days, litters of 1–12+ pups. Global population: ~500 million+. In research, dogs are models for comparative oncology, genetic disease mapping (over 700 heritable disorders, many homologous to human conditions), and behavioral genetics. The genome was sequenced in 2005 (2n=78). Lifespan: 10–13 years (breed-dependent).