My dog, Teddy, is depressed. A beagle-pug mix who usually bounds onto beds and couches with the power and grace of Baryshnikov, he pulled a muscle or strained a tendon while springing into my car the other day, and is hobbling. He's 10, the age at which dogs' bodies start to betray them. Am I anthropomorphizing in reading a perplexed sadness in Ted's soulful brown eyes, or in detecting depression in his hangdog demeanor and sighing exhalations? Of course not. Scientists used to view animals as simple bundles of instincts and learned behaviors, but in recent years, those who closely study species such as apes, crows, dolphins, and even octopuses have become quite certain that our evolutionary brethren think and feel, solve problems creatively, and have inner lives of some mysterious kind.

In a new book, Are We Smart Enough to Know How Smart Animals Are?, primatologist Frans de Waal argues that it's egocentric for Homo sapiens to presume we sit at the pinnacle of evolution, separate and apart from all other creatures. Evolution, he says, is a continuum. The human mind did not spring into being overnight when we began farming or writing or putting on pants. "How could our species arrive at planning, empathy, consciousness, and so on," de Waal asks, "if we are part of a natural world devoid of any and all stepping-stones to such capacities?" Elephants and sea lions mourn their dead. Dogs and dolphins communicate their feelings, and empathize with their own kind and with humans. Chimps outperform people in instantly memorizing a series of numbers. And my dog can experience the same kind of sadness I did when all the basketball mileage on my knees added up, and robbed me of the ability to sprint and leap with abandon and joy. Ted, my good buddy, I know just how you feel.