When Darwin looked at the expressions on people's faces—and at the body language of dogs and cats—he saw evidence for evolution. These pictures, some posed and some candid, appeared in his Expression of the Emotions in Man and Animals (1872). What Darwin himself observed, and what he learned from scientists studying cultures worldwide, convinced him that all humans have the same feelings. What's more, they show them on their faces in similar ways.

Similarly, Darwin thought, animals had recognizable emotions. Chimpanzees could feel disappointment. When they were disappointed, they sulked; and when they sulked, they stuck out their lips like pouting children. Experts of the time believed, wrongly, that we humans had special muscles in our faces so we could express what they called our "exquisite feelings." Darwin's work contradicts this. Our emotions, and the way we express them, connect us with the rest of life on Earth.