Social coordination among organisms, in general, is enabled and further facilitated by communication. Moral behavior is no exception. In sketching the possible roots of moral responses, Darwin (1871) noted the role of organisms being able to interpret the needs of others in order to assist them. Darwin underscored the role of language, but he also understood the role of non-linguistic communication. For example, emotions are typically expressed externally, through anatomically distinct postures or facial configurations. They offer important clues about the internal mental states of other organisms. Darwin followed Descent of Man with a whole volume exploring The Expression of Emotions in Humans and Other Animals (1872).

Figures from Darwin's 1872 Expression of the Emotions in Man and Other Animals,
with photograph (bottom center) of Duchenne's experiments stimulating facial muscles with electrodes