One of the truly remarkable events in human evolution is the unprecedented increase in the size of the brain of "Homo" over a brief span of 2 million years. It would appear that some significant selective pressure or opportunity presented itself to this branch of the hominid line and caused a rapid increase in the brain, introducing a wholly new kind of cognitive orientation toward the external environment. The nature of the causes of this dramatic shift are presently under debate. Arguably, the remarkable increase in the size of the brain of different species of "Homo" arose from the selective advantages of a communication system permitting the exploitation of shared information. To test this evolutionary hypothesis, it is useful to look at patterns of development in humans and in our nearest primate relatives, chimpanzees. Comparison of developmental patterns of human infants and comparable chimps shows that human infants go through a developmental sequence of (1) showing an object to an adult, (2) giving objects to others, (3) pointing to objects in a communicative context, and (4) labeling objects by means of a vocalization. Infant chimps go through the stage of showing, but they do not give, point, or label. Positing the existence of a cooperative strategy in Pliocene hominids is consistent with these developmental differences. (RH)