Within the order Primates, it is clear that the degree of both biological and psychological continuity is a function of genetic relatedness. Although biological continuity between animals and humans has been long recognized, psychological continuity has not-primarily because of the long-standing beliefs that animals cannot think or even feel and that human language results in a psychology that is quite different from that of animals. But now, at the dawn of a new millennium, we have solid evidence for significant psychological continuity between apes and humans. Apes are capable of complex learning, symbolic thought, speech comprehension, basic dimensions of language, basic numeric skills, and planning. Rhesus monkeys (Macaca) also have advanced skills and competencies, revealed by research with computers that requires of them facile use of a joystick that controls a cursor on a monitor. Although apparently not as adroit in symbolic processes, and probably with far less capacity to acquire language than the great apes, they nonetheless are impressive. They can predict events, estimate confidence in making choices, and learn equivalencies and ordinal relationships between arrays of items and Arabic numerals. A comprehensive study with 12 primate species' (total N = 121) ability to transfer learning in relation to brain evolution yielded important insights that pertain to the evolution of humans. A slight increment in the amount initially learned resulted in a remarkable decrease in transfer of learning proficiency in the prosimians and in some small-bodied monkeys. By striking contrast, the larger monkeys and the great apes' transfer of learning increased. This shift, which is qualitative in its effect (i.e., from negative to positive transfer), is believed to reflect a shift from stimulus-response associative learning in the more primitive and small-bodied monkeys to a more rational/ relational learning process, particularly in the great (but not lesser) apes. It is proposed that intelligence (e.g., facile learning and transfer of learning to an advantage rather than a disadvantage) increased fortuitously in its association with both brain size and body size. But selection for intelligence based on body size, because of high metabolic demand, was surely constrained to the great apes. Selection of intelligence based upon brain enlargement apart from body size perhaps was contingent upon some early hominid becoming bipedal. An erect posture would have served to enhance use of the hands and facilitated manual interaction with objects of the environment. Inventions thus became more probable. Here we will emphasize both the evolution and maturation of the primate brain as an information-sensitive structure that is particularly responsive to early rearing. The role of technology in research also will be emphasized.