Two conspicuous human anatomical traits are erect posture and large brain. We are the only vertebrate species with a bipedal gait and erect posture; birds are bipedal, but their backbone stands horizontal rather than vertical (penguins are a trivial exception) and the bipedalism of kangaroos lacks erect posture and is drastically different from our own. Erect posture and bipedal gait entail other morphological changes in the backbone, hipbone, and feet and others.
Brain size in mammals is generally proportional to body size. Relative to body mass, humans have the largest brain. The chimpanzee brain has an approximate volume of 300 cm3; a gorilla’s is slightly larger. The human adult brain is more than three times larger, typically between 1,300 cm3 and 1,400 cm3. The brain is not only larger in humans than in apes but also much more complex. The cerebral cortex, where the higher cognitive functions are processed, is in humans proportionally much greater than the rest of the brain when compared with apes.
Erect posture and large brain are not the only anatomical features that distinguish us from nonhuman primates, even if they may be the most obvious. Other notable anatomical differences include the reduction of the size of the jaws and teeth and the remodeling of the face; reduction of body hair and changes in the skin and skin glands; modification of the vocal tract and larynx, with important implications for spoken language; opposing thumbs that allow precise manipulation of objects; and cryptic ovulation, which may have been associated with the evolution of the nuclear family, consisting of one mother and one father with their children.
Humans are notably different from the apes and all other animals in anatomy, but also and no less importantly in their functional capacities and behavior, both as individuals and socially. Most fundamental are the advanced intellectual faculties, which allow humans to categorize (see individual objects as members of general classes), think in the abstract and form images of realities that are not present (and, thus, anticipate future events and planning future actions), and reason. Other distinctive functional features are self-awareness and death awareness; symbolic (creative) language; tool making and technology; complex and extremely variable forms of cooperation and social organization; legal codes and political institutions; science, literature, and art; and ethics and religion (3).
Humans live in groups that are socially organized, and so do other primates. But primate societies do not approach the complexity of human social organization. A distinctive human social trait is culture, which may be understood here as the set of non–strictly biological human activities and creations. Culture in this sense includes social and political institutions, ways of doing things, religious and ethical traditions, language, common sense and scientific knowledge, art and literature, technology, and in general all of the creations of the human mind. Culture “is a pool of technological and social innovations that people accumulate to help them live their lives” (ref. 4, p. 65). The advent of culture has brought with it cultural evolution, a super organic mode of evolution superimposed on the organic mode, which has, in the last few millennia, become the dominant mode of human evolution. Cultural evolution has come about because of cultural change and inheritance, a distinctively human mode of achieving adaptation to the environment and transmitting it through the generations (3, 5–9).