Humans (Homo sapiens) are the only extant members of the subtribe Hominina. Together with chimpanzees, gorillas, and orangutans, they are part of the family Hominidae (the great apes, or hominids). A terrestrial animal, humans are characterized by their erect posture and bipedal locomotion; high manual dexterity and heavy tool use compared to other animals; open-ended and complex language use compared to other animal communications; larger, more complex brains than other animals; and highly advanced and organized societies.
Human evolution is characterized by a number of morphological, developmental, physiological, and behavioral changes that have taken place since the split between the last common ancestor of humans and chimpanzees. The most significant of these adaptations are 1. bipedalism, 2. increased brain size, 3. lengthened ontogeny (gestation and infancy), 4. decreased sexual dimorphism (neoteny). The relationship between all these changes is the subject of ongoing debate. Other significant morphological changes included the evolution of a power and precision grip, a change first occurring in H. erectus.
Although humans appear hairless compared to other primates, with notable hair growth occurring chiefly on the top of the head, underarms and pubic area, the average human has more hair follicles on his or her body than the average chimpanzee. The main distinction is that human hairs are shorter, finer, and less heavily pigmented than the average chimpanzee's, thus making them harder to see. Humans have about 2 million sweat glands spread over their entire bodies, many more than chimpanzees, whose sweat glands are scarce and are mainly located on the palm of the hand and on the soles of the feet. Humans have the largest number of eccrine sweat glands among species.
The sexual division of humans into male, female, and in some societies other genders has been marked culturally by a corresponding division of roles, norms, practices, dress, behavior, rights, duties, privileges, status, and power. Cultural differences by gender have often been believed to have arisen naturally out of a division of reproductive labor; the biological fact that women give birth led to their further cultural responsibility for nurturing and caring for children. Gender roles have varied historically, and challenges to predominant gender norms have recurred in many societies.
Body modification is the deliberate altering of the human body for any non-medical reason, such as aesthetics, sexual enhancement, a rite of passage, religious reasons, to display group membership or affiliation, to create body art, shock value, or self-expression. In its most broad definition it includes plastic surgery, socially acceptable decoration (e.g. common ear piercing in many societies), and religious rites of passage (e.g. circumcision in a number of cultures).