Human, common name given to any individual of the species Homo sapiens and, by extension, to the entire species. The term is also applied to certain species that were the evolutionary forerunners of Homo sapiens. Scientists consider all living people members of a single species.

Homo sapiens is identified, for purposes of classification, as an animal (kingdom Animalia) with a backbone (phylum Chordata) and segmented spinal cord (subphylum Vertebrata) that suckles its young (class Mammalia); that gestates its young with the aid of a placenta (subclass Eutheria); that is equipped with five-digited extremities, a collarbone, and a single pair of mammary glands on the chest (order Primates); and that has eyes at the front of the head, stereoscopic vision, and a proportionately large brain (suborder Anthropoidea). The species belongs to the family Hominidae, the general characteristics of which are discussed below.

Human Spine Although individual vertebrae move little from one to the next, the human spinal column as a whole is a chain flexible enough to allow us to touch our toes. Its unique S-shape centers the weight of our long bodies over our feet, keeping us from toppling. Animals that walk on all four legs have straighter spines that provide even support for their horizontal bodies.
 

The details of skeletal structure distinguishing Homo sapiens from the nearest primate relatives—the gorilla, chimpanzee, and orangutan—stem largely from a very early adaptation to a completely erect posture and a two-footed striding walk (bipedalism). The uniquely S-shaped spinal column places the center of gravity of the human body directly over the area of support provided by the feet, thus giving stability and balance in the upright position. Other mechanical modifications for bipedalism include a broad pelvis, a locking knee joint, an elongated heel bone, and a lengthened and aligned big toe. Although varying degrees of bipedalism are seen in other anthropoids, all have straight or bowed spines, bent knees, and grasping (prehensile) feet, and all use the hands to bear part of the body weight when moving about.

Complete bipedalism in the human freed the hand to become a supremely sensitive instrument for precise manipulation and grasping. The most important structural detail in this refinement is the elongated human thumb, which can rotate freely and is fully opposable to the other fingers. The physiological requirements for speech were secondarily established by erect posture, which positions the vocal cords for controlled breathing, and by the skilled use of the hands. The latter development occurs in association with the enlargement and specialization of a brain area (Broca's convolution) that is a prerequisite for refined control of the lips and tongue.

The large (averaging 1400 cc/85.4 cu in) brain of Homo sapiens is approximately double that of early human toolmakers. This great increase in size in only 2 million years was achieved by a process called neoteny, which is the prolongation of retention of immature characteristics. The juvenile stage of brain and skull development is prolonged so that they grow for a longer period of time in relation to the time required to reach sexual maturity. Unlike the early human adult skull, with its sloping forehead and prominent jaw, the modern human skull—with biologically insignificant variations—retains into maturity a proportionately large size, in relation to the rest of the body, a high-rounded dome, straight-planed face, and reduced jaw size, all closely resembling the characteristics of the skull in the juvenile chimpanzee. Its enlarged dimensions required adaptations for passage through the birth canal; consequently, the human female pelvis widens at maturity (with some sacrifice in swiftness of locomotion), and the human infant is born prematurely. Chimpanzees are born with 65 percent of their adult brain capacity; Australopithecine, an erect, tool-using near-human of 3 million years ago, was born with about 50 percent; modern human newborns have only 25 percent of adult brain capacity, resulting in an extended period of helplessness. The many neurological pathways to the rapidly growing brain must be organized and coordinated during a prolonged period of dependency on and stimulation by adults; lacking this close external bond in the early years of life, development of the modern brain remains incomplete.

Human Brain

The human brain has three major structural components: the large dome-shaped cerebrum (top), the smaller somewhat spherical cerebellum (lower right), and the brainstem (center). Prominent in the brainstem are the medulla oblongata (the egg-shaped enlargement at center) and the thalamus (between the medulla and the cerebrum). The cerebrum is responsible for intelligence and reasoning. The cerebellum helps to maintain balance and posture. The medulla is involved in maintaining involuntary functions such as respiration, and the thalamus acts as a relay center for electrical impulses traveling to and from the cerebral cortex.
 

The physiological adaptations that made humans more flexible than other primates allowed for the development of a wide range of abilities and an unparalleled versatility in behavior. The brain's great size, complexity, and slow maturation, with neural connections being added through at least the first 12 years of life, meant that learned behavior could largely modify stereotyped, instinctive responses. New environmental demands could be met by rapid adjustments rather than by slow genetic selection; thus, survival in a wide range of habitats and under extreme conditions eventually became possible without further species differentiation. Each new infant, however, with relatively few innate traits yet with a vast number of potential behaviors, must be taught to achieve its biological potential as a human.

Flint Tools

Humans have been toolmakers for at least 2.5 million years. The earliest technology was a tool kit of haphazardly shaped chopping, cutting, and scraping implements fashioned from pebbles. From the later stone ages, archaeologists have identified some 60 or 70 standard kinds of intricate tools with very specific purposes. While the ax-head, arrowhead, scrapers, borers, and flakes in this picture were all made of stone, materials such as bone and ivory were also used. Tools like these can be made by direct percussion (using a hammerstone or other implement to knock flakes from the raw material) or indirect percussion (using the hammerstone to strike a chisel-like tool that is precisely positioned on the raw material).
 

The human species has a unique capability for culture in the sense of conscious thinking and planning, transmission of skills and systems of social relationships, and creative modification of the environment. The integrated patterns of behavior required for planning and fashioning tools were accomplished at least 2.5 million years ago, and some form of advanced code for vocal communication may also have existed at this time. By 350,000 years ago planned hunting, firemaking, and the wearing of clothing were well established, as was possibly ritualized disposal of the dead. Evidence of religion, recorded events, and art date from 30,000 to 40,000 years ago and imply advanced language and ethics for the complex ordering of social groups required for such activities. From about that time the genus Homo began to stabilize into the one generalized species of Homo sapiens.

The preceding description rests on anatomical observation and current scientific theory on the origin of the Homo species. Humankind itself and the essence of being human are also defined in many other ways—religious, social, moral, and legal.