Human skeleton: structure, development, functions, and variation

Anatomical divisions

• Axial skeleton: the central axis formed by the vertebral column and associated structures including the rib cage and skull. It protects vital organs such as the brain, heart and lungs and provides attachment sites for muscles of posture and breathing.
• Appendicular skeleton: the bones that form the limbs and their attachments: the shoulder girdle, the pelvic girdle and the bones of the upper and lower limbs. These bones translate muscular force into movement and manipulate the environment.

Principal functions

• Support: bones form a structural scaffold that maintains body shape and posture.
• Protection: bony cavities shield delicate organs (for example, the skull and rib cage).
• Movement: joints and lever-like long bones enable locomotion and manipulation.
• Hematopoiesis: bone marrow produces red and white blood cells and platelets.
• Mineral storage: bones store calcium, phosphate and other ions released to maintain metabolic balance.
• Endocrine roles: bone-derived hormones influence energy metabolism and mineral homeostasis.

Bone structure ranges from compact cortical bone at the outer surfaces to porous trabecular bone inside; marrow fills cavities and supports blood cell production. During childhood and adolescence growth plates (epiphyseal plates) lengthen long bones; these plates close after puberty. Rates of bone formation and resorption shift with age, nutrition and activity, which is why peak bone mass is usually reached in early adulthood and why older people are more prone to fragility fractures.

Sexual differences in the human skeleton are present but generally subtle compared with some other primates. Population and individual variation is substantial: the male and female skeletons are not so different as in many species, though there are recognisable patterns of variation. Comparative studies in other primates and forensic approaches examine sex differences in skulls, teeth, long bones and the morphology of the pelvis. The shape of the human pelvis differs between typical female and male patterns to accommodate childbirth: females often show wider pelvic dimensions to create a birth canal suitable for a relatively large neonatal head.

Humans lack a penile bone (baculum) that occurs in many other mammals; the evolutionary reasons are debated but some hypotheses link its absence to reproductive and social factors and to adaptations for upright posture—see discussions of adaptation and the upright stance. In clinical practice, disorders such as osteoporosis, osteoarthritis, fractures and congenital anomalies are common concerns. The skeleton is also central to forensic science and archaeology: bone morphology, wear, pathology and chemical signatures provide information about age, sex, diet, activity and health in past and present populations.

Understanding the human skeleton integrates anatomy, physiology, evolution and medicine. Contemporary research continues to refine knowledge about bone biology, the genetic and environmental determinants of skeletal traits, and strategies to prevent and treat skeletal disease.