14.22 - Human Physiology in Relation to Isotopic Studies of Ancient and Modern Humans

Human tissues including bone are constructed of molecules, some ingested heterotrophically, and some synthesized endogenously, under the control of the genome. The internal chemical environment (T, pH, solute activities, etc.) is maintained at a constant level (homeostasis) through compensatory processes, partly controlled by the endocrine system. Tissues that survive for subsequent geochemical analysis (bone, hair, skin) are largely composed of structural proteins (collagen, keratin). Bone is a composite material composed of collagen and a mineral (hydroxyapatite-like) which crystallizes from a supersaturated solution under the control of inhibitory molecules, mainly pyrophosphate. The structure of bone is hierarchical; μm-scale osteons are composed of nm-scale collagen fibrils and mineral crystals, all of which are continuously broken down by osteoclasts and regenerated by osteoblasts. Human diet provides energy mainly in the form of carbohydrates and lipids; energy is stored as endogenously produced lipids. Ingested proteins are hydrolyzed to amino acids, which may be used directly for protein synthesis (especially essential amino acids which cannot be biosynthesized) or may be used as fuel through gluconeogensis.