Bones are a fundamental component of the vertebrate body, providing structure, protection, and enabling movement. Compared to the rapid decay of soft tissues after death, bones exhibit remarkable durability and persistence. This resilience raises questions about how these structures resist pervasive decomposition over long periods. The answer lies in their distinctive composition and mechanisms that counteract organic matter breakdown.
The Unique Makeup of Bone
Bone is a composite material, combining both organic and inorganic components to achieve its strength and resilience. The organic part is primarily type I collagen, a protein constituting about 30% of bone’s mass. Collagen provides flexibility and tensile strength, helping bones resist fracturing under stress. This fibrous protein forms a framework upon which the inorganic components are deposited.
The inorganic component, approximately 60-70% of bone’s dry weight, is largely hydroxyapatite. Hydroxyapatite is a crystalline form of calcium phosphate (Ca₁₀(PO₄)₆(OH)₂), which gives bone its hardness and rigidity. This mineral phase is interwoven with the collagen fibers, creating a strong and synergistic structure that is both tough and resistant to compression.
How Bones Resist Decomposition
Soft tissues like muscles and organs rapidly decompose through autolysis (self-digestion by enzymes) and putrefaction (bacterial breakdown). Microorganisms and enzymes efficiently break down proteins, fats, and carbohydrates, turning tissues into liquids and gases. This leads to significant mass loss and disappearance of most biological material.
Bones resist this rapid microbial and enzymatic breakdown due to their high mineral content. The inorganic hydroxyapatite crystals are not easily consumed or broken down by the bacteria and fungi that thrive on organic matter. While microbes can colonize bone, their ability to degrade the mineral component is limited under most conditions. The collagen, being an organic material, can eventually degrade through the action of collagenase-producing bacteria and other enzymes. When collagen breaks down, the resilient hydroxyapatite mineral remains, preserving the bone’s shape and much of its mass.
Environmental Influences on Bone Preservation
While bone’s mineral composition offers resistance to decay, environmental factors influence its long-term preservation. Soil pH is a factor; acidic conditions dissolve hydroxyapatite, leading to demineralization and deterioration. Conversely, neutral or alkaline soils promote better bone preservation. Moisture levels also play a role; while extremely dry conditions can preserve bones by inhibiting microbial activity, prolonged wetness, especially waterlogging, can lead to mineral leaching and degradation.
Temperature extremes and fluctuations contribute to physical weathering, causing cracking, bleaching, and flaking. Cycles of freezing/thawing or wetting/drying accelerate this physical breakdown. Scavengers and burrowing animals can cause physical damage, scattering and altering skeletal remains and exposing them to further degradation. Certain microorganisms in specific environments can also degrade bone by breaking down organic collagen.