The fossilization potential of organic molecules is extremely variable. Some organic compounds which are very common in living organisms are extremely rare in the fossil record, and are only preserved under exceptional conditions. For example DNA, despite its abundance in living organisms, is apparently only rarely preserved in recognizable form as fossils, primarily because it is concentrated in vulnerable soft-tissues, and the molecule itself is highly reactive and therefore easily broken down. In contrast the recognizable remains of molecules such as proteins, lipids and certain pigments are extremely abundant in the fossil record, to the extent that a group of fossilized molecules derived from bacterial cell walls is present in quantities which exceed that of the total organic carbon in the present day biosphere. Such preservation reflects the inherent relative stability of the molecular structure, or at least the relative stability of the building blocks of which it is constructed. Even simple biological molecules tend to be constructed from smaller molecular subunits, and their long-term stability, and ultimately their fossilization potential, depends both on the strength of the bonding between these smaller modular building blocks as well as on the durability of the building blocks themselves if they become dissociated during or after fossilization. As would be expected from their geological record, the constituent nucleic acids of DNA are much less stable than the amino acids which make up proteins.