Molecular Palaeontology

In the broadest possible sense, molecular palaeontology might encompass any and all chemical traces left by onceliving organismsorby their life processes.Thiswould cover fields ranging from isotope geochemistry and biomineralization to molecular biology. However, in a more restricted sense, molecular palaeontology is the study of complex organic molecules made by once-living organisms. In some cases, molecules are the only remaining clues to the existence of certain organisms at certain times.When complex organic molecules are found in close association with fossils of the organisms that produced them, theymay provide important information on the organisms’ evolutionary relationships, age and/or mode of life. Four major classes of large, complex biomolecules are usually defined: nucleic acids, proteins, lipids and carbohydrates. Of these, nucleic acids have probably the lowest potential for preservation as fossils.Whenwater is present, nucleic acids usually hydrolyse rapidly (Lindahl, 1993). Most proteins are also poor candidates for preservation, although their survivability depends in part on their structures and on associated minerals. In bones, for instance, collagen disintegrates fairly rapidly, while small compact proteins such as osteocalcin may survive largely intact for much longer periods (Bada, 1991). Certain carbohydrates such as chitin and cellulose may be more resistant, although they have received less attention. Lipids are usually chemically modified after death and fatty acids are likely to degrade, but modified sterol lipids are often preserved well enough that their original chemical form can be deduced. Finally, some important molecular fossils do not fit neatly into any of the four classes. These include porphyrins such as chlorophyll and haem; sporopollenins, extremely resistant polymers that form the walls of pollen and spores; and lignin, a complex polymer of phenolic alcohols found in vascular plant tissues. The most abundant ‘molecular fossil’ of all, however, is kerogen, a name given to insoluble, high-molecular weight organic matter of uncertain composition. History 1950s–1996