History of Life from the Hydrocarbon Fossil Record
Certain lipids and biopolymers retain their original structure through sedimentary diagenesis and catagenesis that they can be assigned to a specific biological origin. These “taxon-specific biomarkers” (TSBs) can serve as chemical fossils that trace the
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K. N. Timmis (ed.), Handbook of Hydrocarbon and Lipid Microbiology, DOI 10.1007/978-3-540-77587-4_11, # Springer-Verlag Berlin Heidelberg, 2010
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History of Life from the Hydrocarbon Fossil Record
Abstract: Certain lipids and biopolymers retain their original structure through sedimentary diagenesis and catagenesis that they can be assigned to a specific biological origin. These ‘‘taxon-specific biomarkers’’ (TSBs) can serve as chemical fossils that trace the evolution of life. TSBs in Early Precambrian rocks reveal the early evolution of archaea, cyanobacteria, and eukarya and the development of atmospheric free oxygen. Steroidal TSBs document the changing nature of marine phytoplankton from Neoproterozoic organic-walled acritarchs to the predominance of present day diatoms and terpanoid TSRs reveal the evolution of higher land plants. TSBs, used in conjunction with isotopic analysis, can identify the taxa of enigmatic fossils, provide important clues to the causes of mass extinctions, and describe the global changes in biotic diversity and Earth’s conditions as the biosphere recovers. Biomarkers record the evolutionary history of life on Earth and perhaps, other planets.
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Introduction
Most of life’s chemical components are unstable when deposited in sediments. DNA degrades very rapidly and is restricted to fossils younger than 50 thousand years old preserved in permafrost. Proteins are more stable, but typically lose their diagnostic sequences within 102–105 years. Collagen proteins are stabilized in fossil bones (Collins et al., 2000) and fragments have been successfully sequenced from a 160,000- to 600,000-year-old fossilized mastodon and possibly an exceptionally well preserved, 68 million-year old T. Rex femur (Asara et al., 2007). Many lipids are stable and can survive in sediments for a much longer time (Volkman, 2006). For example, alkenones, which are produced by Prymnesiophyceae phytoplankton, are highly resistant and have been found in sediments as old as 120 Million years (Brassell et al., 2004). Most lipids do not remain intact during burial and lithification of sediments. When exposed to higher temperatures, these compounds undergo chemical and microbial alteration involving the loss of functional groups and hydrogenation, cleavage, cross-linking, condensation, and aromatization reactions. However, certain lipids and biopolymers retain enough of their original molecular structure to assign a specific biologic origin (Peters et al., 2005). These hydrocarbons, termed biomarkers by the geochemical community, can endure exposure to higher temperatures and survive for billions of years. Some ‘‘taxonspecific biomarkers’’ (TSB; Moldowan and Jacobson, 2000) have structures that are specific to a taxonomic group and can serve as chemical fossils that trace the evolution of life. A comprehensive review of biomarkers and their evolution is beyond the scope of this paper and the reader is directed to several recent publications that more broadly cover these topics (Brocks and Pearson, 2005; Brock
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