HR: 09:00h
AN: B31D-04 INVITED     [Abstracts]
TI: Search for the Evolution of Steroid Biosynthesis in the Geological Record
AU: * Brocks, J J
EM: jbrocks@els.mq.edu.au
AF: Australian Centre for Astrobiology, Macquarie University, E8C, Sydney, NSW 2109 Australia
AB: To study the evolution of the structure of organisms we can directly examine fossilized shells, skeletons and petrified cells. In contrast, for the tentative reconstruction of the phylogeny of biosynthetic pathways, such as steroid anabolism, we rely entirely on the comparative molecular biology of living organisms. Thus, without fossil evidence, the times in geological history when successive steps of a metabolic pathway evolved remain particularly elusive. Molecular clocks of genes coding for the enzymes involved in a biosynthetic pathway might provide a rough guess when a natural product first appeared in geological time, but they are intrinsically unreliable without calibration points in the distant past. However, it might be possible to trace the evolutionary history of some biosynthetic pathways directly in the geological record by searching for hydrocarbon biomarkers of anabolic intermediates. Biomarkers are molecular fossils of natural products. They often retain the diagnostic carbon skeleton of their biological precursor and remain stable over hundreds of millions of years enclosed in organic-rich sedimentary rocks. Sterane hydrocarbons are particularly abundant biomarkers and potentially suitable for the search of biosynthetic intermediates. Steranes are the fossil equivalents of functionalized steroids found in eukaryotes and certain bacteria. The biosynthesis of typical eukaryotic steroids such as cholesterol (C$_{27}$), ergosterol (C$_{28}$) and sitosterol (C$_{29}$) from the acyclic precursor squalene (C$_{30}$) involves more than 20 enzymatic steps. The most crucial steps include modification of the carbon skeleton by removal of several methyl groups from the ring system and addition of alkyl groups to the steroid side chain. The evolution of this complex pathway must have occurred over geologically significant periods of time and likely involved several preadaptive intermediates that represented structurally less derived but fully functional lipids. Thus, if a molecular corollary of `ontogeny recapitulates phylogeny' applies, it might be possible to detect a sequence of increasingly modified fossil steroids in the geological record and to create a time frame for the evolution of this fundamental biosynthetic pathway. Here we present first results of an extensive search for the fossil remains of evolutionary intermediate steroids in sedimentary successions of Precambrian age.
DE: 9619 Precambrian
DE: 1055 Organic geochemistry
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting