Fossil Lipids for Life-Detection: A Case Study from the Early Earth Record

Abstract

The geological preservation of lipids from the cell membranes of organisms bestows a precious record of ancient life, especially for the Precambrian eon (>542 million years ago) when Earth life was largely microbial. All organisms produce lipids that, if the lipids survive oxidative degradation, become molecular fossils entrained with information on biological diversity, environmental conditions, and post-depositional alteration history. As with most biosignatures, the molecular fossil record that is indigenous (of the same place) and syngenetic (of the same age) to host rocks can be compromised by the introduction from and reaction with foreign or younger materials (e.g., petroleum or endolithic life). Deciphering the resulting complex pool of organic signals requires tests for the provenance of molecular fossils and the overall quality of the geobiological record itself. This paper reviews the basis for the very existence of a molecular fossil record from lipid biochemistry to mechanisms of organic-matter preservation and geochemical alteration. A systematic approach to resolving the provenance of molecular fossils and historical qualities of the record is presented in a case study of an early Earth record. This example demonstrates the value of geological context and the integration of independent geobiological parameters, which are critical to the detection and understanding of the ecological processes responsible for records of life.

The Sample Analysis at Mars Investigation and Instrument Suite

“…On Earth, organisms leave distinctive chemical records in sediments that reflect biological utilization of highly restricted sets of compounds (e.g. amino acids), biosynthesis of ordered patterns in the length and structure of compounds that are, for example, incorporated in microbial cell membranes (Summons et al 2008) and molecular and isotopic distributions in rock facies that record ecological variance (Eigenbrode 2008). Organic compounds produced by radiation processes in space and then possibly transformed by aqueous processes in the parent body of a carbonaceous meteorite, such as Murchison, demonstrate much different characteristics.…”

Molecular Biosignatures

“…Lipids are significantly different in that there are only two basic building blocks. Lipids are polymers of either acetyl or isopentenyldiphosphate precursors (Eigenbrode 2007). The final products lack a hydrolysable chemical functionality, such as a peptide or glycosidic linkage, at the point where sub-units join and, unlike other proteins and nucleic acids, lipids cannot be depolymerised.…”

The RND-family transporter, HpnN, is required for hopanoid localization to the outer membrane of Rhodopseudomonas palustris TIE-1

“…1A), with the exception that they can be synthesized under strictly anaerobic conditions whereas steroids cannot. Molecular fossils of steroids and hopanoids (steranes and hopanes, respectively) can be found in ancient sedimentary rocks that formed at least 2.7 billion years ago (1)(2)(3). Whereas steroids are well known to play important roles in eukaryotic membrane composition and curvature as well as intercellular signaling pathways (4-6), we know relatively little about the biological functions of hopanoids.…”