Fossil lipids for life-detection: A case study from the early earth record

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.