chapter  8
36 Pages

◾ Fossil Records for Early Life on Earth

ByDAVID WACEY

Body fossils are morphological remains of an organism preserved within a rock. Body fossils can be of varied form, for example, bones or shells. But in the early Archean, organisms with hard body parts did not exist, nor did complex cells with nuclei, so we may only expect to nd very simple and very small (a few micrometers [μm] to a few tens of μm) microbial cells. Such small and delicate structures are prone to decay so one might expect the potential for them to be preserved for over billions of years in the rock record to be very low. is is the case in general, but in some circumstances, even delicate cells can be remarkably preserved (Figure 8.1). Remarkable preservation usually occurs where organisms are rapidly encased in a mineral or rapidly buried to prevent oxidative degradation. e lack of oxygen in the early Archean may fortuitously have facilitated a relatively greater delity of microbial preservation compared to the younger rock record. e quality of preservation also depends on which mineral rst comes in contact with the organism, with silica (SiO2), pyrite (FeS2), calcium phosphate (CaPO4), and some clay minerals being particularly good preservation media for microfossils. e subsequent geological history of the rock is also important in the search for early life, with good preservation favored by rapid occlusion of sediment pore space, followed by minimal postdepositional deformation and metamorphism that would likely destroy delicate cellular body fossils. Certain parts of cells are also more likely to survive in the rock record than others. For example, RNA and DNA molecules, plus other cell contents, are unstable and will be rapidly degraded. e cell membrane is more stable, being made of phospholipids and fatty acids, but is still rather weak so has a rather low chance of preservation. e cell wall, made of stronger peptidoglycan polymer, probably has the best chance of preservation. Extracellular polymeric mucus-like substances that o en form envelopes around cells also have a reasonable chance of being preserved, since they contain functional groups (e.g., carboxyl) on which mineral ions can rapidly nucleate.