Without doubt, the mass extinction at the Permian-Triassic (P-T) boundary was the most important biological event in the history of Phanerozoic evolution. Around 80-90% of families disappeared, and perhaps more than 50% of species diversity. But, the evidence stems largely from marine records. Marine organisms went down at a hasty rate, as evidenced by the superb boundary sequence in China. However, such is the inconsistency of preservation on land that matching evidence is sparse from the terrestrial realm. The best chance of examining the response of land animals to whatever wrought such havoc at sea lies in the Karoo sediments of southern Africa. Roger Smith of the South African Museum and Peter Ward of the University of Washington have combed the mainly fluvial sediments for evidence (Smith, R.M.H. and Ward, P.D. 2001. Pattern of vertebrate extinctions across an event bed at the Permian-Triassic boundary in the Karoo Basin of South Africa. Geology, v. 29, p. 1147-1150). Rather than supporting the general view that terrestrial P-T extinctions took a few million years, they have been able to show that Permian vertebrates disappeared abruptly, to be replaced by a very different fauna equally suddenly in the lowermost Triassic. Only one genus (Lystrosaurus) spans the boundary, and the boundary itself contains no evidence of life. Calculations based on estimates of the rate of sedimentation point to around 50 thousand years for the extinction event, about the same as that affecting marine organisms. Interestingly, the event sharply separates very different sediments, that Smith and Ward interpret as products of perennially wet Permian flood plains and those experiencing ephemeral flow in the Triassic (see End-Permian devastation of land plants in Earth Pages October 2000). Whatever its cause, the stresses placed on land vertebrates seem to have included the sudden onset of aridity.
Mesozoic fossil hunting in Madagascar
Most papers on palaeontology report the details of years of research on what the fossil hunters have found, with mentioning the months of patient searching. John Flynn and André Wyss have provided an insight into the tribulations of palaeontological field work in difficult terrains, as well as a broad account of the context of their finds (Flynn, J.S. and Wyss, A.R. 2002. Madagascar’s Mesozoic secrets. Scientific American, v. 286, February 2002, p. 42-51). Madagascar lingered at the heart of the Gondwana supercontinent until it finally began to split into drifting segments during the early-Triassic. It lay on the eastern flank of an evolving rift basin that filled with mainly terrestrial sediments until the late-Jurassic. This particular basin remained uninterrupted by volcanism or erosion, and so is a repository for organic remains trapped in a continuous sedimentary sequence. This period in geological history, particularly the Triassic, spans the emergence and development of both the dinosaurs and primitive mammals. The wealth of vertebrate fossils that geologists are beginning to unearth suggests that Madagascar may well become the site where the mysterious origins of both are resolved.
The simplest living ecosystem
Hugely complex as life is, at the cell level it has a profound simplicity, at least as regards its fundamental chemistry. Cell metabolism receives its power from the transfer of electrons from a high to a low energy level. High-energy electrons stem from chemically active molecules, atoms or ions able to release them; electron donors or reducing agents. The metabolic path ends in oxidizing agents accepting these electrons. This process of donating and accepting electrons takes the form, in most cell types, of “pumping” hydrogen ions, or protons back and forth across the cell wall to create an electrochemical gradient that is continually charged and discharged. Biochemistry reflects this by the ADP-ATP cycle at life’s core, in many different versions.
The simplest provision of electrons is by hydrogen, and arguably a supply of hydrogen gas is a highly likely precondition for the origin of life. Surprisingly, hydrogen is generated by many geological reactions, although little survives some form of oxidation for long. In a few places hydrogen gas escapes abundantly, as in the weathering of ultramafic rocks by groundwater. The essential process is the breakdown of iron and magnesium silicates to various kinds of clay, by the interaction of hot water with fresh igneous rocks. Geochemists and microbiologists from the USA analysed such a hydrothermal system 200 m beneath a volcanic area in Idaho, and found a thriving and diverse ecosystem dominated by simple organisms that do depend on hydrogen (Chapelle, F.H. et al. 2002. A hydrogen-based subsurface microbial community dominated by methanogens. Nature, v. 415, p. 312-315). More than 90% of the organisms are methane-producing Archaea, which reduce carbon dioxide to methane, using hydrogen. No other hot-spring system comes close to this probably highly primitive community. It is a handy analogue for the kind of ecology that may have developed if life has arisen deep beneath the icy surface of Europa – a target for future NASA missions.
Incidentally, the exploitation of electron and proton transfer that underpins cell metabolism potentially forms a source of electrical power. Younger readers may have experimented with using fruit as the basis of a simple galvanic battery, thereby exploiting low pH conditions. Investigation of the potential of bacteria for direct electricity generation recently made a breakthrough (Bond, D.R. et al. 2002. Electrode-reducing microorganisms that harvest energy from marine sediments. Science, v. 295, p. 483-485). A member of the family Geobacteraceae (Desulfuramonas acetoxidans) has been found to readily produce excess electrons as it metabolises organic material in oxygen-free muds. Daniel Bond and co-workers from the University of Massachusetts and the US Naval Research Laboratory introduced graphite electrodes into airless fish-tank muds and the upper oxygenated sediments. Even with such a crude experiment, sufficient current flowed to power a small calculator. Moreover, D. acetoxidans colonised the electrodes within a matter of days, showing that they were directly involved in the oxidation-reduction system at the root of such a fuel cell. As well as raising the possibility of powering submarine monitoring devices using bioelectricity, such geobacteria are able to metabolise a range of common organic pollutants. Marine organic sediments are virtually limitless, so it is not inconceivable that the process may result in yet another renewable power source, albeit difficult to convert to high-power supplies, with the blessing of pollution control as a sideline.