Robert Williams (Oxford University) and João Fraústo da Silva (Technical University of Lisbon) have an unconventional, but plausible take on the conditions for life’s origin and evolution (Williams, R.J.P & Fraústo da Silva J.J.R. 2003. Evolution was Chemically Constrained. Journal of Theoretical Biology, v. 220, p. 323-343). However life began, presumably as cytoplasm containing DNA, RNA and proteins within a semi-permeable wall, it was surrounded by the chemistry of whatever environment it appeared in. The proto-cell would have drawn hydrogen ions from water, to perform the proton pumping that is essential to all living organisms, and thereby created more oxidising conditions in its immediate vicinity. Oxidation would have generated nitrogen from ammonia, released metals from their sulphides and converted other sulphides to sulphates. Conversely, ions in its surroundings would have been able to “leak” into the cell itself. By creating oxidised radicals, this inward leakage would have rebounded the cell’s activity on itself, with potentially toxic consequences. Survival depended on two things: exploiting the opportunities, such as nitrogen fixation, using oxygen and even photosynthetic chemistry; and fending off potential toxic shock. One of the most interesting aspects is the role assumed by calcium ions. Their presence inside a cell would have precipitated DNA, by binding to it, with fatal consequences. The upshot, according to Williams and Fraústo da Silva, is the special role of calcium as a messenger ion, perhaps having arisen through the necessity to pump it out again. Today, the range of calcium concentrations in cells is extremely limited; too much or too little being fatal. Perhaps a sudden change in the calcium-ion concentration in seawater in the late Neoproterozoic was responsible for the extreme excursions in carbon isotopes that are ascribed to mass extinction and equally massive adaptive radiations. My own stab in the dark, is that a protective response to calcium stress by metazoans at that time may explain the sudden appearance of calcium-rich hard parts, which we know as the Cambrian Explosion. They evolved means of excreting calcium from their many cells, so creating an outer “shell” that eventually developed into “armour” or “armament”.
The delightful aspect of Williams and Fraústo da Silva’s ideas is that they break from pure genetic determinism and the dominance of pure chance in addressing the central issue in the whole of science – the complete interconnectedness of real nature.