Ancient biochemistries and the evolution of parasites.

The characteristic respiratory metabolism of parasites consists of fermentation to carbon-rich, highly reduced volatile fatty acids which are excreted, and electron transport systems emphasising fumarate reductase and b-type cytochromes. The taxonomic groups that contribute major parasites (the heterogeneous protozoa and the helminths) have their evolutionary origins in environments from which oxygen was absent or present in very low concentrations. The Ediacarian period, about 700 million years ago, contains fossils of the appropriate grade of organisation to be contemporaneous with the ancestors of platyhelminths, nematodes and acanthocephalans. With the oxygen transition, carbon flow in the biosphere resulted in conservative, anoxic environments together with oxygen rich ones. The organisms of the former retained their emphasis on anaerobic energy generation, while cytochrome systems were as much concerned with oxygen detoxification as energy generation. Metabolic pathways in the modern parasitic groups are echoes of such ancient biochemistries.