Problems in the rate of evolution in biological systems

Abstract
The earliest fossils date from 1.5 Ga ago. They are tentatively identified as prokaryotic blue-green algae and seem to represent organisms that are already metabolically complicated. We have little idea of the timescale associated with the stages of evolution that preceded them. Presumably they were antedated by bacteria, for which there are no fossil remains. The first bacteria must have been fermentative. Only after the establishment of oxygen-evolving photosynthesis in the blue-green algae could aerobic (respiring) bacteria evolve. The simplest eukaryotes presumably arose by uptake of certain aerobic bacteria into primitive nucleated cells, themselves evolved from a prokaryote, now lost. This line subsequently evolved into the multicellular animal line for which the more recent fossil record is good and enables us to identify eras in which there were successively predominant a series of invertebrates (e.g. corals, insects and molluscs) and vertebrates (e.g. fishes, amphibians, reptiles, birds and mammals). Evolution of the plant kingdom required the uptake by primitive eukaryotes of photosynthetic symbionts. The successive uptake of various photosynthetic prokaryotic and eukaryotic algae into different eukaryotic hosts provided the basis for the multiphyletic origin of the present day plant kingdom. The land plants, derived from the ‘green’ line, first appeared 350 Ma ago; the fossil record indicates a steady progression represented successively by the age of the lycopods and seed ferns, of gymnosperms and of angiosperms. From this record one can recognize the enormous conservatism of certain genera (e.g. Nautilus ; sharks, blue-green algae) an obvious onwards progression with others (e.g. Eohippus to the modern horse) and abrupt changes with many others. It is this apparent inequality in rates of evolution of individual species that makes the temporal evaluation of the evolutionary record so difficult.

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