W Ford Doolittle

Date:
[1980]
Reference:
PP/CRI/H/6/13/5
Catalogue details

Licence: In copyright

Credit: W Ford Doolittle. Source: Wellcome Collection.

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    very beginning, we must conclude that life arose precipitously; a long period of biological (cellular) evolution was preceded by only a very much shorter period of prebiotic evolution. Uving cells diverged very ear ly into th ree lineage s As I remember it, the view of cellular evolution prevalent in the 1960's was that shown in Fig. 1. Life was monophyietic, all living organisms being descendant from an ancient anaerobic heterotrophic prokaryote not unlike a modern Qostndjum. The first eukaryotic cell was probably a unicellular alga, derived from a prokaryotic blue-green alga. From this eukaryotic Uralga arose higher plants and, by loss of photosynthesis, non- photosynthetic protists, fungi and animals. The overwhelming homology of photosynthetic processes in blue-greens and plants and the basic similarities of cellular and genetic organization in all eukaryotes, whether or not they are capable of photosynthesis, seemed to demand such a specific scenario. This scenario no longer appears tenable, not only because evidence for the composite nature (symbiotic origin) of eukaryotic cells is now compelling (see below), but because there appear to be in the contemporary biological world two evolutionary discontinuities more profound than that separating prokaryotic and eukaryotic cellular organisms (Fig. 2). In particular, it seems that living things comprise three distinct evolutionary lineages which diverged at a (presumably very early) stage when genetic organization and the mechanisms and machinery of gene expression were still undergoing rapid ■ Darwinian evolution, in the direction of increased efficiency and accuracy. The profound differences between these lineages are best interpreted as independently achieved solutions to the problems of how genes should best be organized and expressed.
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