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Credit: W Ford Doolittle. Source: Wellcome Collection.
2/23
![It is difficult to remember exactly how one thought about cellular evolution ten or fifteen years ago arid certainly risky to assume that everyone thought about it in that same way. However, it is probably fair- to assume that most biochemists and molecular biologists then accepted the following three notions, (i) The appearance of the first living cell was the cumulative result of very many random events of extremely low probability, and thus the period of biological (cellular) evolution was preceded by a very much longer period of prebiotic evolution involving chance associations of non-biologically synthesized macromolecules accumulating in the Oparin [1] ocean. (i i) The first living cells were what we would now call prokaryotes, and these and their modern prokaryotic descendants, for all their biochemical diversity, represent a single mono- phyletic assemblage of entities which are essentially similar at the most fundamental levels of cellular organization, genetic organization and expression, (iii) Even though the line of demarcation between eukaryotic and prokaryotic cellular organisms is the largest and most profound single evolutionary discontinuity in the contemporary biological world [2], eukaryotes arose rather recently from among the prokaryotes (Fig. 1). The remarkable differences in genetic organization, mechanisms of gene expression and evolutionary versatility which tempt us to consider eukaryotes more advanced than prokaryotes were to be seen as in part the cause and in part the consequence of the transition between prokaryotic and eukaryotic levels of cellular organization. Each of these three earlier views can now be seriously challenged. Although none of the points I would like to make below is uniquely mine, none can be taken as proven, and some remain highly controversial, they](https://iiif.wellcomecollection.org/image/b18175740_PP_CRI_H_6_13_5_0002.jp2/full/800%2C/0/default.jpg)
