• the presence of a cell wall which contains a specific mucopeptide as its strengthening element.

By 1970, Stanier could confidently state that

… advancing knowledge in the domain of cell biology has done nothing to diminish the magnitude of the differences between eukaryotic and prokaryotic cells that could be described some ten years ago: if anything, the differences now seem greater (Stanier, 1970).

But, cautiously endorsing Lynn Margulis' assertion that eukaryotic cells are themselves the result of the fusion of separate (prokaryotic) evolutionary lineages (Margulis, 1970), he went on to note that

… the only major links [between the two cell types] which have emerged from recent work are the many significant parallelisms between the entire prokaryotic cell and two component parts of the eukaryotic cell, its mitochondria and chloroplasts.

This linkage has since been amply supported by molecular sequence data (Gray and Doolittle, 1982), and the endosymbiont hypothesis for the origin of eukaryotic organelles of photosynthesis and respiration has become a basic tenet of the contemporary evolutionary consensus.

Together, the prokaryote/eukaryote dichotomy and the endosymbiont hypothesis for the origin of mitochondria and chloroplasts informed and (no doubt) constrained the biology and molecular biology of the 1960s, 1970s, and early 1980s, providing the framework within which all of the results of biochemists, geneticists, and evolutionists were interpreted (Figure 2). In typical text books from this era, genes in Escherichia coli are compared and contrasted to their counterparts in yeast, mouse, and man, with differences interpreted either in terms of the relatively advanced and complex state of the latter or the admirably streamlined features of the former. The paradigm has been extraordinarily fruitful: without such a grand scheme for organizing our knowledge of cell and molecular biology, we would have become lost in the details. It also seems safe to say that, for the organisms studied by most molecular biologists in those decades, this view of things is substantially correct and invaluable in interpreting the differences in the information-transfer systems of prokaryotes and eukaryotic nuclei, chloroplasts, and mitochondria.

As well, this view was easily consistent with the most straightforward interpretation of the fossil record. As reviewed by Schopf and Knoll elsewhere in this volume, unquestionable prokaryotes, by all available measures indistinguishable from modern cyanobacteria, appeared more than 3.5 billion years ago (Schopf, 1994; Knoll, 1994). Fossils that are undeniably eukaryotic are not seen for another 1 to 1.5 billion years, ample time for the symbioses required by Margulis.

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