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Part I

EARLY EVOLUTION AND THE ORIGIN OF CELLS

Darwin noticed the sudden appearance of several major animal groups in the oldest known fossiliferous rocks. “If [my] theory be true, it is indisputable that before the lowest Cambrian stratum was deposited . . . the world swarmed with living creatures,” he wrote, noting that he has “no satisfactory answer” to the “question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods” (Darwin, 1859, ch. 10). In “Solution to Darwin's Dilemma: Discovery of the Missing Precambrian Record of Life ” (Chapter 2), J. William Schopf points out that, one century later, one decade after the publication of Stebbins' Variation and Evolution in Plants (Stebbins, 1950), the situation had not changed. The known history of life extended only to the beginning of the Cambrian Period, some 550 million years ago. This state of affairs would soon change, notably due to three papers published in Science in 1965 by E.S. Barghoorn and S.A. Tyler (1965), Preston Cloud (1965), and E.S. Barghoorn and J.W. Schopf (1965). Schopf tells of the predecessors who anticipated or made possible the work reported in the three papers, and of his own and others' contributions to current knowledge, which places the oldest fossils known, in the form of petrified cellular microbes, nearly 3,500 million years ago, seven times older than the Cambrian and reaching into the first quarter of the age of the Earth.

Lynn Margulis, M.F. Dolan, and R. Guerrero set their thesis right in the title of their contribution: “The Chimeric Eukaryote: Origin of the Nucleus from the Karyomastigont in Amitochondriate Protists ” (Chapter 3). The karyomastigont is an organellar system composed at least of a nucleus with protein connectors to one (or more) kinetosome. The ances-



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Variation and Evolution in Plants and Microorganisms: TOWARD A NEW SYNTHESIS 50 YEARS AFTER STEBBINS Part I EARLY EVOLUTION AND THE ORIGIN OF CELLS Darwin noticed the sudden appearance of several major animal groups in the oldest known fossiliferous rocks. “If [my] theory be true, it is indisputable that before the lowest Cambrian stratum was deposited . . . the world swarmed with living creatures,” he wrote, noting that he has “no satisfactory answer” to the “question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods” (Darwin, 1859, ch. 10). In “Solution to Darwin's Dilemma: Discovery of the Missing Precambrian Record of Life ” (Chapter 2), J. William Schopf points out that, one century later, one decade after the publication of Stebbins' Variation and Evolution in Plants (Stebbins, 1950), the situation had not changed. The known history of life extended only to the beginning of the Cambrian Period, some 550 million years ago. This state of affairs would soon change, notably due to three papers published in Science in 1965 by E.S. Barghoorn and S.A. Tyler (1965), Preston Cloud (1965), and E.S. Barghoorn and J.W. Schopf (1965). Schopf tells of the predecessors who anticipated or made possible the work reported in the three papers, and of his own and others' contributions to current knowledge, which places the oldest fossils known, in the form of petrified cellular microbes, nearly 3,500 million years ago, seven times older than the Cambrian and reaching into the first quarter of the age of the Earth. Lynn Margulis, M.F. Dolan, and R. Guerrero set their thesis right in the title of their contribution: “The Chimeric Eukaryote: Origin of the Nucleus from the Karyomastigont in Amitochondriate Protists ” (Chapter 3). The karyomastigont is an organellar system composed at least of a nucleus with protein connectors to one (or more) kinetosome. The ances-

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Variation and Evolution in Plants and Microorganisms: TOWARD A NEW SYNTHESIS 50 YEARS AFTER STEBBINS tral eukaryote cell was a chimera between a thermoacidophilic archaebacterium and a heterotrophic eubacterium, a “bacterial consortium” that evolved into a heterotrophic cell, lacking mitochondria at first. Cells with free nuclei evolved from karyomastigont ancestors at least five times, one of them becoming the mitochondriate aerobic ancestor of most eukaryotes. These authors aver that only two major categories of organisms exist: prokaryotes and eukaryotes. The Archaea, making a third category according to Carl Woese and others (Woese et al., 1990), should be considered bacteria and classified with them. The issue of shared genetic organelle origins is also a subject, if indirect, of the paper by Jeffrey D. Palmer and colleagues (“ Dynamic Evolution of Plant Mitochondrial Genomes: Mobile Genes and Introns, and Highly Variable Mutation Rates,” Chapter 4). The mitochondrial DNA (mtDNA) of flowering plants (angiosperms) can be more than 100 times larger than is typical of animals. Plant mitochondrial genomes evolve rapidly in size, both by growing and shrinking; within the cucumber family, for example, mtDNA varies more than six fold. Palmer and collaborators have investigated more than 200 angiosperm species and uncovered enormous pattern heterogeneities, some lineage specific. The authors reveal numerous losses of mt ribosomal protein genes (but only rarely of respiratory genes), virtually all in some lineages; yet, most ribosomal protein genes have been retained in other lineages. High rates of functional transfer of mt ribosomal protein genes to the nucleus account for many of the loses. The authors show that plant mt genomes can increase in size, acquiring DNA sequences by horizontal transfer. Their striking example is a group I intron in the mt cox1 gene, an invasive mobile element that may have transferred between species more than 1,000 independent times during angiosperm evolution. It has been known for more than a decade that the rate of nucleotide substitution in angiosperm mtDNA is very low, 50–100 times lower than in vertebrate mtDNA. Palmer et al. have now discovered fast substitution rates in Pelargonium and Plantago, two distantly related angiosperms. REFERENCES Barghoorn, E. S. & Schopf, J. W. ( 1965) Microorganisms from the Late Precambrian of central Australia. Science 150, 337–339. Barghoorn, E. S. & Tyler, S. A. ( 1965) Microorganisms from the Gunflint chert. Science 147, 563–577. Cloud, P. ( 1965) Significance of the Gunflint (Precambrian) microflora. Science 148, 27–45. Darwin, C. ( 1859) On the Origin of Species by Means of Natural Selection (Murray, London). Stebbins, G. L. ( 1950) Variation and Evolution in Plants (Columbia University Press, New York). Woese, C. R., Kandler, O., & Wheelis, M. L. ( 1990) Towards a natural system of organisms: Proposal for the domains Archaea, Bacteria, and Eucarya. Proc. Natl. Acad. Sci. USA 87, 4576–4579.