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.

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.