[From a colleague:] Hot on the heels of Embley and Martin (Nature 440, 623-630, 30 March 2006), Kurland and colleagues take the plunge and sever the link between eukaryotes and prokaryotes. Their title refers to the “Irreducible Nature of Eukaryote Cells,” which reads like an echo of Mike Behe. The logic of their argument confirms this: the structures and the genetics of eukaryotes mean that an evolutionary pathway from prokaryotes must be rejected. However, they do not again use the word “irreducible” in their paper. What is clear is that the “simple” pathway that the textbooks have proclaimed for years must now be abandoned. Surely there are lessons here about the way darwinism gives false leads in its appetite for a narrative about the origins of complexity.
Evolution by Reduction? (Science Highlights)
The origins of eukaryotes remain controversial and somewhat enigmatic. Kurland et al. (p. 1011) provide a counterpoint to current models in which the eukaryotic cell is derived from structurally and genetically less complex prokaryotic cells. On the basis of genomic and proteomic evidence, they suggest that the essence of eukaryotic cellular complexity existed in the common ancestor of eucarya, bacteria, and archaea, and that the bacteria and archaea have evolved by genome reduction driven by specialization for fast growth and cell division and/or adaptation to extreme environments.
Genomics and the Irreducible Nature of Eukaryote Cells
C. G. Kurland, L. J. Collins, and D. Penny
Science 19 May 2006: Vol. 312. no. 5776, pp. 1011 – 1014
Abstract: Large-scale comparative genomics in harness with proteomics has substantiated fundamental features of eukaryote cellular evolution. The evolutionary trajectory of modern eukaryotes is distinct from that of prokaryotes. Data from many sources give no direct evidence that eukaryotes evolved by genome fusion between archaea and bacteria. Comparative genomics shows that, under certain ecological settings, sequence loss and cellular simplification are common modes of evolution. Subcellular architecture of eukaryote cells is in part a physical-chemical consequence of molecular crowding; subcellular compartmentation with specialized proteomes is required for the efficient functioning of proteins.
Comparative genomics and proteomics have strengthened the view that modern eukaryote and prokaryote cells have long followed separate evolutionary trajectories. Because their cells appear simpler, prokaryotes have traditionally been considered ancestors of eukaryotes (1*4). Nevertheless, comparative genomics has confirmed a lesson from paleontology: Evolution does not proceed monotonically from the simpler to the more complex (5*9). Here, we review recent data from proteomics and genome sequences suggesting that eukaryotes are a unique primordial lineage.
Mitochondria, mitosomes, and hydrogenosomes are a related family of organelles that distinguish eukaryotes from all prokaryotes (10). Recent analyses also suggest that early eukaryotes had many introns (11, 12), and RNAs and proteins found in modern spliceosomes (13). Indeed, it seems that life-history parameters affect intron numbers (14, 15). In addition, “molecular crowding” is now recognized as an important physical-chemical factor contributing to the compartmentation of even the earliest eukaryote cells (16, 17).
Nuclei, nucleoli, Golgi apparatus, centrioles, and endoplasmic reticulum are examples of cellular signature structures (CSSs) that distinguish eukaryote cells from archaea and bacteria. Comparative genomics, aided by proteomics of CSSs such as the mitochondria (18, 19), nucleoli (20, 21), and spliceosomes (13, 22), reveals hundreds of proteins with no orthologs evident in the genomes of prokaryotes; these are the eukaryotic signature proteins (ESPs) (23, 24). The many ESPs within the subcellular structures of eukaryote cells provide landmarks to track the trajectory of eukaryote genomes from their origins. In contrast, hypotheses that attribute eukaryote origins to genome fusion between archaea and bacteria (25*30) are surprisingly uninformative about the emergence of the cellular and genomic signatures of eukaryotes (CSSs and ESPs). The failure of genome fusion to directly explain any characteristic feature of the eukaryote cell is a critical starting point for studying eukaryote origins. . . .