Moreover, large animals and plants generally have tiny populations in comparison with microbes, and cannot acquire life-saving genes by lateral gene transfer, making animals and plants much more vulnerable to extinction. The periodic mass extinctions of plants and animals, followed by evolutionary radiations of hitherto suppressed groups, are not characteristic of microbial evolution-such radiations explore morphological, not metabolic, space. While asserting the unprovable existence of extinct lines of eukaryotes is unsatisfying, if not unscientific, extinction is commonplace, and the argument seems, on the face of it, irrefutable.īut there are several reasons to doubt that prokaryotes have repeatedly given rise to more complex 'protoeukaryotes', which were ultimately all driven to extinction by modern eukaryotes that came to occupy every niche. It is hard to distinguish between eukaryotic and prokaryotic microfossils let alone prove the existence of extinct lines of protoeukaryotes. This cannot be addressed phylogenetically, as any phylogenetic evidence for their existence is lost. However, a single ancestor is perfectly consistent with multiple origins if all 'protoeukaryotic' lines arising later were driven to extinction by fully-fledged eukaryotes already occupying every niche, and if all earlier protoeukaryotes were displaced by modern eukaryotes (or fell extinct for some other reason). Common ancestry is much the most parsimonious explanation. It is implausible that all of these shared properties arose by lateral gene transfer (which is inherently asymmetric in mechanism) or convergent evolution (which implies that traits like intron position are dictated by selective constraints, rather than historical contingency). Common traits range from the conserved position of many introns, to the structure of nuclear pore complexes, to complex traits such as syngamy and two-step meiosis.
There is little doubt that all known eukaryotic cells share a common ancestor that arose only once in four billion years of evolution. Sir Arthur Eddington (1882-1944) The origin of the eukaryotic cell was a unique event We are finding that we must learn a great deal more about 'and'. We used to think that if we knew one, we knew two, because one and one are two. For complete reports see the Reviewers' Comments section. This article was reviewed by: Eugene Koonin, William Martin, Ford Doolittle and Mark van der Giezen. These factors explain the unique origin of eukaryotes, the absence of true evolutionary intermediates, and the evolution of sex in eukaryotes but not prokaryotes. The combination of massive bioenergetic expansion, release from genome-size constraints, and high mutation rate favoured a protosexual cell cycle and the accumulation of eukaryotic traits. The side effect was that a large number of shared eukaryotic basal traits accumulated in the same population, a sexual eukaryotic common ancestor, radically different to any known prokaryote. Unlike prokaryotes, with lower mutation rates and heavy selection pressure to lose genes, early eukaryotes without genome-size limitations could mask mutations by cell fusion and genome duplication, as in allopolyploidy, giving rise to a proto-sexual cell cycle. This energetic transformation was permissive, not prescriptive I suggest that the actual increase in early eukaryotic genome size was driven by a heavy early bombardment of genes and introns from the endosymbiont to the host cell, producing a high mutation rate. The reductive evolution and specialisation of endosymbionts to mitochondria resulted in an extreme genomic asymmetry, in which the residual mitochondrial genomes enabled the expansion of bioenergetic membranes over several orders of magnitude, overcoming the energetic constraints on prokaryotic genome size, and permitting the host cell genome to expand (in principle) over 200,000-fold. Here I argue that the eukaryotic cell originated in a unique prokaryotic endosymbiosis, a singular event that transformed the selection pressures acting on both host and endosymbiont. Prokaryotes show no tendency to evolve greater morphological complexity, despite their metabolic virtuosity.
All eukaryotic cells share a common ancestor that arose just once in four billion years of evolution.
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