Spring, Jürg. “Major transitions in evolution by genome fusions: from prokaryotes to eukaryotes, metazoans, bilaterians and vertebrates.” Journal of structural and functional genomics 3, no. 1 (2003): 19-25.
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The major transitions in human evolution from prokaryotes toeukaryotes, from protozoans to metazoans, from the first animals tobilaterians and finally from a primitive chordate to vertebrates wereall accompanied by increases in genome complexity. Rare fusion ofdivergent genomes rather than continuous single gene duplications couldexplain these jumps in evolution. The origin of eukaryotes was proposedto be due to a symbiosis of Archaea and Bacteria. Symbiosis is clearlyseen as the source for mitochondria. A fundamental difference of highereukaryotes is the cycle from haploidy to diploidy, a well-regulatedgenome duplication. Of course, self-fertilization exists, but thepotential of sex increases with the difference of the haploid stages,such as the sperm and the egg. What should be the advantage of havingtwo identical copies of a gene? Still, genes duplicate all the time andeven genomes duplicate rather often. In plants, polyploidy is wellrecognized, but seems to be abundant in fungi and even in animals, too.However, hybridization, rather than autopolyploidy, seems to be thepotential mechanism for creating something new. The problem withchimaeric, symbiotic or reticulate evolution events is that they blurphylogenetic lineages. Unrecognized paralogous genes or random loss ofone of the paralogs in different lineages can lead to falseconclusions. Horizontal genome transfer, genome fusion or hybridizationmight be only truly innovative combined with rare geologicaltransitions such as change to an oxygen atmosphere, snowball Earthevents or the Cambrian explosion, but correlates well with the majortransitions in evolution.