Chapter 17. The origins of life and Precambrian evolution

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EVOLUTION/LECTURE1 rRNA, appears in Figure 14.16. Figure 14.16.: An estimate of the phylogeny of all living organisms. This tree is based on the analysis of nucleotide sequences of small-subunit rRNAs The whole-life rRNA phylogeny has prompted a dramatic revision of our traditional view of the organization of life, because it reveals that the five-kingdom system of classification [i.e., Plants, Animals, Fungi, Protists (this is the taxomic kingdom that comprises a variety of unicellular and some simple multinuclear and multicellular eukaryotic organisms, they include some algae, the protozoans, and multicellular or multinucleate autotrophs), and Monera (this is taxonomic kingdom that comprises the prokaryotes (bacteria and cyanobacteria))] bears only a limited resemblance to actual evolutionary relationships. 11.4.- The prokaryotes, for example, which are all grouped in the kingdom Monera in the traditional classification, occupy two of the three main branches of the rRNA phylogeny. One of these two branches, the Bacteria, includes virtually all of the wellknown prokaryotes. The other prokaryote branch, the Archaea, is not well known. Many of the Archaea live in physiologically harsh environments like hot springs, and are difficult to grow in culture. They are also known as the Archebacteria 11.5.- As the phylogeny in Figure 14.16 shows, the archebacteria are in fact more closely related to the eukaryotes than they are to the true bacteria. The most inclusive taxonomic units in the new classification are three domains corresponding to the three main branches file:///E|/CH17-PRECAMBRIAN-SPRING-2008/CHAPTER%2017_SPR_2008.HTML[12/8/2011 2:57:27 PM]
EVOLUTION/LECTURE1 on the tree of life: the Bacteria, the Archaea, and the Eucarya. It has been proposed that the these three branches are designated as kingdoms. 11.6.- The universal rRNA phylogeny demonstrates, however, that the Animals, Plants, and Fungi, "the kingdoms" that have absorbed most of the attention of evolutionary biologists, are mere twigs on the tip of one branch of the tree of life. The multicellular, macroscopic organisms in these three "kingdoms" are newcomers on the evolutionary scene, with a relatively recent common ancestor 12.- The origin of organelles 12.1.- Evolution from the common ancestors to the extant Bacteria and Archaea appears to have been a process of gradual refinement. The Eucarya, on the other hand, are dramatically different. Among other things, eukaryotes have complex cells containing a variety of organelles, and dramatically more complicated genomes, in which the coding regions of genes are frequently interrupted by intervening sequences of noncoding DNA (introns). In addition many eukaryotes are multicellular, with differentiated cells and tissues. We will only consider one of these problems here, the evolutionary origin of organelles. 12.2.- The organelles whose evolution is best understood are mitochondrial and chloroplasts. Most Eucarya have mitochondrial and many also have chloroplasts, but some early-branching eukaryotes (Giardia lamblia) have neither. Thus mitochondria and chloroplasts do not appear to be defining characteristics of the Eucarya; instead, they arose within the eukaryotic branch of the tree of life. 12.3.- Superficially, mitochondria and chloroplasts resemble simple bacteria. With the discovery that mitochondria and chloroplasts have their own chromosomes, and that these chromosomes are small circular DNA molecules similar to those of bacteria, biologists began to take seriously an old idea that had been dismissed by all but a persistent few. This idea is that mitochondria and chloroplasts originated as bacteria that lived as internal symbionts of early eukaryotic cells. 12.4.- The definitive test of this hypothesis was to sequence genes from mitochondrial and chloroplasts, such as the genes for the small-subunit ribosomal RNA, then determine their position on the universal phylogeny. If the organelles arose via symbiosis, then their rRNA genes should branch from within the Bacteria; if, instead, the organelles arose independently within the Eucarya, then their rRNA gene should branch from within the Eucarya. The results are in, and they prove that the endosymbiosis theory is correct (Fig. 14.24). Mitochondria are closely related to the protobacteria, previously called purple bacteria. Chloroplasts are cyanobacteria Figure 14.24.: An estimate of the universal phylogeny, showing the locations of the mitochondrial and chloroplasts. The file:///E|/CH17-PRECAMBRIAN-SPRING-2008/CHAPTER%2017_SPR_2008.HTML[12/8/2011 2:57:27 PM]
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Chapter 17. The origins of life and Precambrian evolution

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