Major Evolutionary Transitions.
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It is useful to have a general view of the major steps which have occurred throughout the evolution of life on Earth. For one, it can help focus research to critical evolutionary events that need to be explained -- explanations to questions which may not be readily answered by existing paradigms. For another, it can give a greater meaning to what it is we are studying or what we want to study in biology. A rather good book that describes the various important evolutionary transitions and presents the information in a well-layed out fashion, was written by John Maynard Smith and Eros Szathmary (1995). Here, I will provide an outline of some of their work, since it is relevant to the content of this web site. Table 1.2 on page 6 of their book, lists what the authors considered to be key transtions in evolution. They are as follows:
Before I go on, first a digression. My purpose here is simply to outline some critical events in evolution -- not to provide answers, but to provide questions which need to be answered. My reason for doing this, is to provide a guideline from which we can use to focus in on some particular aspect that seems interesting enough to study. I will admit that I'm not necessarily interested in studying all of these points, but only a select few. For example, I've become fond of studying organelle origins and evolution (and in particular, chloroplasts and plant evolution), however, I'm not really interested in studying human societies and human evolution. I will now describe some key features of these transitions, as well as provide some of the questions that Maynard Smith and Szathmary (1995) have attempted to answer (however, you should be aware that there is a lot more going on than just the information presented here -- this barely scratches the surface). The origin of replicators in the evolution of life. Life requires some form of heritable variation in order to persist. The steps leading up to the origin of replicators deals with "chemical evolution" (I deal with this in another section). With replication, some form of a template is required. So, the question is, how did this template form? In order to answer that, we need to know what sort of molecules were present on the Early Earth and what conditions could lead to the formation of templates. Once replicators exist, they may be under a selective pressure to adapt to changing environmental conditions. That is, once replicators arise, they compete with each other for "nutrients" in order to replicate. The next step then, is encapsulation. Once compartmentalized, a physical barrier exists preventing other replicators from effectively competing with the encapsulated replicator. One problem with being encapsulated, is that nutrients for the replicator would run out, and therefore replication would cease. One possible way out of this dilemma is the "hypercycle". A hypercycle is essentially a small group of replicators, each of which catalyses a reaction to provide the substrates for the succeeding replicator. Ultimately, this group of replicators cooperate together in the form of a continuous cycle. Hypercycles, however, are not stable, since they can easily be destroyed by "parasites" (e.g. a selfish "replicator" that is a good target, but a poor replicase). Therefore, an encapsulated hypercycle may persist. Eventually, these independent replicators became linked to form chromosomes. The mechanism of this linkage is not known for certain, and it would depend on the nature of the replicators. The origins of of replication, transcription, and translation -- and therefore, the origins of DNA, RNA, and protein -- must be accounted for since life on Earth uses these various processes and molecular constituents. RNA is particularly interesting, because RNA can have catalytic activity (i.e. ribozymes), as well as store genetic information. The origins of RNA, however, is problematic, since ribose (the sugar group that is a part of RNA) must somehow be generated abiotically. A phosphate source must also be present, since phosphate is a component of RNA. Ribose is a complex molecule and there is uncertainty as to how it formed on pre-biotic Earth. Clay could have been the first genetic material, which eventually gave rise to RNA, however, Maynard Smith and Szathmary (1995) conclude that we really don't know how the genetic system originated. Once an RNA-based genetic system develops, it not hard to understand why DNA eventually replaced RNA with respect to information storage, and it is easy to see why proteins largely replaced RNA's enzymatic functions. The reasons are, that DNA is relatively more stable than RNA (since DNA is usually double stranded), and that DNA polymerases have a higher copying fidelity than RNA polymerases do. Proteins are composed of more diverse subunits than RNA is -- i.e. there are 20 amino acids and only four bases typically used. Therefore, potentially more enzymatic functions would be possible with proteins because of amino acid diversity. Why are there only four nucleotide used in DNA and RNA and only 20 amino acids in proteins? Why not more or less, in either case? It could be a historical accident, or there could be other reasons. At this point, the reasons are uncertain. Another critical step in the evolution of life, was the transition from prokaryotes to eukaryotes. The prokaryotes originated about 3.5 to 4 billion years ago. The eukaryotes, on the other hand, originated only about one billion years ago. Why did it take so long for eukaryotes to evolve? There could be a number of reasons. One reason could simply be environmental or ecological opportunity -- i.e. oxygen did not reach a significant level until about one billion years ago. Three events are suggested in a possible scenario for eukaryote evolution (ref. 1, pages 124-125). They are 1) loss of the outer wall, 2) organization and transmission of genetic material, and 3) the symbiotic origin of organelles. Another critical event in eukaryote evolution was the origin of mitosis. Mitosis appears to be radically different from bacterial cell division, so how did it evolve? Introns are a feature of eukaryotic genes, but not of prokaryotic genes. There are three classes of introns (ref. 1, pages 133-134): group 1, are self-splicing and mobile; group 2, can be self-splicing, but the mechanisn is different from that in group 1; and in group 3, the mechanism is unknown. What are the actual mechanisms, and how or why did they originate? The origins of sex (and therefore, meiosis) also needs to be determined. How did sexual reproducing entities evolve from asexual entities? The origins of haploids and diploids, as well as crossing-overs, and mating types are described in some detail by the authors. It appears that multi-cellularity arose at least three times independently (i.e. in animals, plants, and fungi), from unicellular eukaryotic ancestors (i.e. protists). Cell differentiation and organismal development, therefore, require some explanation -- in particular: the cell cycle; gene regulation and heredity; and spatial patterns. Studying colonies versus solitary individuals may provide some insight into how multicellularity evolved. The origins of societies, such as those of termites, ants, bees and wasps, must include explanations of behaviours (e.g. rituals), cooperation and altruism. The book ultimately leads up to its logical conclusion: the origins of human societies. The authors contend that language was a key factor in human evolution. How did language originate and evolve? References:
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