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How does the discovery of “extremophiles” change our understanding of the tree of life?

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Introduction

How does the discovery of "extremophiles" change our understanding of the tree of life? Extremophiles have been known about for many years, since the 1950s; at this point in time they were not well understood - scientists have now recognised that places once assumed to be sterile abound with microbial life1. The first classification system was put forward by Carolus Linnaeus (1707-1178); other 'designs' were introduced over the years, until 1937 when the French marine biologist Edouard Chatton suggested the term 'procaryotique' and 'eucaryotique' to distinguish animal and plants cells from those of the bacteria2. From this the 'Five Kingdom' theory was proposed, and it 'worked' for many years - for the most part, all organisms discovered could be categorised within the 'Five Kingdoms'. It was Carl Woese who led the research regarding extremophiles, some 20 years ago, using the technique oligonucleotide cataloging. This technique consisted of breaking a rRNA molecule into fragments (or oligonucleotides) at every G residue. These were then broken into subfragments with enzymes that sliced at different residues; from these Woese reconstructed the sequence of original rRNA fragment. Once they were reconstructed Woese could compare them to other oligonucleotides from other microbes, and thus determine how closely they were related3. ...read more.

Middle

Woese's 'tree of life'11. Since 'extremophiles', ie the methanogens, have been known about for many years, but not properly classified12, Carl Woese has essentially triggered the search for more of the Archaea - by giving them a grouping, there must be more organisms of a similar design (there are in all other phyla). As a result of this many extremophiles are now known. There are hyperthermophiles, psychrophiles, acidophiles, alkaliphiles, and halophiles (to name a few). There are important characteristics of these organisms that enable them to survive in these extreme conditions. For example: hyperthermophiles contain large amounts of the glycolytic derivative, cyclic 2,3-diphosphoglycerate, dissolved in the cytoplasm; this is thought to act as a thermostabilising agent, to prevent denaturation of the enzymes and DNA in the cell13. Acidophiles survive by keeping the acid outside the cell, and halophiles survive in salty solutions by retaining solute from outside the cell (thus becoming isotonic)14. Another reason why Woese's tree of life makes so much sense is that there have been a lot of questions concerning early life. The environment on the earth 4 billion years ago was almost certainly hotter than it is now15, and also 4 billion years ago the earth was being constantly bombarded by meteorites16 (these will also have increased the temperature of environment). ...read more.

Conclusion

This evidence obviously infers that there may be some flaws in Woese's tree of life. As can be seen from this essay, Carl Woese has revolutionised taxonomy for the 1990s. The main point made by Woese is that there are three evolutionary lineages, not two, as was previously thought - the division being between the Archaea, Eucarya and Bacteria. The other important information is that of these three domains the Archaea are closer to the eukaryotes than the bacteria - ie they are closest to the foot of the tree of life. Although classification of species was morphological up until the 1970s (when Woese first put forward his 'tree of life'), there are problems associated with morphological classification - similarities between eubacteria and archaebacteria can be misleading, and some species can be misidentified due to their similar characteristics. As organisms have evolved, there have been some cases of convergence, thus giving the impression that two species are very alike - molecular analysis avoids this possibility by looking at the original building blocks for life; the nucleotide sequences on the genes (specifically the 16S rRNA gene), which will hopefully give the clearest picture of evolution yet. Those people, who are voicing differing opinions from that of Carl Woese at the moment, will probably have to wait another twenty years to have their voice heard. ...read more.

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