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Chapter 24 : Microbes: Earth's First Inhabitants

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Abstract:

The Earth itself is about 4.6 billion years old, and several lines of evidence indicate that life on Earth began approximately 3.5 billion years ago in the form of anaerobic bacteria. Tracing the evolution of higher forms is aided greatly by the study of fossils, but fossils of early microbes are very rare and hard to find. The origin of life on Earth is one of the major unsolved mysteries of science. The first organisms on Earth were anaerobic prokaryotes, and these were the only forms of life on the planet for a very long period. Some investigators looked into hypothetical microbial evolutionary trees based on comparing the structures of genes that code for enzyme proteins required for normal metabolic processes. A major complication has arisen in that new evidence shows that during evolution, extensive transfer of genes (or parts of genes) has occurred among many bacterial species. This phenomenon, known as horizontal gene transfer (HGT), suggests that many, if not most, of the extant bacterial species are genetic chimeras. Since the first complete DNA sequence of a free-living organism, Haemophilus influenzae, was determined in 1995, literally dozens of other genomes, both prokaryotic and eukaryotic, have been investigated and mapped. We can expect that detailed knowledge of the genomes of a wide variety of organisms will soon provide important insights on how early life forms diversified through the ages.

Citation: Gest H. 2003. Microbes: Earth's First Inhabitants, p 169-174. In Microbes. ASM Press, Washington, DC. doi: 10.1128/9781555817855.ch24
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Figure 41

Diagrammatic representation of one theory of how prokaryotes and eukaryotes evolved. This evolutionary tree is based largely on the molecular properties of 16S RNA. Recent investigations, however, indicate that the actual course of evolution of the bacteria was much more complicated than this diagram suggests because of extensive exchange of genes among different species of eubacteria and between eubacteria and archaebacteria.

Citation: Gest H. 2003. Microbes: Earth's First Inhabitants, p 169-174. In Microbes. ASM Press, Washington, DC. doi: 10.1128/9781555817855.ch24
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Citation: Gest H. 2003. Microbes: Earth's First Inhabitants, p 169-174. In Microbes. ASM Press, Washington, DC. doi: 10.1128/9781555817855.ch24

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