Great Moments in the History of Life by George H. Shaw

Author:George H. Shaw
Language: eng
Format: mobi, pdf
Publisher: textseed
Published: 0101-01-01T00:00:00+00:00

Figure 7.1 Phylogenetic tree constructed using sRNA sequencing (NASA Astrobiology Institute)

The distance on the graphic tree between a particular organism and some other to which one wants to compare relatedness is determined by how different the sRNA molecules are from one another, and is approximately proportional to the time span since the division into descendent lines took place. For example, the genetic distance between the branch containing modern plants and green filamentous bacteria runs from near the far right, down and across the major right-hand branch, past where a group of bacteria known as “Archaea” branches, and partway up the left major branch, and finally up the branch at the end of which we find the green filamentous bacteria. Obviously these organisms are not closely related to one another, and in evolutionary terms they parted company a very long time ago.

It is possible to make some estimates of the time spans implicit in this diagram using parts of the fossil record where preserved specimens give us another means of constructing a phylogenetic tree, and relating it to ages that have been measured using radiometric dating methods. This can be done in most detail for a small part of the tree on the right branch, where multicellular animals and plants produced recognizable fossil remains. Figure 7.2 shows a simple tree for animals. Note that this tree only covers about 700 million years, a small fraction of the history of life.

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