Saturday, December 11, 2010

Life Lines 79


Darwin was the first to sketch a simple branched diagram showing how life is connected through evolution. By the time I was a teenager I was enjoying phylogenetic trees that classified all living things into two kingdoms –plants and animal. Some fifty years later that classification has given way to 96 phyla distributed among three domains. The domains are given the technical names eukaryotes, eubacteria, and archaebacteria. You and almost everything you know that is living are eukaryotes. Your cells have nuclei. But bacteria and archaebacteria have a different structure—they do not have nuclei and their cells are much smaller. That cell structure is called prokaryotic (next time you get a strep throat, think of prokaryotes gathering in multitudes reddening those tissues).

In 2006 scientists compared the complete genome sequences of 191 species that shared in common some 31 genes. There were 11 multicelled animals among these 191 and they represented the major animal phyla (worms, jellyfish, mollusks, arthropods, vertebrates, echinoderms, etc.). Most of the completed sequences were from prokaryotes. The most familiar domain of the prokaryotes belong to the eubacteria (the ones dealing with the germ theory, with spoiled food, rotting carcasses, and some commercially useful bacteria that make vinegar and a variety of cheeses, and other dairy products). The ones we almost never encounter live in extreme environments such as ocean vents, geysers, and sometimes miles down in the earth. The reason they are put into two domains has to do with their organization of their genes. The archaebacteria have split genes as do the eukaryotes but they have a prokaryotic organization. Most prokaryotes are killed when scalded but archaebacteria flourish in hot water that can be very hostile. A few of the eubacteria are also capable of thriving in hot water or extreme environments and they are among the oldest of the forms of life on our evolutionary tree. The archaebacteria arose from the eubacteria and the eukaryotes arose from the archaebacteria.

The phylogenetic tree which relates the genomes to their mutational differences, as one form of life arose from an earlier one, gives a picture strikingly similar to the phylogenetic trees constructed from the fossil record. The age of these ancestors is also determined independently by isotopic analysis of the rocks they are in and the mutational differences among their common genes. Those who think evolution is “just a theory” have a lot of explaining to do because that relation between complexity and time fits evolution and not special creation in any of its forms. The evolutionary theory based on the older fossil record predicted the molecular phylogenetic tree would resemble it in its major features and it did. Science makes predictions and the data lets the scientist see if the theory still holds. It does for the phylogenetic theory.

What is remarkable about this computer generated phylogenetic tree is that new genome sequences can be added as all of the 96 phyla are eventually sequenced and the tree will become more elaborate. Just as a genealogist finds family members in the thousands when dedicated to searching for one’s ancestors, the scientist finds the major family members of all life on earth. Some day a fourth domain, the viruses, may be connected to the first and most ancient cells. We are kin to everything alive

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