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Scientists are probably centuries away from drawing the full tree of life. For one thing, they have only discovered a small fraction of the species on Earth--perhaps only ten percent. They are also grappling with the relationships between the species they have discovered. Systematists (scientists who study the tree of life) rely mainly on DNA these days to figure out how species are related to one another. They compare the similarities and differences in a given gene in several different species to figure out which ones share the closest kinship. But they have actually sequenced DNA from relatively few species. And in many cases, that DNA may come from a single gene. *** Here's a quick tour of the tree. Start at middle of the circle. The central point represents the last common ancestor of all living things on Earth. The tree sprouts three deep branches, which between them contain all the species the scientists studied. These deep branches first came to light in the 1970s, and are known as domains. We belong to the red domain of Eukaryota, along with plants, fungi, and protozoans. Bacteria (blue) and Archaea (green) make up the other two domains. *** The long length of the Bacteria branch underscores one of the big messages of this tree: the diversity we can see with the naked eye reflect a pretty paltry snippet of life's genetic diversity. Humans and mushrooms are tucked into a small part of the tree. Meanwhile, bacteria such as ones that cause strep throat (Streptococcous) and the ones that cause food poisoning (Salmonella) are divided by vast evolutionary gulfs. The diversity of microbes did not stop evolving billions of years ago. Escherichia coli, for example, emerged relatively recently, specializing on the warm guts of mammals and birds. As the scientists point out, this tree challenges the traditional way that biologists classify living things into species, genus, family, class, phylum, and kingdom. Scientists named many of these groups in the eighteenth and nineteenth centuries, when they could only sort species by what they could see with the naked eye or a crude microscope. But there's a vast amount of hidden biochemical diversity in living things, and that diversity is reflected in this new tree. The scientists compared the genetic distance among different groups. Animals in different phyla are separated by much less genetic distance than bacteria that are in the same phylum. If scientists were classifying life from scratch today based on genetic distance, they'd probably downgrade animal phyla to classes. _______________________________________________ Marxism-Thaxis mailing list [email protected] To change your options or unsubscribe go to: http://lists.econ.utah.edu/mailman/listinfo/marxism-thaxis
