| Today the fields of classification and taxonomy
are undergoing a real revolution. When I was a kid we just thought in terms of two
kingdoms -- plants and animals. Then when I was in college we were taught about five
kingdoms. In an important college textbook from the year 2000 six kingdoms were recognized Nowadays scientists have realized that life on Earth isn't as simple as to neatly fit into two, five, six or even several kingdoms. For example, loosly based on a paper by FD Ciccarelli et al in a 2006 paper in Science 311 (5765): 1283–7, here is a redrawn chart showing how some of the top specialists, at least in the year 2006, thought the Earth's living things might be related:
Notice how plants and animals are stuck toward the top of the tree, highlighted with yellow. To better get a feeling for this tree, you might want to Google some of the names in the chart, such as Crenarchaecota and Chlamydiae. Part of many new concepts about the Tree of Life is that all living things fall into these three basic groups (not kingdoms):
It's now believed that the first simple bacteria emerged at least as far back as 3.5 billion years ago, about a billion years after the Earth's formation. The Eukarya materialized only about 2 billion years ago, maybe earlier. On top of all this fast-changing thinking, many scientists are pretty sure that one way certain species increased their complexity was by taking into their own bodies the one-celled bodies of other species and then, instead of digesting them, adopting them as permanent, genetically reproducible parts of themselves. For example, because they have their own DNA, it's thought that both chloroplasts and mitochondria are derived from ancestors that were once free-roaming species. Chloroplasts are the pigment-filled green items in the cells of green plants in which photosynthesis takes place, and mitochondria are particles in cells of both plants and animals, and they process energy. Amazing, heady stuff, no? If you can handle the technical names of the major plant groupings, such as Filicopsida, Gnetales, Euangiosperms, and the like, you can't do better than to browse the Tree of Life website. For example, at that site if you want to see a frequently updated, current chart showing how the Eudicots (most flowering plants) are thought to be subdivided, click here. If you want to begin with "Life on Earth" and then work your way down, first through the three basic groupings mentioned above, and then, clicking on links as they appear, from Eukaryotes, for example, on down through the various groupings, perhaps Animals/ Bilateria/ Deuterostomia, etc. (We're on our way to human beings there... ), click here. You may enjoy taking a look at Amazon.com's write-up for the books Fundamentals of Molecular Evolution and the rather less expensive Patterns in Evolution : The New Molecular View. |
