| 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 Well, just recently scientists have realized that life on Earth isn't as simple as to fit into two, five, six or maybe even several kingdoms. For example, based on Fundamentals of Molecular Evolution, Second Edition, by Dan Graur and Wen-Hsiung Li (Sinauer Associates, Inc., Publishers, Sunderland, Massachusetts, 2000), here is a redrawn chart showing how some of the top specialists, at least in the year 2000, thought the Earth's living things might be related:
Notice how plants and animals are stuck way over on the left side, 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 Flavobacteria Also notice the idea that all living things fall into 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. If you just need to do a paper on "The Kingdoms of Living Things," then you might want to use the scheme shown below, keeping in mind that it's a bit outdated and that not everyone would agree :
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 living on their own. Of course 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. 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. |
