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The Three Domains of Life
Moffett Field - Nov 6, 2001 When scientists first started to classify life, everything was designated as either an animal or a plant. But as new forms of life were discovered and our knowledge of life on Earth grew, new categories, called "Kingdoms," were added. There eventually came to be five Kingdoms in all - Animalia, Plantae, Fungi, Protista, and Bacteria. The five Kingdoms were generally grouped into two categories called Eukarya and Prokarya. Eukaryotes represent four of the five Kingdoms (animals, plants, fungi and protists). Eukaryotes are organisms whose cells have a nucleus -- a sort of sack that holds the cell's DNA. Animals, plants, protists and fungi are all eukaryotes because they all have a DNA-holding nuclear membrane within their cells. The cells of prokaryotes, on the other hand, lack this nuclear membrane. Instead, the DNA is part of a protein-nucleic acid structure called the nucleoid. Bacteria are all prokaryotes. However, new insight into molecular biology changed this view of life. A type of prokaryotic organism that had long been categorized as bacteria turned out to have DNA that is very different from bacterial DNA. This difference led microbiologist Carl Woese of the University of Illinois to propose reorganizing the Tree of Life into three separate Domains: Eukarya, Eubacteria (true bacteria), and Archaea. Archaea look like bacteria - that's why they were classified as bacteria in the first place: the unicellular organisms have the same sort of rod, spiral, and marble-like shapes as bacteria. Archaea and bacteria also share certain genes, so they function similarly in some ways. But archaeans also share genes with eukaryotes, as well as having many genes that are completely unique. Archaea are so named because they are believed to be the least evolved forms of life on Earth ('archae' meaning 'ancient'). The ability of some archaea to live in environmental conditions similar to the early Earth gives an indication of the ancient heritage of the domain. The early Earth was hot, with a lot of extremely active volcanoes and an atmosphere composed mostly of nitrogen, methane, ammonia, carbon dioxide, and water. There was little if any oxygen in the atmosphere. Archaea and some bacteria evolved in these conditions, and are able to live in similar harsh conditions today. Many scientists now suspect that those two groups diverged from a common ancestor relatively soon after life began. Millions of years after the development of archaea and bacteria, the ancestors of today's eukaryotes split off from the archaea. So although archaea physically resemble bacteria, they are actually more closely related to us! If not for the DNA evidence, this would be hard to believe. The archaea that live in extreme environments can cope with conditions that would quickly kill eukaryotic organisms. Thermophiles, for instance, live at high temperatures - the present record is 113�C (235�F). In contrast, no known eukaryote can survive over 60�C (140�F). Then there are also psychrophiles, which like cold temperatures - there's one in the Antarctic that grows best at 4�C (39�F). As a group, these hard-living archaea are called "extremophiles." There are other kinds of archaea extremophiles, such as acidophiles, which live at pH levels as low as 1 pH (that's about the same pH as battery acid). Alkaliphiles thrive at pH levels as high as that of oven cleaner. Halophiles, meanwhile, live in very salty environments. But there are also alkaliphilic, acidophilic, and halophilic eukaryotes. In addition, not all archaea are extremophiles. Many live in more ordinary temperatures and conditions.
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