300 million years ago
In the late Paleozoic Era, less than a third of a billion years ago, microbes were carried ashore in the intestines of roving aimals. As complex as it is compared to an inanimate droplet of chemicals, the bacterial cell is quite a simple affair among life forms. DNA floats unpackaged in bacteria. Cells with nuclei such as ours and those of all animals, by contrast, are studded with mitochondria and plastids, and held together by a network of reticular structures and by the streaming, pulsing cytoplasm around them. The nuclear DNA, much of it repetitive, is coiled tightly into chromosomes wheich are contained in a membrane-bounded nucleus. Genetic evidence fingerprints the origin of nucleated (or "eukaryotic") cells from mixtures of living bacteria. Living corporations (that remained because they were good metabolic fits), some of the mergers began as hostile takeovers of one organism by another. But over hundreds of millions of years they became so interwoven that it took the electron microscope and biochemical analysis to trace the multiple origins of the nucleated cell. Modern kinds of bacteria suggest what happened. Bdellovibrio and Daptobacter are oxygen-using bacteria that break into other bacteria, multiplying inside: these look similar to the ancestors of the oxygen-using bacteria that became the mitochondria. The originally invaded bacterium was a larger being like Thermoplasma, an organism intolerant of concentrated atmospheric oxygen that dwells in hot and acidic waters such as found in Yellowstone National Park. Using a primitive sulfur metabolism, this ancient oxygen-intolerant part of the partnership may date back to the origins of life itself in and around iron sulfide structures. Plants have an even more complicated partnership: their green parts, or chloroplasts, have been traced to free-living bacteria such as Prochloron. The addition of a photosynthetic bacterium to the mix made it unnecessary to actively seek food. This was a further refinement and arguably an advance upon the more frenetic lifestyle that led to animals. Learn More
© 2005 by Sciencewriters
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