Microbial Evolution and the Inadequacy of the Plant Animal Divide

Unlike many previous taxonomies of living organisms, the five kingdoms taxonomy reflects current understandings of evolutionary history. Both paleontological evidence in the form of microfossils, as well as biochemical (metabolic) and genetic comparative studies of living organisms, show that the first organisms for which there is evidence on earth are bacteria. The biggest division between types of organisms is not between plants and animals (as an ancient perspective cognizant of green growing, and breathing moving, living things suggested before knowledge of cells or evolution) but rather between prokaryotes and eukaryotes. Prokaryotes are all organisms without nuclei (pro = "before"; karyon = "kernel") in their cells; eukaryotes (eu = "true"; karyon = "kernel") all organisms with nuclei in their cells. Additional differences are the lack of true chromosomes in prokaryotes and the differential presence, in eukaryotes only, of gene-containing plastids (for example, the chloro-plasts of plant cells) and gene-containing mitochondria (in virtually all eukaryotes) outside the nuclei. Eukaryotic cells also are generally much larger in size.

The difference between prokaryotes and eukaryotes is really striking for anyone positing a gradual view of evolution. In this case, however, there is no "missing link": abundant genetic, comparative metabolic, microbial ecological, and other studies show, beyond a reasonable shadow of a doubt, that bacteria came together in symbiosis to produce the larger eukaryotic cells with nuclei, chromosomes, and, more often than not, mitosis (and sometimes meiosis)—characteristic chromosomal pairings and separations during cell division. Bacteria, by contrast, divide by simple cell division. The mitochondria and plas-tids (which may be purple-brown phaeoplasts or red rhodoplasts as well as the more familiar green chloroplasts) have an independent bacterial origin: the reason they continue to possess separate DNA, their own timetable of reproduction, and their own binary fissionlike mode of reproduction outside the nucleus is that their ancestors were bacteria. The symbiotic origin of eukaryotic cells thus shows neither plants nor animals, but rather bacteria, to be the primeval form of cell. Not all bacteria merged by any means, and free-living bacteria today have descended from bacterial ancestors assumed to have first evolved on earth more than 3.5 billion years ago. But among those bacteria that did merge were cells with nuclei and mitochondria (descended from aerobic, oxygen-metabolizing bacteria)— the first eukaryotes.

Circumstantial and genetic evidence suggests that the ancestors to the host cells were heat-resistant, oxygen-poisoned fermenting cells similar to the modern Thermoplasma, which survives the heat and acid of geysers in Yellowstone National Park. Although evidence is not definitive, these hosts of the ancestors of mitochondria had probably already established still another symbiosis—with highly motile spirochetes, known to feed on, attach, and sometimes inhabit other cells—

before becoming a breeding ground for oxygen-breathing bacteria. The hosts preferentially survived because the surface of the planet, as geological evidence such as red beds of rust (iron oxide) shows, was becoming much richer in oxygen. Oxygen, produced by photosyn-thetic bacteria, the ancestors to plastids, as a result of microbial evolution to use water (H2O) for hydrogen to build cells, is a highly reactive gas still toxic to many cells (called anaerobes). The ability to metabolize oxygen thus conferred an evolutionary advantage on anaerobes containing oxygen-breathing (and thus detoxifying) mitochondrial ancestors in their cells. The host-mitochondrial ancestor assemblages were the first O2-respiring (or aerobic) protoctists, amoebalike cells. They were the ancestors to animals and fungi. Meanwhile, photosynthetic bacteria, when they were eaten but not digested, joined the host-respiring bacteria assemblage and created another sort of eukaryotic cell—one ancestral to algae and plants.

The upshot of all this microbial evolution, from a taxonomic standpoint, is that plants and animals are not an ancient all-encompassing division but relatively recent branches on the tree of life. Our ideas about how life has evolved have changed how life is best divided. The bacterial symbiotic ancestry of eukaryotic cells renders obsolescent older forms that continue to be used out of ignorance combined with linguistic momentum. (Language also has its rudimentary organs.) Thus it is now, for example, inaccurate to speak of "one-celled animals"; you are more a giant colonial amoeba, because amoebalike cells were your ancestors, than are amoebas (protists, one-celled protoctists) one-celled animals: pro-toctists evolved not only into animals but also into fungi and plants. A more technical term, protozoa, is also best avoided for similar reasons: protozoa comes from the Greek words for "first animals." Microbes such as Chlamydomonas, a single cell with green plastids that swims through the water, shows the inadequacy of the plant-animal divide.

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