Life on a Young Planet Read Online Free PDF Page B

biological importance, eukaryotes would seem to have a decisive edge; eukaryotic organisms display a variety of form that ranges from scorpions, elephants, and toadstools to dandelions, kelps, and amoebas. In contrast, prokaryotes are mostly minute spheres, rods, or corkscrews. Some bacteria form simple filaments of cells joined end to end, but very few are able to build more complicated multicellular structures.
    Size and shape surely favor eukaryotes, but morphology provides only one of several yardsticks for measuring ecological significance. Metabolism—how an organism obtains materials and energy—is another, and by this criterion, it is the prokaryotes that dazzle with theirdiversity. Eukaryotic organisms basically make a living in one of three ways. Organisms like ourselves are heterotrophs ; we gain both the carbon and energy needed for growth by ingesting organic molecules made by other organisms. To obtain energy, our cells use oxygen to break down sugars to carbon dioxide and water, a process called aerobic (oxygen-using) respiration . In a pinch, we can gain a bit of energy from a second metabolism called fermentation , an anaerobic (no-oxygen) process in which one organic molecule is broken into two others—only brewer’s yeast and a few other eukaryotes make much of a living this way. The third principal energy metabolism found in eukaryotes is the photosynthesis performed by plants and algae: chlorophyll and associated pigments harvest energy from sunlight, enabling plants to fix carbon dioxide into organic matter. In order to convert light into biochemical energy, plants need an electron—water supplies the needed charge, producing oxygen as a byproduct.
    A Christmas Carol , Charles Dickens’s classic tale of redemption, opens with an admonition for readers to pay close attention to a particular fact: “Old Marley was as dead as a door-nail.… This must be distinctly understood, or nothing wonderful can come of the story I am going to relate.” The early history of life has its own “Jacob Marley” facts that, like the old miser’s death in Dickens’s story, need to be understood if the narrative is to make sense. First of these is the metabolic diversity of prokaryotic microorganisms, key to any exploration of early biological history. We must come to grips with the many ways that prokaryotes make a living and how these tiny organisms fit onto the Tree of Life before we put our boots back on and return to the field as paleontologists.
    Like eukaryotes, many bacteria respire using oxygen. But other bacteria can respire using dissolved nitrate (NO 3 -) instead, and still others use sulfate (SO 4 2- ) ions or the metallic oxides of iron and manganese. A few prokaryotes can even use CO 2 to react with acetic acid, generating natural gas, which is methane (CH 4 ). Prokaryotic organisms have also evolved a galaxy of fermentation reactions.
    Bacteria ring changes on the theme of photosynthesis, as well. The cyanobacteria, a group of photosynthetic bacteria tinted blue-green by chlorophyll and other pigments, harvest sunlight and fix CO 2 much like eukaryotic algae and land plants. However, when hydrogen sulfide(H 2 S, well known for its “rotten egg” smell) is present, many cyanobacteria use this gas rather than water to supply the electrons needed for photosynthesis. Sulfur and sulfate are formed as by-products, but oxygen is not.
    The cyanobacteria comprise only one of five distinct groups of photosynthetic bacteria. In the other groups, electron supply by H 2 S, hydrogen gas (H 2 ), or organic molecules is obligatory, and oxygen is never produced. These photosynthetic bacteria harvest light using bacteriochlorophyll rather than the more familiar chlorophyll. Some employ the same biochemistry as cyanobacteria and green plants to fix carbon dioxide, but others have distinctively different pathways, and still others rely on carbon already packaged into organic molecules.
    Bacterial variations on the