summarize the difference between animals, (metazoans) and protozoans. All three are eukaryotes—organisms with large cells that contain a nucleus as well as other smaller organelles, such as mitochondria. Animals and “metazoans” are the same thing. All are composed of more than a single cell during their lives except at fertilization. Protozoans can seem animal-like, in that many are capable of movement and relatively complex behavior. But all of them are composed of only a single cell. Nevertheless, they are far larger and far more complex than bacteria.) Yet if that was agreed on, the reason for this was not. Life was capable of oxygenic photosynthesis, but there should have been far more life than all evidencesuggests. Animals need a good 10 percent of the atmosphere to have oxygen (we are at 21 percent today) and the “photosynthesizers” were not doing their job. The answer, when it finally came, was once again the element that runs through the history of these pages in an ever-repeating pattern: sulfur, usually in the guise of its most toxic and at the same time life-giving form—hydrogen sulfide, molecule of life and death. In a 2009 paper published in the Proceedings of the National Academy of Sciences , 2 Harvard paleobiologist Andy Knoll and his colleagues showed that oxygen levels should have been higher during the boring billion, but were not. Something was holding them back. The long interval devoid of any kind of real intermediary between the single-celled organisms of the 2.3-billion-year-old great oxidation event and the appearance of larger, multicellular creatures of far, far later in time was real.
There were no life forms that we might call complex in this long interval of time (although we hope it is clear from preceding chapters that even the simplest life forms on Earth are unbelievably complex when viewed at the molecular and chemical scale!). And the reason was an overabundance of single-celled sulfur-using bacteria that were competing with the oxygen-releasing forms. Thus it was that two very different life forms competed for resources coveted by all life—spaceand nutrients. The sulfur-requiring microbes, called green and purple sulfur bacteria, are still alive today, but only in the most toxic of places—shallow-water lakes and some seaways that have no oxygen but are shallow enough so that sunlight can penetrate to the levels of the bacteria, allowing photosynthesis. But the problem is that this kind of photosynthesis does not split water apart, and thus does not produce oxygen as a by-product.
Our new model for the rise of atmospheric oxygen and some of the related events.
Fundamentally, it seems that life was lazy . Splitting water is actually a difficult task, which generates all sorts of nasty, toxic compounds. Using H 2 S for photosynthesis instead of H 2 O results in less toxic sulfur compounds, and even many strains of cyanobacteria—if given the choice—will shut down their oxygen-generating machinery and use H 2 S rather than water.
For most of the boring billion the oceans were stratified, with a thin top layer of oxygenated, clean surface water where single-celled green algae took sunshine and used that energy for cell growth, all the while releasing oxygen. But beneath them, perhaps only ten or twenty feet below, was a totally different layer of seawater, and this layer would have extended all the way to the deepest ocean bottom. It would have been purple in color in its uppermost, shallowest regions, stained this color by the untold numbers of the purple sulfur microbes. The water they lived in would have been a fatal poison for most ocean life of our world, as it was filled with toxic hydrogen sulfide all brewed in a near-boiling miasma of liquid brimstone. Even in death they would have helped rob the world of oxygen (unconsciously, of course, although some microbial specialists actually seem convinced that the microbes have always been some kind of sneaky smart).
There were no life forms that we might call complex in this long interval of time (although we hope it is clear from preceding chapters that even the simplest life forms on Earth are unbelievably complex when viewed at the molecular and chemical scale!). And the reason was an overabundance of single-celled sulfur-using bacteria that were competing with the oxygen-releasing forms. Thus it was that two very different life forms competed for resources coveted by all life—spaceand nutrients. The sulfur-requiring microbes, called green and purple sulfur bacteria, are still alive today, but only in the most toxic of places—shallow-water lakes and some seaways that have no oxygen but are shallow enough so that sunlight can penetrate to the levels of the bacteria, allowing photosynthesis. But the problem is that this kind of photosynthesis does not split water apart, and thus does not produce oxygen as a by-product.
Our new model for the rise of atmospheric oxygen and some of the related events.
Fundamentally, it seems that life was lazy . Splitting water is actually a difficult task, which generates all sorts of nasty, toxic compounds. Using H 2 S for photosynthesis instead of H 2 O results in less toxic sulfur compounds, and even many strains of cyanobacteria—if given the choice—will shut down their oxygen-generating machinery and use H 2 S rather than water.
For most of the boring billion the oceans were stratified, with a thin top layer of oxygenated, clean surface water where single-celled green algae took sunshine and used that energy for cell growth, all the while releasing oxygen. But beneath them, perhaps only ten or twenty feet below, was a totally different layer of seawater, and this layer would have extended all the way to the deepest ocean bottom. It would have been purple in color in its uppermost, shallowest regions, stained this color by the untold numbers of the purple sulfur microbes. The water they lived in would have been a fatal poison for most ocean life of our world, as it was filled with toxic hydrogen sulfide all brewed in a near-boiling miasma of liquid brimstone. Even in death they would have helped rob the world of oxygen (unconsciously, of course, although some microbial specialists actually seem convinced that the microbes have always been some kind of sneaky smart).
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