How about ozon layer theories, now ..?
Oxygen-Breathing Organisms
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As the ozone layer matured sometime in the past, survival no longer meant protection by a layer of water, or by some rock or other object acting as a barrier against what must have earlier been a truly hellish world. Life became possible on the surface of the water and eventually on the surface of the land. Organisms were on their way toward spreading at will, populating nearly every available nook and cranny on planet Earth. In short, life could invade areas where no life had existed before.
None of this happened overnight. The ozone layer needed time to thicken enough to screen out most of the harmful ultraviolet radiation. The process was an accelerating one: Oxygen-producing autotrophs had an increased chance for survival and therefore replication. The more offspring they produced, the more oxygen they dumped into the atmosphere. And more oxygen meant more ozone, more protection from solar radiation, and enhanced opportunities for survival. But it still took time for the protective ozone to cumulate. Perhaps as much as 2 billion years after the onset of photosynthesis were needed since dissolved iron in the oceans would have combined with any free oxygen, removing it from the atmosphere until waters everywhere were saturated.
Models of Earth’s early atmosphere imply that the ozone layer started to form, or at least oxygen gas had initially begun to rise, somewhat >2 billion years ago. Deposits of oxidized iron (called “red-bed” sediments or banded-iron formations) in the geological record of that date, now mined for their metal to make steel, support the view that oxygen was then hardly 1% of Earth’s air, well below the ~20% we enjoy today. Some of the most ancient fossils, dating back earlier than this as noted shortly, do show evidence for chlorophyll products, suggesting that oxygen was then being released into the atmosphere—but to what extent is unknown. Other models imply that oxygen didn’t reach its current levels nor did the ozone layer become a fully effective shield of solar radiation until ~0.5 billion years ago. Fossil evidence also supports that argument as life rather suddenly became varied and widespread ~550 million years ago, before which only primitive life forms existed. Shortly thereafter, a rapid surge in numbers and diversity of complex living organisms came forth—a population explosion of the first magnitude.
Figure 6.1 plots the overall expansion of life in relatively recent times, as revealed by the fossil record. Since ~550 million years ago, the number and diversity of life forms have risen. The one major exception, represented by the dip in the plot ~250 million years ago, probably resulted from a widespread ecological crisis discussed later in this BIOLOGICAL EPOCH. The demise of the dinosaurs and many other life forms ~65 million years ago, also noted later in this epoch, was so small by comparison that it doesn’t even register on this plot.
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Had nothing changed, Earth’s simplest life forms would have proceeded toward an evolutionary dead end—starvation. Earth would be a barren, lifeless rock, and our story aborted. Fortunately, something did change. It had to; nothing fails to change. And one change that did occur enabled the story to continue—not by some design and not solely by chance, rather more likely by the usual mixture of chance and necessity operating over long durations. At least partly, successful evolution is often a case of being at the right place at the right time.
Photosynthesis
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Other cells—the forerunners of plants, called autotrophs (for they were self-nourishing)—invented a new way to get energy, thereby conceiving a unique opportunity for living. (Some researchers claim that the first cells were likely already autotrophic, acquiring energy directly from the environment and skipping altogether the heterotrophic stage.) This novel biological technique employed carbon dioxide (CO2), the major waste product of the fermentation process. While the earliest cells were busily eating organic molecules in the sea and thus polluting the atmosphere, more advanced cells were learning to use these pollutants to extract energy. In this case, the energy wasn’t derived from the consumed gas, but from another well-known source—the Sun. This newly invented process is photosynthesis, perhaps the greatest single metabolic invention in history.
The key here is the chlorophyll molecule, a green pigment having its atoms arranged so that light, when striking the surface of a plant, is captured within the molecule. Advanced cells containing chlorophyll thereby extract energy from ordinary, gentle sunlight (not harsh ultraviolet radiation) by means of a chemical reaction that exploits that sunlight to convert carbon dioxide and water into oxygen and carbohydrates; simplified, it’s symbolized by the formula:
carbon dioxide (CO2) + water (H2O) + sunlight --> oxygen (O2) + carbohydrate (CH2O).
The oxygen gas escapes into the atmosphere, while the synthesized carbohydrate (sugar) is used for food. This, then, is another way a cell can “eat,” or extract energy from its environment—hence its name: photo, meaning “light”; synthesis, “putting together.”
How did some protoplant, microbial cells develop photosynthesis? To be sure, it was primitive bacteria that invented photosynthesis, not plants per se, which emerged much later. But how they actually did it, biologists are again uncertain, other than to presume that random events first altered the DNA molecules in some early cells, which then determinedly sucked up the needed solar energy to survive. They no longer had to compete for the organic acids and bases in the primal ocean. They were selected by Nature to endure because they adapted to the changing environment. And with photosynthesis came a big advantage since the new cells could persist on merely inorganic matter. The autotrophs were clearly more fitted for survival during what was probably the first ecological crisis on our planet.
Photosynthesis freed the early life forms from total dependence on the diminishing supply of organic molecules in the oceanic broth. Fermentation within heterotrophs was no longer needed for survival. Early cells able to utilize sunlight overspread the watery Earth. In time—much time—the autotrophs changed into not only many types of bacteria but also all the varied types of plants now strewn across the face of our planet.
The photosynthetic process continues to this day as plants routinely use sunlight to produce carbohydrates as food (for both metabolic function as well as cellulose structure). The plants, in turn, release oxygen gas that animals, including ourselves, breathe. Photosynthesis is, in fact, the most frequent chemical reaction on Earth. In round numbers, each day ~400 million tons (~1012 kilograms) of carbon dioxide mix with ~200 million tons of water to make ~300 million tons of organic matter and another 300 million tons of oxygen gas. Yet despite these large numbers, it’s still the small but abundant stuff that does much of it: Fully half of today’s global photosynthesis and oxygen production is accomplished by single-celled marine plankton living in the top oceanic layer where enough light penetrates to support their growth.
By loss of their food source, the ancient and primitive heterotrophs were naturally selected to die. The better adapted autotrophs were naturally favored to live. Life on Earth was on its way toward using a primary and plentiful source of energy—that of our parent star—in a reasonably efficient and direct manner. It all began not quite 3 billion years ago.
Photosynthesis over eons of time is, by the way, partly responsible for the fossil fuels. Dead, rotted plants, buried and squeezed below layers of dirt and rock, have chemically changed over megacenturies into oil, coal, and natural gas. Such fossil fuels, with their vast quantities of solar energy trapped in carbohydrates, have made industrial civilization possible. But those fuels are virtually nonrenewable, at least over time scales shorter than tens of millions of years. Billions of years of energy deposits in rotted organisms will be depleted shortly—oil and gas in the 21st century and coal not more than a few hundred years thereafter. Once again, things will have to change, just as they’ve changed in the past.
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http://www.tufts.edu/as/wright_cente...ext_bio_1.html
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23-April-2007, 10:48 PM
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