Roughly when did the transition from an oxygen-free atmosphere to an oxygenated one occur?
112629
Very roughly 2 billion years ago.  But you should be aware of two points. \n\nThe first is that the date is a matter of considerable debate and it could easily be half a billion years either side of that.\n\nThe second is that the transition was extremely slow, starting with the addition of oxygen by inorganic processes and then taking a massive leap with the evolution of oxygen producing autotrophs.
45254
Although in the 1950s' experiments, the early atmosphere of Earth was thought to be highly reducing, with methane, ammonia and hydrogen, currently many researchers consider that this early atmosphere was not so reducing, and it was dominated by carbon dioxide (CO2) rather than methane (CH4), and molecular nitrogen (N2) instead of ammonia (NH3). There would also have been sulphur gases from volcanic activity.
1005803
It was first suggested in the 1920s that simple organic molecules could form and spontaneously polymerize into macromolecules under the conditions thought to exist in primitive Earth's atmosphere. At the time life arose, the atmosphere of Earth is thought to have contained little or no free oxygen, instead consisting principally of CO2 and N2 in addition to smaller amounts of gases such as H2, H2S, and CO. Such an atmosphere provides reducing conditions in which organic molecules, given a source of energy such as sunlight or electrical discharge, can form spontaneously. The spontaneous formation of organic molecules was first demonstrated experimentally in the 1950s, when Stanley Miller (then a graduate student) showed that the discharge of electric sparks into a mixture of H2, CH4, and NH3, in the presence of water, led to the formation of a variety of organic molecules, including several amino acids. Although Miller's experiments did not precisely reproduce the conditions of primitive Earth, they clearly demonstrated the plausibility of the spontaneous synthesis of organic molecules, providing the basic materials from which the first living organisms arose.
363702
changes oxygen into carbon dioxide, the carbon come from the burning fuel
449167
One of the major gases is inert-Argon, and doesnt really seem to cycle-it makes up about 1%.  The primary gas is nitrogen, a small amount of which is fixed in the roots of plants (or by symbiotic organisms in those roots) and lifeforms such as blue-green algae-it makes up about 78%.  The greatest cycling gas is oxygen, which makes up about 21%.  Free oxygen, as found in our atmosphere, is extremely rare and likely would not be found in the absence of life-Oxygen in very reactive-many elements and compounds readily oxidize, such as iron turning to rust.  On Earth, plants take in carbon dioxide and split it, using the carbon to build things such as carbohydrates- the oxygen is releases, and used by aerobic lifeforms for respiration.  The level of carbon dioxide is about 0.1%- This is also very unusual, since in other planets with an atmosphere, carbon dioxide is a major gas...again, low CO2 is probably a sign of life.  The CO2 is low because the plants use it-it is returnd to the atmosphere by respiration.
562710
July 1884\n\nI hope this helps ... the link below explains more and is very informative as well.\n\n- Alan
795053
While, indeed, Michał Sędziwój first wrote about oxygen. \n\n A Swedish person named Carl Wilhelm Scheele was a little more scientific in his description and used scientific methods. But, he did not published his findings until after the discovery of Oxygen by Joseph Priestley. Priestley published his discovery, consequently Priestley is usually given the credit.\n\nGet an "A".
259202
Here is a link that might help.  It is part of a general page on the Ozone Layer...\n\nhttp://en.wikipedia.org/wiki/Ozone_layer#Ozone_depletion\n\nhttp://en.wikipedia.org/wiki/Ozone_layer
1027730
Oxygen was first discovered by Michał Sędziwój, a Polish alchemist and philosopher in the late 16th century. Sędziwój assumed the existence of oxygen by warming nitre (saltpeter). He thought of the gas given off as "the elixir of life".\nOxygen was again discovered by the Swedish pharmacist Carl Wilhelm Scheele sometime before 1773, but the discovery was not published until after the independent discovery by Joseph Priestley on August 1, 1774, who called the gas dephlogisticated air (see phlogiston theory). Priestley published his discoveries in 1775 and Scheele in 1777; consequently Priestley is usually given the credit. It was named by Antoine Laurent Lavoisier after Priestley's publication in 1775.
862118
Because they can happen with oxygen present.  These processes require another electron acceptor to replace oxygen as in aerobic respiration.  The word anaerobic literally means without the presence of oxygen
1103330
My thought is air as well. Air is composed of around 2/3s part of Nitrogen. Oxygen is a much smaller amount
945520
The Earth's primitive atmosphere was much different from today's and consisted primarily of ammonia, methane, and trace amounts of carbon dioxide and water vapor.  There was little, if any, free oxygen present.
382716
It was warmer and wetter than today, and there were higher levels of oxygen.  The higher levels are oxygen were what allowed invertabrates, such as early scorpions, to grow to such a large size.
1136574
look up bernoulis theorum...this will tell you all you need to know reference changes in pressure with respect to aeronatical engineering.\n\nGood Luck
222105
a) cyanobacteria \n\nOur atmosphere would have virtually no free oxygen nor any eukaryotes if not for them....
548720
HISTORY OF OXYGEN\n\nIt's difficult to think of free oxygen, which makes up 21 percent of earth's atmosphere, as a poisonous gas. Virtually all organisms alive today need oxygen to survive. Even plants, which expel oxygen into the atmosphere, need oxygen to live. There was a time early in earth's history, however, when living organisms not only did not need oxygen, but were killed when exposed to the element. \n\nThree and a half billion years ago, Earth's atmosphere contained almost no free oxygen. Instead, it consisted mainly of carbon dioxide, perhaps as much as 100 times more carbon dioxide than contained in today's atmosphere. During this time, Earth's only life forms were aquatic, one-celled organisms -- primitive forms of bacteria -- that extracted energy from a variety of sources. To most of these organisms, free oxygen was poisonous. \n\nAt about the same time, new types of primitive bacteria evolved. Unlike their predecessors, these bacteria could convert energy directly from the Sun, through photosynthesis. One type, bluish-green organisms called cyanobacteria, used sunlight to convert carbon dioxide and water into glucose (sugar). These organisms expelled oxygen gas -- a by-product of photosynthesis -- into the oceans. \n\nAs the cyanobacteria multiplied and spread around the globe, they released more and more free oxygen into the oceans. This oxygen combined with the molecules of living bacteria, killing them off as it did so. It also combined with other matter, such as the iron dissolved in the ocean. Only after the oxygen combined with all of the matter available did free oxygen begin to accumulate in the oceans. And when the oceans became saturated with oxygen, it began to make its way into the atmosphere. \n\nOrganisms that don't need free oxygen to survive are called anaerobic; all living things that need free oxygen, including us, are aerobic organisms. As the oceans and atmosphere filled with oxygen gas, aerobic life evolved further, at some point adding a nucleus to the cell (to protect the cell's DNA from the harmful effects of oxygen) and eventually to multi-cellular organisms. Anaerobic life, on the other hand, was pushed back to areas where there was little or no oxygen. Today, anaerobic organisms like those that existed more than three and a half billion years ago live in oxygen-free places such as volcanic hot vents, where they get their energy in ways other than from carbon dioxide, water, and sunlight.\n\nIMPORTANCE OF OXYGEN\n\nCirculatory System, or cardiovascular system, in humans, the combined function of the heart, blood, and blood vessels to transport oxygen and nutrients to organs and tissues throughout the body and carry away waste products. Among its vital functions, the circulatory system increases the flow of blood to meet increased energy demands during exercise and regulates body temperature. In addition, when foreign substances or organisms invade the body, the circulatory system swiftly conveys disease-fighting elements of the immune system, such as white blood cells and antibodies, to regions under attack. Also, in the case of injury or bleeding, the circulatory system sends clotting cells and proteins to the affected site, which quickly stop bleeding and promote healing.\n\nRespiratory System, in anatomy and physiology, organs that deliver oxygen to the circulatory system for transport to all body cells. Oxygen is essential for cells, which use this vital substance to liberate the energy needed for cellular activities. In addition to supplying oxygen, the respiratory system aids in removing of carbon dioxide, preventing the lethal buildup of this waste product in body tissues. Day-in and day-out, without the prompt of conscious thought, the respiratory system carries out its life-sustaining activities. If the respiratory system’s tasks are interrupted for more than a few minutes, serious, irreversible damage to tissues occurs, followed by the failu
160712
Oxygen was first discovered by Michał Sędziwój, Polish alchemist and philosopher in late 16th century. Sędziwój assumed the existence of oxygen by warming nitre (saltpeter). He thought of the gas given off as "the elixir of life".\n\nOxygen was again discovered by the Swedish pharmacist Carl Wilhelm Scheele sometime before 1773, but the discovery was not published until after the independent discovery by Joseph Priestley on August 1, 1774, who called the gas dephlogisticated air (see phlogiston theory). Priestley published his discoveries in 1775 and Scheele in 1777; consequently Priestley is usually given the credit. It was named by Antoine Laurent Lavoisier after Priestley's publication in 1775.