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Margulis was doing reserarch on the originof eukaryotic cells.

What Is Endosymbiotic Hypothesis

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Endosymbiont theory for origin of mitochondria

has been a prominent hypothesis that posited that behaviorally modern humans suddenly appeared. It was once considered an abrupt event that began about 50-40 kya, but as new archeological finds are amassed, as well as recent advances in genetic research and other areas, the story is familiar. Although on the geological timescale the event was abrupt, radical, and unprecedented in life’s history on Earth, the “ramping” period seems to have lasted longer than initially thought. A likelier story is that in East Africa, which conforms to a . inherited culture and tools from their ancestors and continued along the path of inventing more complex technologies and techniques, exploiting new biomes, and reaching new levels of cognition. There does not seem to be any or development that needs to invoke divine or extraterrestrial intervention to explain the appearance and rise of . Some migrated past their African homeland during the of 130 kya to 114 kya and brought along their technology. Although they may have disappeared and perhaps became Neanderthal prey, vestiges of their fate are probably yet to be discovered. They may have contributed to the biological and technological wealth of Eurasian humans and may have begun to drive vulnerable species to extinction with their new tools and techniques. However, Africa remained the crucible of primate biological and technological innovation, as it almost always had to that time. By 70-60 kya, isolated African humans reached a level of sophistication called behavioral modernity. Art was in evidence, needles made clothes and other sophisticated possessions, and they mastered language, which was probably a unique trait among land animals. They made tools of a sophistication far advanced over other humans, which probably included projectile weapons that radically changed the terms of engagement with prey animals, predators, and other humans.

The mitochondria and chloroplasts have their own genomes, but can not live outside their host cell.

Birds are warm-blooded and today’s reptiles are cold-blooded. is a vast, complex issue, and warm-bloodedness or cold-bloodedness appears to be a result of evolutionary cost-benefit outcomes. The first vertebrates that , the first dominant reptiles had , and therapsids may have at least dabbled in chemical means of internal temperature regulation, although the evidence is thin. But the evidence for dinosaurian internal temperature regulation is strong, and the surviving therapsid line, the mammals, also developed internal temperature regulation.

History: The Formation of the Endosymbiotic Hypothesis

The Triassic began hot and ended hot, and the Jurassic and Cretaceous were also hot, so staying warm was not a significant issue for dinosaurs. stayed cool by becoming aquatic, and for land-based dinosaurs, features such as plates apparently replaced the sails of for both heating and cooling, and like the synapsid sail, those plates may have also been used for display. Also, like the cliché, many large herbivorous dinosaurs lived near cooling swamps, although the issue has been controversial. Cooling swamps and protective water holes that we see in the tropics today were a major aspect of Mesozoic landscapes. But the thermoregulatory aspect that most work is directed toward today is how dinosaurs kept warm. There is compelling evidence that dinosaurs regulated their body temperature in myriad ways, including internal chemistry. All bipedal animals today are endotherms and they all have four-chambered hearts, as dinosaurs did. , dinosaurs living near the poles (, ), and of dinosaur bones all support the idea that , but one of the more intriguing areas is that of . Like tree rings, bones have seasonal growth rings and they have been read for many dinosaur fossils. They have been used to determine dinosaurian life expectancies. could live to be about 30, giant could live to be 50, and smaller dinosaurs, as with smaller mammals, lived shorter lives. The tiny ones only lived three-to-four years and the mid-sized ones lived seven-to-fifteen years. Growth rates also provide thermoregulation evidence. Tyrannosaurs had juvenile growth spurts and largely stopped growing as adults, and sauropods had growth rates equivalent to today’s whales, which are Earth’s fastest growing animals. But there is also evidence of ectothermic dynamics. The great size of dinosaurs would have led to relatively easy ways to stay warm, as large animals have a greater mass-to-surface area ratio, like the way in which . Also, in the generally hot Mesozoic times, staying warm would have been fairly easy, particularly for huge dinosaurs.

The view of dinosaurian intelligence has also changed radically in the past generation, as evidence has been discovered that some dinosaurs were significantly (particularly the ), as well as evidence for , and . Dinosaurs had the first hands, even with . Recent work on encephalization suggests that animals were well on their way toward human-level encephalization hundreds of millions of years ago, and were prevented from attaining it far earlier, , due to the Permian extinction. The world might be populated with sentient, civilized, and even space-faring reptiles today if events had played out slightly differently, such as that asteroid missing Earth 66 mya (or technologically advanced dinosaurs preventing its impact).

Endosymbiosis - The Appearance of the Eukaryotes

, a great clade of herbivorous dinosaurs, appeared , but were initially marginal dinosaurs and did not begin becoming abundant until the late Jurassic. If dinosaurs all have the same common ancestor, ornithischian dinosaurs quickly diverged, with their different hips, and so far, there is no good evidence that ornithischians breathed with the air sac system, and they became the dominant herbivores in the relatively high-oxygen Cretaceous. The ornithischian advantage was a superior eating system. Ornithischians were the only dinosaurs that chewed their food. Chewing squeezes more calories from plant matter and may be why ornithischians surpassed sauropods in the Cretaceous. Sauropods did not chew their food but had rock-filled , as birds and reptiles do today. in the late Triassic. Only rare ornithischians had gizzards. Sauropods also had the smallest proportional brains of any dinosaur. The most encephalized dinosaurs were , some of which were featured as clever killers in . Theropods were the most encephalized dinosaurs, which is an early example of predators having larger brains in order to outsmart their prey. were in second place only to theropods in encephalization and were among the most successful Cretaceous herbivores. A fascinating aspect of some ornithopods was their seeming ability to communicate by bugling with a . This kind of evidence strongly supports the idea of herd behavior in herbivorous dinosaurs. There is also evidence of a , which has been keenly contested (, ) in recent years.

The ecosystems may not have recovered from Olson’s Extinction of 270 mya, and at 260 mya came another mass extinction that is called the mid-Permian or extinction, or the , although a recent study found only one extinction event, in the mid-Capitanian. In the 1990s, the extinction was thought to result from falling sea levels. But the first of the two huge volcanic events coincided with the event, in . There can be several deadly outcomes of major volcanic events. As with an , massive volcanic events can block sunlight with the ash and create wintry conditions in the middle of summer. That alone can cause catastrophic conditions for life, but that is only one potential outcome of volcanism. What probably had far greater impact were the gases belched into the air. As oxygen levels crashed in the late Permian, there was also a huge carbon dioxide spike, as shown by , and the late-Permian volcanism is the near-unanimous choice as the primary reason. That would have helped create super-greenhouse conditions that perhaps came right on the heels of the volcanic winter. Not only would carbon dioxide vent from the mantle, as with all volcanism, but the late-Permian volcanism occurred beneath Ediacaran and Cambrian hydrocarbon deposits, which burned them and spewed even more carbon dioxide into the atmosphere. Not only that, great salt deposits from the Cambrian Period were also burned via the volcanism, which created hydrochloric acid clouds. Volcanoes also spew sulfur, which reacts with oxygen and water to form . The oceans around the volcanoes would have become acidic, and that fire-and-brimstone brew would have also showered the land. Not only that, but the warming initiated by the initial carbon dioxide spike could have then warmed up the oceans enough so that methane hydrates were liberated and create even more global warming. Such global warming apparently warmed the poles, which not only melted away the last ice caps and ended an ice age that had , but deciduous forests are in evidence at high latitudes. A 100-million-year Icehouse Earth period ended and a 200-million-year Greenhouse Earth period began, but the transition appears to have been chaotic, with wild swings in greenhouse gas levels and global temperatures. Warming the poles would have lessened the heat differential between the equator and poles and further diminished the lazy Panthalassic currents. The landlocked Paleo-Tethys and Tethys oceans, and perhaps even the Panthalassic Ocean, may have all become superheated and anoxic as the currents died. Huge also happened, which may have and led to ultraviolet light damage to land plants and animals. That was all on top of the oxygen crash. With the current state of research, all of the above events may have happened, in the greatest confluence of life-hostile conditions during the eon of complex life. A recent study suggests that the extinction event that ended the Permian may have lasted only 60,000 years or so. In 2001, a bolide event was proposed for the Permian extinction with great fanfare, but it does not appear to be related to the Permian extinction; the other dynamics would have been quite sufficient. The Permian extinction was the greatest catastrophe that Earth’s life experienced since the previous supercontinent existed in the .

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Also, the formation of Pangaea ( regarding what processes ) may have led to the dynamics that broke it apart. The Hawaiian Islands are that began forming more than 80 mya, and is due to a hotspot bubbling up from Earth’s mantle. Although the is , a prominent hypothesis is that the formation of Pangaea plugged hotspots and prevented heat from venting from Earth’s core, which led to a swelling and fracturing Pangaea. Part of the evidence for that hypothesis was relatively sudden and widespread volcanism sprouting up around Pangaea, which followed a known fracture pattern around such crustal upwellings. The volcanism and resultant fracture lines formed today’s continents. As can be seen in the during the late Permian, what became China and Siberia were on the northeast margins of Pangaea, bordering the Paleo-Tethys Ocean, and two volcanic events arising from China and Siberia are currently favored as key proximate causes of the Permian extinctions.

endosymbiont hypothesis of mitochondrial origin ..

When sea levels rise as dramatically as they did in the Cretaceous, coral reefs will be buried under rising waters and the ideal position, for both photosynthesis and oxygenation, is lost, and reefs can die, like burying a tree’s roots. About 125 mya, reefs made by , which thrived on , began to displace reefs made by stony corals. They may have prevailed because they could tolerate hot and saline waters better than stony corals could. About 116 mya, an , probably caused by volcanism, which temporarily halted rudist domination. But rudists flourished until the late Cretaceous, when they went extinct, perhaps due to changing climate, although there is also evidence that the rudists . Carbon dioxide levels steadily fell from the early Cretaceous until today, temperatures fell during the Cretaceous, and hot-climate organisms gradually became extinct during the Cretaceous. Around 93 mya, , perhaps caused by underwater volcanism, which again seems to have largely been confined to marine biomes. It was much more devastating than the previous one, and rudists were hit hard, although it was a more regional event. That event seems to have , and a family of . On land, , some of which seem to have , also went extinct. There had been a decline in sauropod and ornithischian diversity before that 93 mya extinction, but it subsequently rebounded. In the oceans, biomes beyond 60 degrees latitude were barely impacted, while those closer to the equator were devastated, which suggests that oceanic cooling was related. shows rising oxygen and declining carbon dioxide in the late Cretaceous, which reflected a general cooling trend that began in the mid-Cretaceous. Among the numerous hypotheses posited, late Cretaceous climate changes have been invoked for slowly driving dinosaurs to extinction, in the “they went out with a whimper, not a bang” scenario. However, it seems that dinosaurs did go out with a bang. A big one. Ammonoids seem to have been brought to the brink with nearly marine mass extinctions during their tenure on Earth, and it was no different with that late-Cretaceous extinction. Ammonoids recovered once again, and their lived in the late Cretaceous, but the end-Cretaceous extinction marked their final appearance as they went the way of and other iconic animals.

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