Four and a half billion years ago, the debris and dust left from the formation of the sun coalesced to form our home planet. 3. 5 billion years ago, the first living organisms appeared on Earth. About 230 million years ago, Dinosaurs diverged from their Archosaurs ancestors during the middle to late Triassic period. For 160 million years they have dominated our planet. They are dubbed the most successful species to have lived on Earth. However, 65 million years ago, the most recent mass extinction seemed to have caused all of them to die-off.
What caused the demise of the dinosaurs and 60% of life on Earth at the time? How did it affect life on Earth afterwards? Are all of the Dinosaurs dead? Will this happen to humans in the near future? The Cretaceous-Tertiary extinction event, or the K-T event, is the name given to the mass extinction of the dinosaurs and other species that took place about 65 million years ago. For many years, paleontologists believed this event was caused by climate and geological changes that interrupted the dinosaurs’ food supply.
However, in the 1980s, father-and-son scientists Luis (1911-88) and Walter Alvarez (1940- ) discovered in the geological record, a distinct layer of iridium (an element found in abundance only in space) that corresponds to the precise time the dinosaurs died. This suggests that a comet, asteroid or meteor impact event may have caused the extinction of the dinosaurs. In the 1990s, scientists located the massive Chicxulub Crater at the tip of Mexico’s Yucatan Peninsula, which dates to the period in question.
Dinosaurs roamed the earth for 160 million years until their sudden demise approximately 65 million years ago, in an event now known as the Cretaceous-Tertiary, or K-T, extinction event. (“K” is the abbreviation for Cretaceous, which is associated with the German word “Kreidezeit. “). Besides dinosaurs, many other species of mammals, amphibians and plants died out at the same time. Over the years, paleontologists have proposed several theories for this extensive die-off.
One early theory was that small mammals ate dinosaur eggs, thereby reducing the dinosaur population until it became unsustainable. Another theory was that dinosaurs’ bodies became too big to be operated by their small brains. Some scientists believed a great plague decimated the dinosaur population and then spread to the animals that feasted on their carcasses. Starvation was another possibility: Large dinosaurs required vast amounts of food and could have depleted all the vegetation in their habitat. Many of these theories are easily dismissed.
If dinosaurs’ brains were too small to be adaptive, they would not have flourished for 160 million years. Also, plants do not have brains nor do they suffer from the same diseases as animals, so their simultaneous extinction makes these theories less plausible. For many years, climate change was the most credible explanation for the dinosaurs’ demise. Dinosaurs thrived in the planet’s consistently humid, tropical climate. Yet, in the late Mesozoic Era that corresponds with the extinction of the dinosaurs, evidence shows that the planet slowly became cooler.
Lower temperatures caused ice to form over the North and South poles and the oceans to become colder. Because the dinosaurs were cold-blooded (meaning they obtained body heat from the sun and air) they would not have been able to survive in significantly colder climates. Yet some species of cold-blooded animals, such as crocodiles, did manage to survive. Also, climate change would have taken tens of thousands of years, giving the dinosaurs sufficient time to adapt.
In 1956, Russian astronomer Joseph Shklovsky (1916-85) became the first scientist to consider the extinction was due to a single catastrophic event when he theorized that a supernova (the explosion of a dying star) showered the earth in radiation that could have killed the dinosaurs. Once again, the problem with the theory was explaining why dinosaurs died out and other species did not. Also, scientists said that such an event would have left evidence on the surface of the earth–trace amounts of radiation dating back to the Cretaceous Period.
None was found. Luis Alvarez was a Nobel Prize-winning physicist, inventor and pioneer in the field of radiation and nuclear research. He and his son, noted geologist Walter Alvarez, were conducting research in Italy when they discovered a centimeter-thick layer of iridium-enriched clay at the K-T boundary. Iridium is rare on earth, but more common in space. The Alvarezes published their findings in 1981, postulating that the thin layer of iridium was deposited following the impact of a large meteor, comet or asteroid with the earth.
Furthermore, this bolide impact (the meteor, comet or asteroid colliding with the earth’s surface) could have caused the extinction of the dinosaurs. At the time, the Alvarez theory was so farfetched from prevailing hypotheses that it was derided. Slowly, other scientists began finding iridium evidence at various places around the globe that corroborated the Alvarez theory. There was, however, no smoking gun in the form of an impact site. Then in 1991, a massive meteor crater 110 miles in diameter was discovered on the edge of the Yucatan Peninsula, extending into the Gulf of Mexico.
The Chicxulub Crater, as it was dubbed, was named for a nearby village. Scientists believe the bolide that formed it was roughly 6 miles in diameter, struck the earth at 40,000 miles per hour and released 2 million times more energy than the most powerful nuclear bomb ever detonated. The heat would have broiled the earth’s surface, ignited wildfires worldwide and plunged the planet into darkness as debris clouded the atmosphere. Colossal tsunamis would have washed over the continents, drowning many forms of life. Shock waves would have triggered earthquakes and volcanic eruptions.
The resulting darkness could have lasted for months, possibly years. It would have plunged the earth’s temperatures into the freezing zone, killing plants and leaving herbivores with nothing to eat. Many dinosaurs would have died within weeks. The carnivores who feasted on the herbivores would have died a month or two later. Overall, the loss of biodiversity would have been tremendous. Only small scavenging mammals that could burrow into the ground and eat whatever remained would have survived. The iridium layer plus the Chicxulub Crater were enough evidence to convince many scientists that the bolide impact theory was credible.
It explained much of what previous theories could not. Before 2000, arguments that the Deccan Traps flood basalts caused the extinction were usually linked to the view that the extinction was gradual, as the flood basalt events were thought to have started around 68 million years ago and lasted for over 2 million years. However, there is evidence that two-thirds of the Deccan Traps were created in only 1 million years about 65. 5 million years ago, and so these eruptions would have caused a fairly rapid extinction, possibly over a period of thousands of years, but still longer than would be expected from a single impact event.
The Deccan Traps could have caused extinction through several mechanisms, including the release of dust and sulphuric aerosols into the air, which might have blocked sunlight and thereby reduced photosynthesis in plants. In addition, Deccan Trap volcanism might have resulted in carbon dioxide emissions, which would have increased the greenhouse effect when the dust and aerosols cleared from the atmosphere. Before the mass extinction of the dinosaurs, the release of volcanic gases during the formation of the Traps contributed to an apparently massive global warming.
Some data points to an average rise in temperature of 8 °C (14 °F) in the last half million years before the impact at Chicxulub. In the years when the Deccan Traps theory was linked to a slower extinction, Luis Alvarez replied that paleontologists were being misled by sparse data. While his assertion was not initially well-received, later intensive field studies of fossil beds lent weight to his claim. Eventually, most paleontologists began to accept the idea that the mass extinctions at the end of the Cretaceous were largely or at least partly due to a massive Earth impact.
However, even Walter Alvarez has acknowledged that there were other major changes on Earth even before the impact, such as a drop in sea level and massive volcanic eruptions that produced the Indian Deccan Traps and these may have contributed to the extinctions. Graeme T. Lloyd et al (2008) noted that, in the Mid Cretaceous, the flowering, angiosperm plants became a major part of terrestrial ecosystems, which had previously been dominated by gymnosperms such as conifers. Dinosaur coprolites (fossilized feces) indicate that, while some ate angiosperms, most herbivorous dinosaurs mainly ate gymnosperms.
Statistical analysis by Lloyd et al. concluded that, contrary to earlier studies, dinosaurs did not diversify very much in the Late Cretaceous. Lloyd et al. suggested that dinosaurs’ failure to diversify as ecosystems were changing doomed them to extinction. While some mammals seized the day and diversified after the mass extinction, they largely were evolutionary dead-ends, scientists said. Researchers led by Olaf Bininda-Emonds used DNA from some of the 4,500 species of mammals on Earth and fossils of extinct animals to devise a family tree tracing mammalian evolutionary history.
Mammals from the major groups around today arose tens of millions of years before the extinction and survived the calamity. They remained secondary to now-extinct mammal forms and did not start diversifying and asserting themselves until about 55 to 50 million years ago, the study found. The dinosaurs ruled the planet from about 225 million years ago until their demise along with the flying reptiles called pterosaurs, the marine reptiles called mosasaurs and a bunch of other animals. The first mammals appeared roughly 220 million years ago and the first directly related to today’s mammals arose about 125 million years ago.
Nevertheless, these furry little creatures remained largely an evolutionary afterthought, doing their best to avoid becoming dinosaur prey before the extinction. Some of the mammals that flourished in the time shortly after the dinosaurs died included cat-sized, rodent-like Ptilodus, squirrel-like primate relative Plesiadapis, dog-sized meat eaters called creodonts and the lion-sized carnivorous Andrewsarchus, known for its fearsome jaws. Most of the present-day groups of placental mammals arose between 100 and 85 million years ago during the Cretaceous Period.
Placental mammals give birth to live young and include rodents, carnivores, primates and hoofed mammals. Other types of mammals include marsupials, like kangaroos, and the very rare monotremes like the platypus that lay eggs. The mass extinction event paved the way for mammals to dominate. If dinosaurs did not die out, the consequences would lead to a dramatic decrease of mammals in the world’s ecosystem. Mammals would not be given a good chance to evolve and human beings may not have even evolved. There have been a total of 5 mass extinctions throughout the history of the Earth.
The K-T event has wiped out 60% of the species on Earth. There may be a possibility of another mass extinction in our near future. The force of the explosion would have killed off everything within 500 km. Smoke and ash would have clogged the planet’s atmosphere, blocking out sunlight and blocking photosynthesis for many months, causing the collapse of food chains. Paleontologist David Raup estimated that extinction-sized strikes happen, on average, once every 100 million years. Supervolcanoes are giant eruptions of lava that can last a million years and cover a million km2 of the surface.
Supervolcanoes ooze lava from long fractures in the Earth’s crust and give off trillions of tonnes of carbon dioxide, sulphur dioxide, fluorine and chlorine. This pollution would cause erratic fluctuations in temperature and acid rain on global scales. Supervolcanoes have been cited as possible causes for mass extinction events at the end of the Permian, Triassic, and Cretaceous periods. With an average of one every eight to ten million years, we’re not due another for several million, says Paul Wignall of the University of Leeds. In any case, distortion of the Earth’s crust should give 10,000 years notice, he says.
Many scientists believe that humans are the deadliest cause of extinctions and we may be in the midst of a mass extinction of our own making – the Holocene event. It’s estimated that humans contribute to 99% of all modern species extinctions, by overexploiting resources, polluting and destroying habitats, enhancing the spread of disease and pest species, and accelerating climate change. Two 2004 papers in Science documented steep declines in organisms as diverse as plants, birds and insects over the last 20 years. If these trends continue, more than half of the current species living today will have gone extinct.
Contemporary Chinese History
Contemporary Chinese History.
The 6-pages essay needs to answer two questions: 1. Please explain the dynamics between Chinese government and popular nationalism in the post-Tiananmen era. Use examples from the course material to illustrate your points.(Hint: lectures, Osnos and Zheng) 2. Your friend says, “China is an authoritarian state. The party-state exercises absolute control over society and does not allow any expression of public opinion.” Do you agree with this comment? Or do you see a more complex and nuanced picture when it comes to the relationship between the Chinese government and the Chinese public? Please use examples to illustrate your points. (Hint lectures, Osnos, and Shapiro). The format of the essay is not restricted. Needed reading material: Evan Osnos, Age of Ambition, Chapter 10, 4, 5, 18, 9, 23, 11, 13-15. Judith Shapiro, China’s Environmental Challenges: 2, 3, 5,6. Yongnian Zheng, Contemporary China: a History since 1978, chapter 7.
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