Evolution: Extinction: What Killed the Dinosaurs? - PBS
That is because our ancestors seemed particularly fixated on âslaying the dragonâ!So, what happened to the dinosaurs?
What caused the extinction of the dinosaurs?
Habilines and australopithecines coexisted, and the went extinct about 2.0 mya. Robust australopiths survived to about 1.2 mya (, ), and habilines , so they overlapped the tenure of a species about which there is no doubt of its genus: , which first appeared about 2.0-1.8 mya, and the first fossils are dated to 1.8 mya. is the first human-line species whose members could pass for humans on a city street, if they dressed up and wore minor prosthetics on their heads and faces. had a protruding nose and was probably relatively hairless, the first of the human line to be that way. That was probably related to shedding heat in new, hot environments, as well as cooling its large brain (molecular data with head and body lice supports arguments that the human line became relatively hairless even before australopiths). There are great controversies about that overlap among those three distinct lines that might all have ancestral relationships. Oldowan culture was a multi-species one. There is plenty of speculation that the rise of and its successors caused the extinction of other hominids, driving them to extinction by competition, predation, warfare, or some combination of them. What is certain is that “competing” protohumans went extinct after coexisting with the human line for hundreds of thousands of years. The suspicion that evolving humans drove their cousins to extinction becomes more common as the timeline progresses toward today.
After as little as a half-million years of bedraggled survivors adapting to ice age seas, the ice sheets retreated and the oceans rose. The of the time may have also changed, and upwelling, anoxia, and other dramatic chemistry and nutrient changes happened. Those dynamics are suspected to be responsible for the second wave of extinctions. There also seem to have been .Atmospheric oxygen levels may have fallen from around 20% to 15% during the Ordovician, which would have contributed to the mass death. Seafloor anoxia seems to have been particularly lethal to continental-shelf biomes, possibly all the way to shore. It took the ecosystems millions of years to recover from the Ordovician-Silurian mass extinction, but basic ecosystem functioning was not significantly altered in the aftermath, which is why a has been proposed as a more significant extinction event. The were laid down by the . Most oil deposits were formed in the era of dinosaurs and the processes of oil deposit formation were similar; they were related to oceanic currents. When currents came to shore via the bottom and the prevailing winds blew the top waters offshore, it became a and anoxic sediments could form. When the winds blew onshore and left via the bottom, the waters became clear and are known as nutrient deserts. The oscillation between nutrient traps and nutrient deserts can be seen in oil deposit sediments. In the mid-20th century, Soviet scientists revived an old hypothesis that oil was , a variation of which was also championed by , but improving tools and investigation invalidated those hypotheses. No petroleum geologists today seriously consider the abiogenic origin of hydrocarbons. Oil sediment formation events seem related to mantle and crust processes that created high sea levels and anoxic events, and the last great one was in the , which formed more than 10% of the world's oil deposits.
Dinosaur extinction one more hypothesis
What was most relevant to humans, however, was the almost-complete extinction during the Kellwasser event of the tetrapods that had come ashore. Tetrapods did not reappear in the fossil record until several million years after the Kellwasser event, and has even been referred to as the Fammenian Gap (the is the Devonian’s last age). The Kellwasser event also appeared to be a period of low atmospheric oxygen content, and some evidence is the lack of charcoal in fossil deposits. Recent research has demonstrated that getting wood to burn at oxygen levels of less than 13-15% may be impossible. Because all periods of complex land life show evidence of forest fires, it is today thought that oxygen levels have not dropped below 13-15% since the Devonian, but during the “charcoal gap” of the late Devonian, when the first landlubbing tetrapods went extinct, oxygen levels reached their lowest levels since the , which must have impacted the first animals trying to breathe air instead of water. During the , there is no charcoal evidence at all, which leads to the notion that oxygen levels may have even dropped below 13%. This drop may be related to severe climatic stresses on the new forests, which are probably related to the ice age that the forests helped bring about due to their carbon sequestering. That is an attractively explanatory scenario, but the continues. The first seed plants probably appeared before the Kellwasser event, but it was not until after the Fammenian Gap that seed plants began to proliferate.
Peter Ward led an effort to catalog the fossil record before and after Romer’s Gap, which found a dramatic that did not resume until about 340-330 mya. Romer’s Gap seems to have coincided with low-oxygen levels of the late Devonian and early Carboniferous. If coincided with a halt in colonization, just as the adaptation to breathing air was beginning, the obvious implication is that low oxygen levels hampered early land animals. Not just the lung had to evolve for the up-and-coming amphibians, but the entire chest cavity had to evolve to expand and contract while also allowing for a new mode of locomotion. When amphibians and splay-footed reptiles run, they cannot breathe, as their mechanics of locomotion prevent running and breathing at the same time. Even walking and breathing is generally difficult. This means that they cannot perform any endurance locomotion but have to move in short spurts. This is why today’s predatory amphibians and reptiles are ambush predators. They can only move in short bursts, and then have to stop, breathe, and recover their oxygen deficit. In short, they have no stamina. This limitation is called . The below image shows the evolutionary adaptations that led to overcoming Carrier's Constraint. Dinosaurs overcame it first, and it probably was related to their dominance and the extinction or marginalization of their competitors. (Source: Wikimedia Commons)
New evidence supports asteroid theory of dinosaur extinction
The rise of life was based on . Just as the remained basically unchanged since the Cambrian Explosion, energy systems form the foundations for all ecosystems and civilizations. While the superstructure change and can seem radical at times, the foundation dictates what kind of superstructure exist. A huge superstructure built on a small foundation, if it can be built at all, will not be very resilient (the first earthquake or storm levels it), and will not last long. Today, industrialized civilization is burning through its foundational energy sources a million times as fast as they were created and will largely . On the , the rise and fall of humanity may happen in the blink of an eye and create more ecosystem devastation than the asteroid that wiped out the dinosaurs; it would happen faster than all previous mass extinctions other than that asteroid’s effect. Arthropods may then come to rule the world once again.
The , like the prior , was more than one event and had more than one cause. The is what most people think about when mass extinctions are mentioned (as it was Hollywood-spectacular and ended one fascinating line of animals and paved the way for mammals to dominate), and it led to the existence of humans, but the Permian extinction was the Big One. Before the began lifting in the 1970s and 1980s, specialists generally thought that the Permian extinction only impacted the oceans and left terrestrial ecosystems unaffected. The picture has radically changed since the 1980s, and the terrestrial extinctions are now acknowledged as similarly catastrophic. The Permian extinction is Earth’s only mass extinction of insects, and although plants are not normally vulnerable to mass extinctions, land plants also barely survived the Permian extinction. But the extinction came in phases, and each may have had different causes. There is great ongoing controversy and research regarding the issues.
Evolution: Extinction: What Killed the Dinosaurs ? - PBS
05/01/2018 · What Killed the Dinosaurs ..
What caused the extinction of the dinosaurs? If the dinosaurs cohabitated with humanity, what happened to them?
Dinosaur Extinction - Facts and Theories for Kids
27/05/2016 · New evidence supports asteroid theory of dinosaur extinction
A collection of fossil dinosaur skeletons
Though the general opinion is that an impact event was the primary cause of dinosaur extinction, ..
Dinosaur Extinction: When and how did dinosaurs become extinct
But the branch of the that readers might find most interesting led to humans. Humans are in the phylum, and the last common ancestor that founded the Chordata phylum is still a mystery and understandably a source of controversy. Was our ancestor a ? A ? Peter Ward made the case, as have others for a long time, that it was the sea squirt, also called a tunicate, which in its larval stage resembles a fish. The nerve cord in most bilaterally symmetric animals runs below the belly, not above it, and a sea squirt that never grew up may have been our direct ancestor. Adult tunicates are also highly adapted to extracting oxygen from water, even too much so, with only about 10% of today’s available oxygen extracted in tunicate respiration. It may mean that tunicates adapted to low oxygen conditions early on. Ward’s respiration hypothesis, which makes the case that adapting to low oxygen conditions was an evolutionary spur for animals, will repeatedly reappear in this essay, as will . Ward’s hypothesis may be proven wrong or will not have the key influence that he attributes to it, but it also has plenty going for it. The idea that fluctuating oxygen levels impacted animal evolution has been gaining support in recent years, particularly in light of recent reconstructions of oxygen levels in the eon of complex life, called and , which have yielded broadly similar results, but their variances mean that much more work needs to be performed before on the can be done, if it ever can be. Ward’s basic hypotheses is that when oxygen levels are high, ecosystems are diverse and life is an easy proposition; when oxygen levels are low, animals adapted to high oxygen levels go extinct and the survivors are adapted to low oxygen with body plan changes, and their adaptations helped them dominate after the extinctions. The has a pretty wide range of potential error, particularly in the early years, and it also tracked atmospheric carbon dioxide levels. The challenges to the validity of a model based on data with such a wide range of error are understandable. But some broad trends are unmistakable, as it is with other models, some of which are generally declining carbon dioxide levels, some huge oxygen spikes, and the generally relationship between oxygen and carbon dioxide levels, which a geochemist would expect. The high carbon dioxide level during the Cambrian, of at least 4,000 PPM (the "RCO2" in the below graphic is a ratio of the calculated CO2 levels to today's levels), is what scientists think made the times so hot. (Permission: Peter Ward, June 2014)
Cretaceous–Paleogene extinction event - Wikipedia
Mass extinction events may be the result of multiple ecosystem stresses that reach the level where the ecosystem unravels. Other than the meteor impact that destroyed the dinosaurs, the rest of the mass extinctions seem to have multiple contributing causes, and each one ultimately had an energy impact on life processes. The processes can be complex and scientists are only beginning to understand them. This essay will survey mass extinction events and their aftermaths in some detail, as they were critical junctures in the journey of life on Earth.
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