posts_gdocs: 1wTcR50pw-YBNbDANNB1oy8ubzy_DiTFcyqBggjSXsBE
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| 1wTcR50pw-YBNbDANNB1oy8ubzy_DiTFcyqBggjSXsBE | mass-extinctions | article | {
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"text": "There have been five big mass extinctions in Earth's history \u2013 these are called the \"Big Five\". Understanding the reasons and timelines of these events is important to understand the speed and scale of species extinctions today.",
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"text": "When and why did these mass extinction events happen?",
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"text": "What is a mass extinction?",
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"text": "First, we must be clear on what we mean by \"mass extinction\". Extinctions are a normal part of evolution: they occur naturally and periodically over time.",
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"text": "There\u2019s a natural background rate to the timing and frequency of extinctions: 10% of species are lost every million years, 30% every 10 million years, and 65% every 100 million years.",
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"text": " It would be wrong to assume that species going extinct is out of line with what we would expect. Evolution occurs through the balance of ",
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"text": " \u2013 the creation of new ones.",
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"text": "Extinctions occur periodically at what we would call the \"background rate\". We can therefore identify periods of history when extinctions were happening much faster than this background rate \u2013 this would tell us that there was an additional environmental or ecological pressure creating more extinctions than we would expect.",
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"text": "However, mass extinctions are periods with ",
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"text": " higher extinction rates than normal. They are defined by both magnitude and rate. Magnitude is the percentage of species that are lost. Rate is how quickly this happens. These metrics are inevitably linked, but we need both to qualify as a mass extinction.",
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"text": "In a mass extinction, at least 75% of species go extinct within a relatively (by geological standard) short period of time.",
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"text": " Typically less than two million years.",
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"text": "The five mass extinctions",
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"text": "There have been five mass extinction events in Earth\u2019s history, at least since 500 million years ago. We know very little about extinction events in the Precambrian and early Cambrian earlier, which predate this.",
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"text": " These are called the \"Big Five\" for obvious reasons.",
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"text": "In the chart, we see the timing of events in Earth\u2019s history.",
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"text": " It shows the changing extinction rate (measured as the number of families that went extinct per million years). Again, note that this number was never zero: background extinction rates were low \u2013 typically less than 5 families per million years \u2013 but ever-present.",
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"text": "We see the spikes in extinction rates marked as the five events:",
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"text": " (444 million years ago; mya)",
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"text": " (200 mya) \u2013 many people mistake this as the event that killed off the dinosaurs. But in fact, they were killed off at the end of the Cretaceous period \u2013 the fifth of the \"Big Five\".",
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"text": " (65 mya) \u2013 the event that killed off the dinosaurs.",
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"text": "Finally, at the end of the timeline, we have the question of what will come. Perhaps we are headed for a sixth mass extinction. But we are currently far from that point.",
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"text": "There are a range of trajectories that the extinction rate could take in the decades and centuries to follow; which one we follow is determined by us.",
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"text": "All of the \"Big Five\" were caused by some combination of rapid and dramatic changes in climate, combined with significant changes in the composition of environments on land or the ocean (such as ocean acidification or acid rain from intense volcanic activity).",
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"text": "In the table here, I detail the proposed causes for each of the five extinction events.",
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1 | 2024-03-09 18:21:06 | 2022-11-30 11:54:16 | 2024-03-09 18:27:51 | listed | ALBJ4LuqiePBzN1VBmWjOHiMFab8IwIap0hSNnN2G3GSyk3XbKfOrakF2Om693xAfgmTLmITq99AHBSiSIT2aQ | There have been five big mass extinctions in Earth's history – these are called the "Big Five". Understanding the reasons and timelines of these events is important to understand the speed and scale of species extinctions today. When and why did these mass extinction events happen? # What is a mass extinction? First, we must be clear on what we mean by "mass extinction". Extinctions are a normal part of evolution: they occur naturally and periodically over time.1 There’s a natural background rate to the timing and frequency of extinctions: 10% of species are lost every million years, 30% every 10 million years, and 65% every 100 million years.2 It would be wrong to assume that species going extinct is out of line with what we would expect. Evolution occurs through the balance of _extinction_ – the end of species – and _speciation_ – the creation of new ones. Extinctions occur periodically at what we would call the "background rate". We can therefore identify periods of history when extinctions were happening much faster than this background rate – this would tell us that there was an additional environmental or ecological pressure creating more extinctions than we would expect. However, mass extinctions are periods with _much_ higher extinction rates than normal. They are defined by both magnitude and rate. Magnitude is the percentage of species that are lost. Rate is how quickly this happens. These metrics are inevitably linked, but we need both to qualify as a mass extinction. In a mass extinction, at least 75% of species go extinct within a relatively (by geological standard) short period of time.3 Typically less than two million years. # The five mass extinctions There have been five mass extinction events in Earth’s history, at least since 500 million years ago. We know very little about extinction events in the Precambrian and early Cambrian earlier, which predate this.4 These are called the "Big Five" for obvious reasons. In the chart, we see the timing of events in Earth’s history.5 It shows the changing extinction rate (measured as the number of families that went extinct per million years). Again, note that this number was never zero: background extinction rates were low – typically less than 5 families per million years – but ever-present. We see the spikes in extinction rates marked as the five events: 0. **End Ordovician** (444 million years ago; mya) 1. **Late Devonian** (360 mya) 2. **End Permian** (250 mya) 3. **End Triassic** (200 mya) – many people mistake this as the event that killed off the dinosaurs. But in fact, they were killed off at the end of the Cretaceous period – the fifth of the "Big Five". 4. **End Cretaceous** (65 mya) – the event that killed off the dinosaurs. Finally, at the end of the timeline, we have the question of what will come. Perhaps we are headed for a sixth mass extinction. But we are currently far from that point. There are a range of trajectories that the extinction rate could take in the decades and centuries to follow; which one we follow is determined by us. <Image filename="Big-Five-Mass-Extinctions.png" alt=""/> # What caused the five mass extinctions? All of the "Big Five" were caused by some combination of rapid and dramatic changes in climate, combined with significant changes in the composition of environments on land or the ocean (such as ocean acidification or acid rain from intense volcanic activity). In the table here, I detail the proposed causes for each of the five extinction events.6 |**Extinction Event**|**Age(mya)**|**Percentage of species lost**|**Cause of extinction**| |End Ordovician|444|86%|Intense glacial and interglacial periods created large sea-level swings and moved shorelines dramatically. The tectonic uplift of the Appalachian mountains created lots of weathering, sequestration of CO2, and with it, changes in climate and ocean chemistry.| |Late Devonian|360|75%|Rapid growth and diversification of land plants generated rapid and severe global cooling.| |End Permian|250|96%|Intense volcanic activity in Siberia. This caused global warming. Elevated CO2 and sulfur (H2S) levels from volcanoes caused ocean acidification, acid rain, and other changes in ocean and land chemistry.| |End Triassic|200|80%|Underwater volcanic activity in the Central Atlantic Magmatic Province (CAMP) caused global warming and a dramatic change in the chemical composition of the oceans.| |End Cretaceous|65|76%|Asteroid impact in Yucatán, Mexico. This caused a global cataclysm and rapid cooling. Some changes may have already pre-dated this asteroid, with intense volcanic activity and tectonic uplift.| Jablonski D (1986) [Mass and background extinctions: the alternation of macroevolutionary regimes](https://science.sciencemag.org/content/231/4734/129.abstract?casa_token=EoFixpArJnsAAAAA:R4ejNN9Ccy8BjYWiJJMfWUToj6qJSJFQ8jWGiMsL_x2OoBRfsrBdze0p8n6kvYdps25LiL8hcRKwrcy9). _Science_ 231:129–133 Raup DM (1991) [A kill curve for Phanerozoic marine species](https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=A+Kill+Curve+For+Phanerozoic+Marine+Species+David+M.+Raup+Paleobiology+Vol.+17%2C+No.+1+%28Winter%2C+1991%29%2C+pp.+37-48+%2812+pages%29&btnG=). _Paleobiology_. 17:37–48. We can see a 75% reduction in species in two ways: high extinction or very low speciation rates. If speciation – the creation of new species – slows down a lot, the extinction rate does not need to be as high as we would expect in order to deplete species numbers by 75%. These events are sometimes called "mass depletions" but are treated the same way as mass extinctions. Jenkins RJF (1989) The supposed terminal Precambrian extinction event in relation to the Cnidaria. Memoirs of the Association of Australasian Paleontologists 8:307–317. This data and detail comes from multiple sources: Barnosky, A. D., Matzke, N., Tomiya, S., Wogan, G. O., Swartz, B., Quental, T. B., ... & Ferrer, E. A. (2011). Has the Earth’s sixth mass extinction already arrived? Nature, 471(7336), 51-57. McCallum, M. L. (2015). Vertebrate biodiversity losses point to a sixth mass extinction. Biodiversity and Conservation, 24(10), 2497-2519. Howard Hughes Medical Institute. Barnosky, A. D., Matzke, N., Tomiya, S., Wogan, G. O., Swartz, B., Quental, T. B., ... & Ferrer, E. A. (2011). [Has the Earth’s sixth mass extinction already arrived?](https://www.nature.com/articles/nature09678). _Nature_, _471_(7336), 51-57. | There have been five mass extinctions in Earth's history |