What caused Earth’s 5 mass extinctions?
Hint: It wasn't always an asteroid.
Tom Hartsfield, 08/15/22
l Earth’s lifeforms have undergone five mass extinctions that we know of.
l What sorts of events wipe out the majority of species on Earth over a brief period of time?
l These events have guided evolution and led to human domination. They remain mysteries, with one clear exception.
Life on Earth began under mysterious circumstances billions of years ago. The most ancient dated microbial fossils suggest that life is at least 3.5 billion years old, during which time it occasionally experienced monumental setbacks. While species naturally come and go, several mass extinctions have occurred, resulting in the disappearance of many or most species on Earth.
Late Ordovician (443 million years ago)
The first mass extinction on record divides the Ordovician period from the succeeding Silurian period. At this stage of history, nearly all life was still in the sea. Molluscs and various hard-shelled simple creatures like trilobites were dominant. The first fishes with jaws appeared, destined to be the ancestors of nearly all modern vertebrates. The first plant fossils on land appear to date from this period, indicating what was to come.
The Ordovician extinction wiped out something like 85% of all marine species. (由於年代過於久遠，原因不明；2 – 4次相同。)
Surprisingly, this extinction did not drive the dominant species of Earth along a new direction. Most existing forms — clearly including our vertebrate ancestors — persisted in smaller numbers. They recovered to roughly their previous patterns within a few million years.
Late Devonian (372 million-359 million years ago)
During the Devonian period, colonization of land grew as plants and insects took to terra firma. Plants developed seeds and internal vascular systems to transport and store water. They did not yet face substantial competition from land-based herbivorous animals, and plants’ explosive growth might have reduced atmospheric carbon dioxide levels and led to global cooling. After the Devonian extinction event, tetrapods — ancestors to the first amphibians, and later to reptiles, birds, and finally mammals — began to dominate the land.
The extinction that closed the Devonian period appears to have begun with the Kellwasser Event, continued slowly for a few million years, and closed with the Hangenberg Event.
Many theories, but no clear answer will remain a theme of these mass extinctions, with one glaring exception. (指第5次，也是最近的一次。)
Permian-Triassic (252 million years ago)
The most brutal mass extinction occurred roughly 250 million years ago, and it took out the majority of species on the planet. Some scientists believe that as many as 90% to 96% of all marine species may have disappeared, while others claim it was probably closer to 80% to 85%. At least 70% of land vertebrates went extinct as well.
Atmospheric carbon isotopes shifted, and giant volcanic eruptions occurred in modern China and Siberia. Coal beds might have burned, and microbes might have bloomed, changing the atmosphere with their metabolic processes. A number of current lines of thought speculate that some combination of these factors combined to warm the climate. In any case, this extinction did alter the course of life. It took land creatures millions of years to recover, and they did so with new forms.
Triassic-Jurassic (201 million years ago)
The most widely believed causal factor is interruptions in the composition of the atmosphere by volcanic activity that occurred around this time in the Central Atlantic Magmatic Province. Magma welled up across modern North America, South America, and Africa as these masses began to split apart. As they drifted away, these continental masses each carried a piece of the original field across what was becoming the Atlantic Ocean.
Cretaceous-Paleogene (66 million years ago)
This is the one you are most likely familiar with: the end of the dinosaurs and the beginning of the modern (Cenozoic) era. Unlike the others, the cause of this extinction has become very clear to nearly all. Geological sediment strata around the world exhibit a layer of rock containing greatly elevated levels of the element iridium, a heavy metal that is extremely rare within the crust of the planet. Iridium is much more common in asteroids. The layer depth corresponds to the time of the extinction event. A 2016 drilling experiment at the Chicxulub crater in Mexico removed cores from the impact structure. Under analysis, these revealed iridium anomalies and other elemental signatures tying the crater to the worldwide iridium-rich layer.
Regardless of why these events occurred, their steerage of dominant lifeforms is the plot line that has placed us here today.
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