mass extinctions have dominated earth's history -- 9/12/18
Today's selection -- from Origin Story: A Big History of Everything by David Christian. Mass extinctions have dominated the history of our earth:
"[Life form changes] changes did not take the smooth, stately forms that Darwin and his generation expected of evolution. Instead, the history of big life was an unpredictable and dangerous rollercoaster ride. Asteroid impacts, sudden shifts in Earth's innards, changes in the planet's atmosphere, and massive volcanic eruptions sent evolution careering down new and unexpected pathways. Evolution was 'punctuated,' as Niles Eldredge and Stephen Jay Gould argued in a famous article published in 1972. Like the cliche about the life of a soldier, evolution in the Phanerozoic meant long periods of boredom punctuated by moments of terror and life-threatening violence. The violence is most apparent in periods of mass extinctions. ...
"During mass extinctions, whole groups of species vanished suddenly and apparently randomly. ...
"The first mass extinctions happened back in the Archean eon. The great oxygenation event, 2.5 billion years ago, surely killed off many bacterial organisms for which oxygen was toxic. Indeed, this may have been the greatest mass extinction of all. Many groups of species also perished during the snowball-Earth episodes late in the Proterozoic eon, and we know that many disappeared at the end of the Ediacaran period. Since then, we know of at least five mass-extinction events during which more than half of all existing types of species disappeared.
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| Visual representation of the history of life on Earth as a spiral |
"The Cambrian explosion ended in a series of extinction events starting about 485 million years ago. Many species of trilobites walked the plank. So did many of the stranger Cambrian species, whose fossils have been found in the Burgess Shale in Canada and in the Chengjiang region in China. The Ordovician period also ended in a mass-extinction event 450 million years ago, when 60 percent of all genera may have vanished.
"The greatest of all mass extinctions came at the end of the Permian period, 248 million years ago. This time, more than 80 percent of all genera vanished, including the last of the trilobites. The precise causes of this mass extinction remain uncertain. It might have been due to rising magmas that broke through the crust in massive volcanic eruptions that sent enough ash into the air to block photosynthesis. We find modern evidence of this in a large volcanic region of Siberia known as the Siberian Traps. The eruptions pumped huge amounts of carbon dioxide into the atmosphere, so when the dust settled, carbon dioxide levels spiked, oxygen levels fell, and the oceans warmed. When Earth burped, the biosphere shuddered. By some estimates, oceans may have been as warm as thirty-eight degrees Celsius, hot enough to kill most marine organisms and stop nearly all photosynthesis in the seas. Warmer oceans could hold less oxygen and support less life, and deep beneath their surface, thawing balls of frozen methane known as clathrates may have released huge bubbles of methane. This was a greenhouse mass extinction; it killed by heating rather than freezing. In an extreme greenhouse world, large organisms survived only in the cooler polar environments in the far north and south of the vast supercontinent of Pangaea.
"Beneath the violent changes of the early Phanerozoic, a new biosphere was building. The spread of plants, fungi, and animals onto land transformed the Earth's surface. Particularly important was the spread of photosynthesizing plants onto land, because they consumed huge amounts of carbon dioxide and released huge amounts of oxygen. That reset the biosphere's thermostats, creating a new climatic regime with higher oxygen levels and lower carbon dioxide levels than ever before. In its essential features, that regime has lasted until today."
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