字幕表 動画を再生する 英語字幕をプリント Over 99% of the animal species that have ever lived are now extinct. And sometimes, an event occurs causing changes so drastic that most species are completely wiped out within a short period of time. So here are the 5 most incredible mass extinctions ever to occur, and a look at whether the 6th might be happening right now. 440 million years ago was the The Ordovician extinction. At that time, most creatures swam or crawled in shallow seas. As newly created volcanic rock was worn down by water and wind, it reacted with carbon dioxide and absorbed it. As a result, carbon dioxide levels dropped, temperatures fell and water got locked away as ice. This caused ocean levels to drop and shallow seas to drain and after several cycles of growing and shrinking glaciers, about 86 percent of species were lost forever. Then over millions of years the oceans slowly repopulated with fish and the land was colonized by early plants. These plants were then eaten by the first crawling and flying insects. Then 374 million years ago these new plants contributed to the next mass extinction. The plants absorbed enough carbon dioxide to create another round of global cooling. They changed soil causing nutrients to wash into the ocean creating enormous amounts of algae which sucked up oxygen. More than half of ocean species essentially choked to death. Although somewhere in the world, a small family of fish with foot-like fins and lungs managed to scrape by. Over the next 100 million years, these creatures would evolve into amphibians, reptiles and nearly all modern land animals. 250 million years ago is the Permian extinction. The single worst mass extinction in history, 70 percent of life on land and over 95 percent of life in the oceans was wiped out. Billions of tonnes of volcanic gases destroyed the ozone layer and the average ocean temperature hit 40 degrees Celsius, which is hotter than most hot tubs. Acid rain fell all over the planet, devastating life on land. Life had only 50 million years to recover until the fourth mass extinction came. At this time early small dinosaurs roamed the land. A huge volcanic rift opened in the middle of the planet; eventually splitting the Americas from Europe and Africa, and forming the Atlantic Ocean. The volcanoes spewed out carbon dioxide, increasing temperatures and killing about 80 percent of the species around at the time. Yet in this newly emptied world dinosaurs did extremely well and during the next 135 million years, they grew to become some of the largest land animals the world had ever seen. But all good things must come to an end. Many scientists believe that the extinction of dinosaurs was caused by an asteroid the size of a small town crashing into what is now Mexico’s Yucatan peninsula. The impact shot millions of tons of dust into the atmosphere, blocking out sunlight and spelling an end to all large dinosaurs, the small ones that survived evolved into birds. The world was now primed for a small, scuffling, rat-like creature, the ancestor of all modern mammals. Within 50 million years, its descendants - from wildebeest and whales to horses and us humans have diversified and grown to dominate nearly every available environment on earth. So where does that leave us now, 65 million years later? Well, human dominance of earth has led to big changes too, except the changes of today are happening over decades, not millennia. Carbon dioxide levels, implicated in so many of the past extinctions, have climbed at least 25 percent in just the last 50 years, almost no time in geological terms. In addition to climate change, we’ve exterminated hundreds of species by hunting, fishing, habitat destruction and pollution. It’s been estimated that current species extinction rates are between 100 and 1000 times higher than the natural background rate, and that if all the species that are currently threatened by declining populations actually do go extinct, we may reach the level of a true mass extinction in just a few hundred years. And while all the mass extinctions of the past have had some survivors, it’s worth remembering that the creatures at the top of the food chain are usually hit the hardest. We may be setting the stage for history to repeat itself, and for some small, unexpected organism to replace us as the planet’s dominant form of life. Curious about the coolest animals that existed during these mass extinctions? Check out our new video “5 Insane Creatures We Wish Still Existed” over on our AsapTHOUGHT channel which explores some of the neatest and most absurd creatures through history that are no longer with us. There's a Link in the description for that video. And subscribe for more weekly science videos!