Over 99% of the animal species that have even 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 even to occur,and a look at whether the 6th might be happening right now.
99%曾经存在过的动物 如今都已经灭绝了,有时 一件事情的发生造成了剧烈的变化,会导致大部分生物在短期内彻底消失,今天我们来讲讲已经发生的5次大灭绝,以及很可能就快来临的第六次。
4.4 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 CO2 and absorbed it.
4.4亿提前 发生了奥陶纪大灭绝,当时 大多数的生物都生活在浅水区,新形成的火山岩在水与风的侵蚀下,与二氧化碳发生反应 将其吸收。
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.
导致二氧化碳含量降低，从而造成了气温降低 水面结冰，海平面因此而下降 浅海地区海水排空，在冰川数次涨跌循环之后，约86%的物种灭绝了，在数百年之后 海洋中重新出现了鱼儿，陆地上也开始出现早期植物。
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 Co2 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.
它们成了早期爬行和昆虫的食物，直到3.74亿年前，这些新出现的植物造成了第二冷饮大灭绝，它们吸收了太多的二氧化碳，造成了第二次全球降温，植物还改变了土壤 营养盐被冲刷进海里，造成藻类大量生长 它们吸收了许多氧气。
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.
超过半数的海洋生物血缺氧窒息而死， 但是在世界上某个地方，一种长着脚状鳍和肺的鱼类勉力熬过了这个劫难，在之后的1亿多年里，这些生物进化成了两栖动物，爬行动物和接近现代的陆地生物，2.5亿年前 发生了二叠纪大灭绝。
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 CO2,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 the 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.Whthin 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,64 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 highter 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.