Extinctions, where entire species① are wiped② out, are not unusual③ in our Earth ’s history. In fact, over 90 percent of all species that ever lived are now extinct. In the last 500 million years, there have been five times when life on Earth has almost ended. These include the third mass④ extinction, known as the “Great Dying”， which killed 90% to 96% of all species. And the fifth event, which occurred 66 million years ago, caused the death of the dinosaurs. The reasons for extinctions are varied⑤. According to scientists, they include volcanic eruptions, asteroids⑥ hitting the Earth from space, changes in sea levels, the decrease⑦ of the oxygen content of the sea and global warming. Many scientists say we are now entering the Earth ’s sixth mass extinction. This time, human activity will be to blame⑧.
Although it is not unusual for species to die out⑨ naturally, the rate at which this is now happening is cause for concern. A 2015 study by scientists who were based at Brown University and Duke University in the US, looked at how quickly species die out due to natural causes, that is, the “background extinction rate”. They found that human activity is causing species to die out 1,000 times faster than normal.(1) There is a long list of reasons why so many species are dying out: air and water pollution, forests being destroyed, factory farming and overfishing. It is clear that human activity has negatively affected⑩ all other species on Earth, including animals and plants.
(1)本句是一个主从复合句。句中that引导宾语从句；1,000 times faster是倍数的表示方法。
If a sixth mass extinction occurs, scientists who have studied the issue believe that up to? three quarters of all species on Earth could die out. Of course, as we humans depend on? so many species for our survival, we would also be at risk? of dying out. Our world and our lives depend on the balance in nature between animals and plants. Without forests, we would have no air to breathe, and without clean water we would be unable to survive.(2)
(2)本句是一个并列句。由and连接两个并列分句，其中without与no构成双重否定。
Can a sixth extinction be avoided? Experts say that it is not yet too late, if we take action? now.This would mean we have to significantly change the way we live.(3) For example, we need to take steps? to save endangered? species, including setting up special areas where plants and animals can be protected. We need to change how we work and travel by using cleaner, more natural sources of energy, such as wind and solar? energy. We also need to improve our farming methods to reduce water, air and land pollution. In short?， if we want to survive, we need to stop destroying the planet that we live on and start to protect it.
