If you could travel back 65 million years ago,you would be able to witness the fifth mass extinction on地球.It was at that time that a major event -- likely the impact from a彗星or giant meteor-- wiped out more than half of all life on Earth [source:崔]Among the life forms that died out were the恐龙.
As enormous as that extinction event was,an event 250 million years ago dwarfed it.The Permian extinction saw 95 percent of all marine life and nearly all land life die out [source:美国公共广播电视公司]The world might look very different today had this灭绝没有发生。Scientists believe enormous volcanic eruptions in what is now Asia caused this mass extinction.
今天，the Earth could be facing its next mass extinction.Depending upon which expert you ask,you may hear the extinction rate for species is 100 to 1,000 times greater than it normally should be [source:康纳]Harvard biologist Edward O.Wilson once estimated that as many as 30,000 species go extinct every year.
Why are so many life forms dying out?Several factors could contribute to extinction,but one seems to dominate all others: human interference.Whether it comes in the form of hunting,destruction of habitat or introducing an alien species to a new environment,humans have made an enormous impact on life on Earth in general.The history of our meddling goes back 100,000 years,when humans first started to migrate out of Africa.And then really picked up around 10,000 years ago when humans developed agriculture.
Since that time,human activities have wiped out thousands of species of plants and animals.If the trend continues,we could see a mass extinction that could threaten our own survival.We'll learn more in the next section.