盘点5次生物大灭绝

　　在地球的发展史上，生命从无到有，再到多样化，经历了长达数亿年的时间。为了更直观地理解地球的演化历史，法国科学家里夫形象地把46亿年的时间压缩成了一天：这一天的前1/4 的时间，地球上是一片死寂;时针指向凌晨6时，最低级的藻类开始在海洋中出现，它们持续的时间最长;一直到了20时，软体动物才开始在海洋与湖沼中活动;23时30分，恐龙出现，但只“露脸”了仅仅10分钟便匆匆离去;在这一天的最后20分钟里，哺乳动物出现，并迅速分化;23时50分，灵长类的祖先登场，在最后的2分钟里，它们的大脑扩大了3倍，成为人类。

　　Causes for mass extinction

　　Some of the hypotheses for the causes of mass extinction events are:.

　　1 Impact events. The impact of a sufficiently large asteroid or comet could create large tsunamis, global forest fires, and reduction of incoming sunlight due to large amounts of dust and smoke in the atmosphere. Taken together, it is not surprising that these and other related effects from an impact event might be sufficiently severe as to disrupt the global ecosystem and cause extinctions. Only for the End Cretaceous extinction (about 65 mya) is there strong evidence of such an impact. Circumstantial evidence of such events is also given for the End Ordovician extinction (about 444 mya), End Permian extinction (about 251 mya), End Jurassic extinction (about 145 mya), and End Eocene extinction (about 40 mya).

　　2 Climate change. Rapid transitions in climate may be capable of stressing the environment to the point of extinction. However, it is worth observing that the recent cycles of ice ages are believed to have had only very mild impacts on biodiversity. Extinctions suggested to have this cause include: End Ordovician (about 444 mya), End Permian (about 251 mya), and Late Devonian (about 360 mya).

　　3 Volcanism. The formation of large igneous provinces through the outflow of up to millions of cubic kilometers of lava in a short duration is likely to poison the atmosphere and oceans in a way that may cause extinctions. This cause has been proposed for the End Cretaceous extinction (about 65 mya), End Permian extinction (about 251 mya), End Triassic extinction (about 200 mya), and End Jurassic extinction (about 145 mya).

　　4 Gamma ray burst. A nearby gamma ray burst (less than 6,000 light years distance) could destroy the ozone layer and sufficiently irradiate the surface of the Earth to kill organisms living there. From statistical arguments, approximately 1 gamma ray burst would be expected to occur in close proximity to Earth in the last 540 million years. This has been suggested as a possible explanation for the End Ordovician extinction (about 444 mya). However, a recent study by leading gamma ray burst researchers says that gamma ray bursts are not possible in metal rich galaxies like our own (Stanek et al. 2006).

　　5 Plate tectonics. The opening and closing of seaways and land bridges may play a role in extinction events as previously isolated populations are brought into contact and new dynamics are established in the ecosystem. This is most frequently discussed in relation to the End Permian extinction (about 251 mya).

　　Other hypotheses, such as the spread of a new disease or simple competition following an especially successful biological innovation are also considered. However, it is often thought that the major mass extinctions in Earth's history are too sudden and too extensive to have resulted solely from biological events.

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