The extinction of biological species is not an unusual event--every life comes to an end sooner or later. Most species will exist for only several million years, and then they will disappear forever: the great dinosaurs, for example.
生物滅絕本來並不稀奇，因為每個個體遲早要死亡，大多數物種也只存在數百萬年，之後就永遠消失，像是一度稱霸地球的大型恐龍。