Q. The Ordovician extinction (440 million years ago), the Devonian extinction (370 mya), the Permian extinction (250 mya), the Triassic extinction (220 mya), and the Cretaceous extinction (65 mya, ending the age of the dinosaurs): Each wiped out close to half of Earth species then alive, due possibly to meteor impact, global glaciation, super-volcanoes, etc. Another is the more recent Holocene extinction. What calamity caused this one?
A. "Caused" isn't the right word, because this Sixth Great Extinction is ongoing, says physicist Stephen Webb in "If the Universe Is Teeming with Aliens... Where Is Everybody?" It's happening right now! This epoch encompasses the last 10,000 years, up to the present day. Unlike previous extinctions, "in this case the cause is clear: human activity. We hunt species to extinction, introduce alien species into new ecosystems and cause havoc, and most importantly, we destroy habitats."
By some estimates, species are disappearing 120,000 times the "normal" rate. Many in destroyed rain forests were never even documented as species in the first place. If these activities continue, says Webb, global atmosphere and climatic effects seem certain to occur. Will Homo sapiens then join the list of disappearers?
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