Tapestry of death, part 1

You will never convince some palaeontologists that an impact killed the dinosaurs unless you find a dinosaur skeleton with a crushed skull and a ring of iridium round the hole.-Frank Kyte

This discussion about mass extinctions on Earth may not sound like a space-science related topic, but evidence has been mounting for several decades that points to a "cosmic" hand in some, perhaps all, of our planet's megadeaths-so far. Since its final accretion four billion years ago, our planet has been rocked by five major extinctions. While these extinctions brought Earth's biosphere to near collapse, life amazingly bounced back and continued on.

Now a new, "sixth extinction" has been proposed by several respected scientists including biologist Edward O. Wilson and paleoanthropologist Richard Leakey.

Hominid fossil-hunter Leakey-who wrote a thoughtful 1996 book titled "The Sixth Extinction" with science writer Roger Lewin-and others contend that the Earth is in the midst of another great die off of life; humans may be among the victims.

Of this proposed "sixth extinction", Dr. Leakey said, "For the sake of argument, let's assume the number (of species becoming extinct) is 50,000 a year. Whatever way you look at it, we're destroying the Earth at a rate comparable with the impact of a giant asteroid slamming into the planet, or even a shower of vast heavenly bodies."

We'll leave the "sixth extinction" debate to others and instead look at prehistoric extinction events 1 through 5 to see if there's a cosmic thread in Earth's ancient tapestry of death:

•Extinction 1-known as the Cambrian-Ordovician Extinction Event-occurred 488 million years B.C. It marked the demise of the great Cambrian Explosion of life. In geological terms, the Cambrian Period is the first geo period of the long Paleozoic Era. The Cambrian lasted from 542 million years ago to 488 million years or so ago.

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