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.
This first planetwide extinction occurred at the close of the Cambrian about 100 million years after the first plantewide explosion of sea life.
Evolution was a speed demon back in the early Cambrian. All kinds of sea life-in the forms of crablike trilobites, bivalve brachiopods, and other critters-expanded and filled the shallows. This is beautifully described in the 1989 award-winning science book "Wonderful Life" by the late geo scientist Stephen Jay Gould.
As far as we know, no living thing yet occupied the Earth's barren supercontinents of Gondwanaland and Laurentia (although it's suspected microbes were already breaking down terrestrial shield rocks).
So what caused this great explosion of "Wonderful Life"? Well, paleontolgists guess that environment, oxygen and climate played big roles. Ocean temperatures and oxygenation were ideal for life. But then something mysterious occurred and crashed the system.
There's some evidence that a global ice age at the end of the Cambrian may have chilled the seas and reduced the oxygen content of the water. But what caused the ice age? Here are a few suggestions offered: cosmic impact (asteroid, comet or NEO swarm), climate change, supervolcanism, gamma ray bursts, plate tectonics-or maybe a combination of some, or all, of the above.
Not satisfied with such vagueness in the physical sciences? Well, think about today's climate change/global warming tussle; there's just no consensus regarding the Cambrian-Ordovician extinction.
•Extinction 2-known as Ordovician-Silurian Extinction-occurred about 444 million years B.C. This extinction may actually have been a series of events. No matter, it was the second greatest of the five extinction events on Earth. Again, sea life was affected, and again the event(s) ushered in a deep ice age. Gone was the "greenhouse" Earth of the warm Ordovician Period. Atmospheric CO2 crashed, too, and with it went many species. Cosmic or terrestrial agent to blame? It's still a mystery.
•Extinction 3-known as the Late Devonian Extinction-happened about 364 million years B.C. The bio marker of this event is the great die off of the agnathan or jawless fishes that filled the Devonian seas. But there was land life, too, with the first primitive forests (the Gilboa, N.Y., tree fossils mentioned here a few weeks ago), insects and lungfishes. Again, everything from cosmic impact to climate change are blamed, but there's no definitive smoking gun.
Next week: Extinctions 4 and 5 and cosmic agents of change.
What's in the Sky: On Aug. 10, the Moon is at perigee, the point where our natural satellite's orbit is closest to Earth.
Lou Varricchio, M.Sc., was a science writer at the NASA Ames Research Center. He is a NASA-JPL Solar System Ambassador in Vermont and is the recipient of the U.S. Civil Air Patrol's Brigadier General Charles E. "Chuck" Yeager Aerospace Education Achievement Award. His second book, "Seeing Stars", an illustrated collection of his newspaper columns, will be published next year.