Scarred By The Past

The planet's recent run-ins with Mother Nature are nothing compared with what the Earth has been through over millions of years of history

March 23, 2007|By Robert S. Boyd | Robert S. Boyd,McClatchy-Tribune

Tornadoes, hurricanes, blizzards, floods, wildfires, earthquakes, tsunamis: Mother Nature seems to have it in for our world these days.

In a way, though, we live in a relatively peaceful time. While it's no comfort to those hurting or grieving now, Earth saw far greater catastrophes in its long and troubled past.

The planet has been frozen, roasted, smothered, battered, shaken and half-drowned. Entire species have been obliterated. So far, fortunately, that doesn't include Homo sapiens, but we've had a close call.

And these are all natural calamities, not those caused by humans, such as war, terrorism or the Holocaust.

"The history of life may have been shaped by major catastrophes to a far greater extent than previously realized," Trevor Palmer, a biologist at Britain's Nottingham Trent University, wrote in his 2003 book, Perilous Planet Earth.

Some disasters struck in recent centuries and are well recorded. Others occurred thousands or millions of years ago, but their scars litter the Earth. Memories of them may survive in myth and story, such as the lost city of Atlantis or Noah's flood.

Palmer thinks that myths are "possible recollections of catastrophes in ancient times."

For example, a tremendous flood drowned 60,000 square miles along the shores of the Black Sea 5,600 years ago. Geologists William Ryan and Walter Pitman of Columbia University suggest that this was the origin of the biblical story of Noah's flood and of a similar tale in the even older Sumerian epic of Gilgamesh.

Scientists have found no evidence of a worldwide flood, but shallow seas drowned large parts of North America, Africa and Eurasia 65 million years ago. The famous white chalk cliffs of Dover were created by the shells of sea animals.

Researchers have collected evidence of at least five major extinctions of living organisms, dated at 65 million, 200 million, 250 million, 360 million and 440 million years ago.

In the most recent episode, an asteroid 6 miles across slammed into what's now the Yucatan Peninsula, speeding the deaths of the dinosaurs and many other creatures.

The biggest extinction of all, 250 million years ago, is known as "The Great Dying" because more than 80 percent of the species then alive disappeared.

This was a "far greater crisis than the extinction of dinosaurs 65 million years ago," said Douglas Erwin, a paleobiologist at the National Museum of Natural History in Washington. "Plants and animals came closer to complete elimination than at any point since they first evolved," he wrote in his 2006 book, Extinction.

Earth's calamities included collisions with asteroids and comets, eruptions of super-volcanoes, huge lava flows engulfing millions of square miles, shattering earthquakes and devastating tsunamis.

A huge volcanic explosion 75,000 years ago at Mount Toba, in what's now Indonesia, blasted an estimated 700 cubic miles of ash and dust into the atmosphere, shrouding the entire Earth.

The Toba eruption coincided with the beginning of the latest Ice Age and may have almost wiped out our ancestors. DNA evidence shows that the human population shrank to about 10,000 at that time, according to Jelle de Boer, an earth scientist at Wesleyan University in Middletown, Conn.

"If such an evolutionary bottleneck did indeed occur, humankind must have come close to extinction," de Boer wrote in his 2002 book, Volcanoes in Human History.

Most catastrophes aren't bad enough to cause an extinction, but they create tremendous havoc. Sometimes they've contributed to the rise and fall of civilizations.

A rash of climate disasters about 2,300 years ago may have led to multiple civilization collapses in Egypt, the Middle East, India and China, according to Benny Peiser, an anthropologist at Liverpool John Moores University in England.

Peiser speculated that the troubles may have been caused by the breakup of a comet in the Earth's atmosphere, like the one that hit Jupiter in July 1994.

A volcanic eruption on the island of Thera in the eastern Mediterranean during the second millennium B.C. destroyed the Minoan civilization on Crete. It also may have ruined agriculture in Egypt for years, perhaps leading to the exodus of the Jews, Peiser said.

The constant asteroid threat stems from the fact that the inner solar system is littered with rubble left over from the formation of the planets. Mountain-size chunks of rock, nickel or iron regularly cross Earth's path and occasionally run into it.

"We live in a cosmic shooting gallery," Clark Chapman, a planetary scientist at the Southwest Research Institute at Boulder, Colo., told a Planetary Defense Conference this month in Washington.

According to Palmer, more than 140 impact craters scar the Earth's surface, five of them 60 to 120 miles across. Presumably there are many more hidden under the ocean or obliterated by the passage of time.

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