by Robert S. Boyd
KNIGHT RIDDER WASHINGTON
August 17, 1999
in the Herald-Leader
Recent scientific discoveries are
shedding new light on why great empires such as Egypt, Babylon and
Rome fell apart, giving way to the periodic "dark ages'' that
punctuate human history.
At least five times during the last 6,000 years, major environmental
calamities undermined civilizations around the world. Some
researchers say these disasters appear to be linked to collisions
with comets or fragments of comets such as the one that broke apart
and smashed spectacularly into Jupiter five years ago.
The impacts, yielding many megatons of explosive energy, produced
vast clouds of smoke and dust that circled the globe for years,
dimming the sun, driving down temperatures and sowing hunger,
disease and death.
The last such global crisis occurred between A.D. 530 and 540 - at
the beginning of the
Dark Ages in Europe - when Earth
was pummeled by a swarm of cosmic debris.
In a forthcoming book, Catastrophe, the Day the Sun Went Out,
British historian David Keys describes a 2-year-long winter
that began in A.D. 535.
Trees from California to Ireland
to Siberia stopped growing
Plague and famine decimated
Italy, China and the Middle East
Keys quotes the writings of a
6th-century Syrian bishop, John of Ephesus:
"The sun became dark... Each day it
shone for about four hours and still this light was only a
A contemporary Italian historian,
Flavius Cassiodorus, wrote:
"We marvel to see no shadows of our
bodies at noon. We have summer without heat.''
And a contemporary Chinese chronicler
"Yellow dust rained like snow.''
Researchers say similar environmental
calamities occurred around,
Each led to the collapse of urban
societies in widely scattered portions of the globe.
Destructive as they were, the natural disasters that have plagued
Earth since the dawn of human civilization are but popguns compared
with the truly titanic catastrophes of prehistoric eras.
There have been at least five of these monster events, each of which
wiped out most of the creatures living at the time, the fossil
The best known was a 6-mile-wide meteor that smashed into what is
now the Gulf of Mexico 65 million years ago. The collision wreathed
the planet in clouds of dust, poisoned the atmosphere and drove the
dinosaurs, then rulers of the Earth, into extinction. Traces of the
enormous crater, at least 100 miles across, created by the impact
were found in 1990.
Even that wasn't the biggest blow the Earth has suffered. The mother
of all extinctions, which wiped out 90 percent of living species,
happened about 245 million years ago.
Paleontologists say other mass
extinctions occurred about,
Although the evidence is debated, a growing number of researchers
contend that most, if not all, of these ecological disasters are
connected to bombardments from space.
"Recent evidence is converging on
the conclusion that mass extinctions coincided with comet or
asteroid impacts, and that periodic comet showers, triggered by
the Solar System's motions through the Milky Way galaxy, may
provide a general theory to explain impact-related mass
extinctions,'' said Michael Rampino, a geologist at New York
"After an impact, the dense dust cloud that is created by the
impact spreads through the atmosphere, cuts out sunlight,''
Rampino said. "This stops photosynthesis and causes the climate
to get cold and dark, leading to the mass extinction of large
numbers of organisms.''
These disasters, while terrible for
their victims, opened the way for the survivors to flourish,
diversify and - for humans - take over the world.
"We mammals may owe our pre-eminent
position atop the Earth's food chain to a collision some 65
million years ago that wiped out most of our competition,
including the dinosaurs,'' said Donald Yeomans, a NASA
astronomer who tracks comets and asteroids.
These discoveries are lending weight to
a revised theory of evolution.
Instead of proceeding gradually by a
series of tiny changes, as
Charles Darwin proposed 140 years ago,
life developed in a series of starts and stops, biologists now
They call it "punctuated evolution,''
periods of slow development interrupted by wholesale extinctions and
"It may take millions of years, but
as the new organisms fill all the new niches that were emptied
out, a whole new biosphere is created,'' Rampino explained.
"Earth is currently enjoying a quiescent period,'' said Robert
Shoch, a Boston University geologist. "But around 2200 A.D., it
is likely that a new flow of comet fragments will enter
Earth-crossing orbits and pose a real threat to our planet.''
The bigger the object and the faster it
travels the more damage it causes. A direct hit is not required;
simply passing through one of the streams of cosmic rubble littering
the inner solar system can have unpleasant consequences.
The civilization-shattering events of the historic era,
"must have been near misses, because
if we had been hit by a full-blown comet in the past 10,000
years or so, we wouldn't be here today,'' said Mike Baillie, a
British archaeologist who studies tree rings.