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
- Crops failed
- Plague and famine decimated Italy, China and the Middle East
"The sun became dark... Each day it shone for about four hours and still this light was only a feeble shadow.''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 reported,
"Yellow dust rained like snow.''Researchers say similar environmental calamities occurred around,
- 3200 B.C.
- 2300 B.C.
- 1628 B.C.
- 1159 B.C.
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 record shows.
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 214 million, 360 million and 440 million years ago.
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 University.
"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 believe.
They call it "punctuated evolution,'' periods of slow development interrupted by wholesale extinctions and recoveries.
"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.