SAN FRANCISCO -- All life on Earth may owe its origin to fire and ice, as a rain of immense meteorites shattered the thick ice sheathing a primordial ocean more than 4 billion years ago, according to a new theory.
In a sharp departure from the conventional scientific view of how life began, several leading authorities told the American Association for the Advancement of Science this past weekend that such cataclysmic impacts may have intermittently melted frozen oceans and triggered in their depths a cascade of chemical reactions that spontaneously created the elemental building blocks of life.
Redolent with religious and philosophical implications, the origin of life is the fundamental mystery of organic chemistry. By what process were the molecules that led to living things first assembled?
The new theory was developed by Jeffrey Bada, an authority on marine chemistry at the Scripps Institution of Oceanography, and Stanley Miller, a University of California chemistry expert who first demonstrated in the laboratory how all the elements needed for life could spontaneously combine to create the amino acids, simple sugars and DNA bases necessary for life.
To frame their theories about the beginning of life, scientists only have the rules of chemistry and informed speculation about the state of the early Earth. There are no fossils, or direct geological or astronomical evidence to guide them as they try to understand chemical reactions that took place when the Earth was barren and the sun was a more faint, cooler star.
Some scientists have suggested that primeval lightning may have sparked the crucial chain reaction in an atmosphere rich in ammonia and methane -- according to the famous experiment Mr. Miller carried out in 1953, but on a planetary scale. Other experts have proposed that comets or cosmic dust might have carried the seeds of life to Earth.
Still others argue that the sun was so faint in its youth that the Earth must have been a frozen ball -- unless it was warmed by a "greenhouse" atmosphere rich in carbon-dioxide. That, however, would have smothered any organic chemical reaction.
The new theory is an effort to reconcile the requirements of organic chemistry with what planetary scientists have determined about the violent conditions on primitive Earth 4 billion years ago.
The result is a plausible explanation of how, under a devastating celestial bombardment, life began over and over again.
"There is no evidence that the origin of life took place only once. The origin of life may have taken place several times, and several times it may have been annihilated by impacts," said Mr. Bada, who presented the new theory Sunday at the annual conference the nation's oldest and largest organization of scientists.
Their presentation drew a packed auditorium of planetary scientists and chemists in large part because of Mr. Miller's stature as a leader of such theorizing for four decades.
Messrs. Bada and Miller believe that the organic molecules that serve as seeds of life arose at the bottom of ancient oceans crusted with ice 1,000 feet thick. At that time, the sun was so faint that the temperature at Earth's surface never rose higher than 40 degrees below zero Fahrenheit, according to planetary scientists.
The ice would have served as a blanket to keep most of the ocean warm and fluid. Scientists believe those ancient oceans could have been more than two miles deep.
Battered by the fiery impacts of asteroids greater than 50 miles in diameter, the ice would have thawed and refrozen repeatedly, perhaps as often as every 100,000 years.
Under the new theory, any organic material would have been trapped beneath the ice when the oceans refroze, preventing the raw material for organic molecules from being destroyed in the atmosphere. Thus -- in that rich, protected stew -- the first self-replicating molecules could have come into being, Messrs. Miller and Bada said.