Science / Medicine : Heat From Asteroids Killed Early Life Forms, Scientists Say : The Earth: New theory points to collisions that vaporized the oceans, sterilizing the planet.

Life may have arisen on Earth several times during the planet’s first billion years, only to be wiped out by the cataclysmic impact of asteroids, a team of researchers headed by geophysicist Norman Sleep of Stanford University reported last week.

Asteroids landing in the Earth’s oceans would have vaporized them, creating an atmosphere of scalding steam that would have sterilized the planet, they said. Only when such bombardments stopped about 3.8 billion years ago, they reported in the British journal Nature, would life have been able to begin evolving to its current complexity.

Sleep and his colleagues said their calculations show that a 260-mile-wide asteroid striking Earth at a speed of 3,672 m.p.h. would create so much heat that the oceans would boil dry. Such an asteroid would be roughly the size of the larger asteroids in our solar system.

The team acknowledged no direct evidence exists of such a devastating asteroid hitting Earth. That is not surprising, however, because virtually all rocks and geologic formations older than about 3.5 billion years have been obliterated by the constant activity of the tectonic plates that have formed the planet’s continents.

They noted, however, that the moon bears scars of heavy asteroid bombardment, with the last major impact occurring about 3.8 million years ago.

“Earth should have been hit with many more projectiles than the moon as it has more surface area and larger gravity. Statistically, Earth is also likely to get hit with larger objects,” the team wrote.

Most scientists think Earth and nearby planets were born about 4.5 billion years ago from material left over as the cooling sun shrank. The oldest known evidence of life dates back about 3.56 billion years to fossils found in Australia by UCLA paleontologist J. William Schopf. Some rocks found in Greenland, dating back about 3.77 billion years, also have mineral ratios indicating they may have been formed from remains of living organisms.

The new calculations show the hot rock vapor produced by a large asteroid would evaporate all ocean water and thereby sterilize the planet with hot steam, researchers said. It would take 1,000 to 2,000 years for Earth to cool down enough again to support life.

Furthermore, the researchers noted that the impact of an average-sized asteroid--about 114 miles across--would be powerful enough to vaporize surface waters containing the plankton crucial to other life forms. The impact of such events would only last about 300 years.

Because heavy asteroid bombardment of Earth apparently ended about 3.8 billion years ago, researchers said: “Our estimate for the time of the origin of contiguous life converges on this date.”