Are we the sons and daughters of comets?

Scientists have long speculated the ingredients for life arrived on Earth aboard comets or asteroids that pelted the planet some four billion years ago. While the early, prebiotic Earth appears to have been deficient in the building blocks for life, comets and asteroids are rich in organic fodder needed to create sugars and amino acids.

But many scientists have found the idea implausible because if the early Earth's atmosphere was as thin as today's, the asteroids would have been incinerated as they streaked through the skies. Anything that did not burn up in the atmosphere would have vaporized upon impact.

However, new calculations by Christopher Chyba of Cornell University and his colleagues, published in the current issue of the journal Science, show that if the Earth's atmosphere was rich in carbon dioxide, comets and meteorites would have been slowed down dramatically by the dense atmosphere and the bigger objects would have survived impact.

Chyba believes at least 2 million pounds of organic material could have reached the Earth each year during the period of heavy extraterrestrial bombardment 4 billion years ago. That just might have been enough to get life rolling.