Every day, unheralded and largely unseen, hundreds of tons of dust and rock land on Earth from space. Most of this alien rain is either too small to notice or comes down where nobody is around. But occasionally, extraterrestrial debris makes a big impression.
Around 50,000 years ago, a body of iron and nickel weighing millions of tons slammed into the dry plain near present-day Winslow, Arizona, and carved out a rocky amphitheater almost a mile wide and as deep as a 60-story building. In 1908, what seems to have been a chunk of a comet exploded over Siberia, felling or stripping hundreds of square kilometers of trees, burning reindeer to death, and sending the tents of nomads flying through the air. Had it fallen over a major city, the results would have been catastrophic.
More recently, on January 18, 2000, a meteorite the size of a small truck and weighing at least 200 tons streaked across the skies of northern Canada, broke up in the atmosphere, and scattered thousands of fragments near Lake Tagish in the Yukon. Scientists were delighted when pieces of this object were recovered in a still-fresh, frozen state, because the Lake Tagish meteorite is of a rare and important kind. Locked within it, billions of years old, are carbon chemicals that formed in the lonely void between the stars.
Today the amount of material delivered from space each year is quite small. But in the remote past, the rain of extraterrestrial debris would have been a torrent. What role did it play in biological developments here?
As early as the first half of the nineteenth century, scientists knew that some meteorites contained organic matter. Called carbonaceous chondrites, they account for only a small percentage of all meteorites found, yet they offer