Writing in the latest issue of the technical journal Science, a team of researchers reported that analysis of lunar material brought back during the Apollo program suggests that the Earth and moon went through a sustained bombardment by meteors and comets roughly 400 million years ago. Assuming this finding is correct (dating methods for lunar materials are disputed), it could bear on several aspects of the mystery of the origin of life--including whether Earth was "seeded" by organic compounds from elsewhere in the universe.
First, a thumbnail sketch of what science thinks it knows about the rise of life. Earth appears to have formed about 4.5 billion years ago. Initially, intense radiation from the developing sun, combined with constant asteroid strikes, made Earth uninhabitable. Somewhat more than 3 billion years ago, the high radiation and asteroid fusillade ended; it's at that time that life began, but in extremely rudimentary forms that were less complex than even the single-celled organisms of today.
For the next 2 billion years, life barely evolved; the planet was covered with life-forms similar to algae, but nothing swam or squawked or breathed. Then about 500 million years ago, the "Cambrian Explosion" occurred. Complex organisms, including fish, seemed to arise very rapidly compared with the previous 2 billion years of little change. Following the Cambrian Explosion, land plants, then land fauna, then thunder lizards, then flowering plants, and finally mammals acquired their being. From our perspective, the big event came when humanity's primate ancestors diverged from the ape family, probably 3 or 4 million years ago.
And what caused the explosion of new life forms in the Cambrian period? Here, there are a few sketchy theories. Earth's early atmosphere contained almost no oxygen; it might have taken 2 billion years of exhaling by algae-like rudimentary life-forms to add enough oxygen to the biosphere for fish and animals to become possible. Also, Earth was warming during this period, and warmth is conducive to life. (The primeval sun was "faint," about 25 percent less powerful than it is today.) But these seem, at best, clues. Something else, and something important, might have happened around the Cambrian point.
Enter the moon-rocks bombardment, which seems to have occurred around the time of the Cambrian Explosion. (The timing doesn't match exactly, but all dating from ancient eras is approximate.) Could asteroids and comets falling on Earth have seeded it with complex organic compounds that accelerated evolution? One of the biggest surprises astronomers encountered in this century was the discovery of vast clouds of organic compounds floating in deep space. These materials in space are not alive; they are "organic" as chemists use the term, meaning complex carbon compounds. But if a bombardment of rocks and comets from elsewhere in the Milky Way arrived at Earth about the time of the Cambrian Explosion, it might have brought along seed compounds from the deep-space organic clouds, adding complexity to life.
In this regard, it is important that researchers assume the rock bombardment that occurred around the Cambrian age came from beyond the solar system. The asteroids that fell on Earth in its dawn era, billions of years ago, were probably debris left over from the formation of this solar system, and thus unlikely to have born complex organic compounds. But the rocks that seem to have hit around the Cambrian time period probably would have come from far away (we'll skip the scientific reasons why) and thus might have carried complex organic compounds from distant points in the galaxy. A bombardment of materials from deep space arrives at Earth; "suddenly," in geologic terms, evolution accelerates. Perhaps a coincidence, perhaps not.