It Came from Outer Space!

How life began is among the greatest of mysteries. The Bible, Qur'an, and other sacred texts say God created the living things of Earth, but offer few clues as to the manner in which this miracle was accomplished. Evolutionary biology assumes life commenced through accidents of chemistry, but even the most ardent Darwinians admit they have no idea what the initial process was. Paleontologists puzzle over the lack of any physical evidence of how the spark of biology was lit. For years, a small minority of researchers have suggested that the reason we see no evidence on Earth of how life began is that we're looking in the wrong place--life began somewhere else and was transplanted here. This concept, called "panspermia," has been backed by figures as respectable as Francis Crick, codiscoverer of the double helix of DNA. Now a new study of moon rocks is lending indirect credence to this maverick idea.

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.

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