Rock of Ages

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As gems go, it wasn't much, just a dustlike grain of zircon. But the tiny crystal put a gleam in scientific eyes last week. Some 400 million years older than any previously discovered terrestrial rock, it could rewrite Earth's history--upsetting the timetable for the appearance of oceans and continents, challenging ideas about the formation of the moon and, most important, pushing back by several hundred million years the genesis of life.

Discovered in northwestern Australia, the prize specimen was dated by two separate scientific teams as either 4.3 billion or 4.4 billion years old. That puts it within a geological blink of Earth's fiery birth out of a swirling cloud of solar dust and gases 4.56 billion years ago. But how could any crystalline object solidify under such torrid conditions? The answer, the scientists reported in Nature, is that the planet was already bathed in cooling water.

If that is indeed the case, Earth's oceans and crust appeared only 200 million to 300 million years after its formation, when the planet was still being bombarded by large objects from space. One Mars-size chunk, by that calculation, would have slammed into Earth only 50 million years before the crystal formed, ejecting enough material to create the moon. Says University of Wisconsin (Madison) geologist John Valley: "Perhaps the moon formed earlier than we thought, or by a different process."

Even more intriguing is what this rock means for life. If Earth wasn't a seething ball of magma at the time, but cool enough for water, a crust and a biosphere, life might have arisen a half-billion years earlier than previously believed.