Rock Of Ages

A surprise discovery could rewrite history

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...

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