By BRUCE HALLETT PRESIDENT

REPORTERS DO SOME OF THEIR best work between assignments, when a sixth sense leads them to the next big story--sometimes simply by nudging them to call the right source and ask, "What's new?" Senior correspondent Madeleine Nash made one of those breakthrough calls last summer. But Nash was sniffing out a story that's 543 million years old.

"What's new about the Cambrian?" she asked David Jablonski, a paleontologist at the University of Chicago.

"You must come down here today, now, this very minute," Jablonski replied. He told Nash that a colleague had just returned from an expedition to Namibia, where he had unearthed fossils that shed amazing new light on evolution's "Big Bang"--the unparalleled explosion of life forms that sprang into being during the Cambrian period. So began an odyssey that led Nash to labs, museums and rocky outcroppings across the country, and culminated in the extraordinary report in this week's issue.

"I've had my eye on the Cambrian for a long, long time," says Nash, who specializes in telling complex science stories in a way that's not just comprehensible but compelling. The key, she says, is a sense of adventure. "People think of science as a body of knowledge, but in truth it's a process of discovery. When things don't make sense, that's when they get really exciting."

"She gets stirred up by a big story, so her nickname is Mad Nash," says deputy International editor Charles Alexander. "But I like to call her Mad Dash because she'll drop everything instantly and go anywhere in the world to report it."

To communicate her sense of excitement, Nash uses bold metaphorical leaps and descriptive passages that can bring to life a half-billion-year-old fossil ("plump Aysheaia," she writes, "prancing on caterpillar-like legs"). These gifts, says senior editor Philip Elmer-DeWitt, make Nash "a national treasure--one of the few who can see the cutting edge of science, report it deeply, and then write about it with grace and style."

Nash began her Time career 25 years ago as a "clip girl" in the Nation section and quickly moved up to correspondent, covering a variety of beats before devoting herself to science and writing and reporting a dozen or more memorable cover stories--so many, in fact, that she's lost track of the number. Like any great reporter, she works the beat around the clock. Her husband Thomas Nash is a particle physicist at the Fermi National Accelerator Laboratory.