GUERNEVILLE, Calif. - Inside a stainless-steel tank at a brew pub here overlooking the redwood-rimmed Russian River, a 45-million-year-old yeast proves its mettle.

And the remarkably resilient prehistoric microbe hasn't just garnered a devoted pack of Fossil Fuels Beer fans, it's also providing palpable proof of the tenacity of life on this planet.

When the Australian-born owner of Stumptown Brewery, Peter Hackett, first learned of the ancient yeast, he doubted this long-extinct strain would ferment anything drinkable. It took the urging of Chip Lambert, an East Bay microbiologist who confirmed the yeast's ancient lineage in his Berkeley lab, to give it a shot. Lambert is also one of the principals of Fossil Fuels Brewing Co., which plans to make the brew available to more pubs and restaurants in the fall.

''I had very low expectations,'' said Hackett, when he made his first batch of beer in 2006 using the prehistoric yeast. Extracted from a chunk of 45-million-year-old Burmese amber, its revival from deep dormancy by a scientist from California Poly-San Luis Obispo riveted the scientific world.

Beer made with the ancient yeast has only been brewed commercially at Stumptown and at Kelley Brothers Brewing Co in Manteca, Calif., where a 22-ounce mug sells for $5. The beer's public debut, however, came in 1997, when a small, experimental batch made by the yeast's discoverer was served at the casting party for ''The Lost World: Jurassic Park,'' a sequel to the 1993 dino-thriller. Those films portray the fictional resurrection of the fearsome animals using DNA from blood in a dinosaur-feeding mosquito entombed in amber.

The films took their inspiration from the work of scientists who were finding insects, plant parts and other ancient material preserved in fossilized tree resin, or amber, including Raul Cano at Cal Poly, who discovered the ancient yeast. Cano has revived hundreds of strains of microbes encased in amber, and is now ''a rock star in the single-cell organism world,'' Hackett said.

The yeast was actually trapped in the gooey tree resin during the Eocene epoch, 20 million years after the last dinosaur perished. High temperatures and warm oceans created a balmy environment throughout the Earth, with palm trees growing in what's now Alaska. And, fortuitously, the yeast is an ancient relative of today's Saccharomyces, or brewer's yeast. With 1,500 known species of yeast, a far less useful variety might easily have been captured in the resin.

It's by far the oldest yeast revived, although even older bacteria - 65 million years old - have been resurrected, said Cano. A master stock of the ancient yeast is kept at Cal Poly. To get enough yeast for brewing commercial size lots of beer, a few yeast are removed from that stock and allowed to replicate.

Dogfish Head Brewery in Delaware serves a line of ''ancient ales,'' with the oldest of the brewery's yeast derived from a 9,000-year-old tomb in China.

But when three more zeros are added to the microbe's age, ''it's like suddenly, whoa, that's an incomprehensible number,'' said Hackett. ''And it really is.''

Cano, the Cal Poly scientist, first had to overcome doubts that the ancient Saccharomyces he was cultivating wasn't in fact just a product of contamination by a modern strain. Lambert, a microbiologist then working at Xoma in Berkeley, was brought in to verify Cano's work.

''I was as skeptical as anyone else,'' said Lambert, who co-owns Fossil Fuels Brewing Co. with Cano and Oakland, Calif., lawyer Scott Bonzell. But he said a sterile verification process showed conclusively that this yeast was of ancient origin.

Scientists have long known that microbes evolved strategies for remaining dormant for hundreds or thousands of years, and then coming back to life under the right conditions. The single-cell creatures employ a variety of mechanisms, collectively known as ''starvation-survival strategies,'' said Cano. Those multiple strategies include microbes in the nutrient-deficient deep sea shutting down their metabolic processes until they once again encounter food or sunlight. In that dormant state, they can remain undetectable even under microscopes- a condition called ''cryptobiosis,'' which translates to ''hidden life.''

But at the time of Cano's discoveries, in the 1990s, few if any thought life so ancient was capable of renewal. Cano said work in reviving prehistoric microbes has expanded understanding of cryptobiosis and the genetic basis by which organisms extend their life span.

Amber, however, provides perfect long-term storage, said Lambert. The resin absorbs virtually all the water from the microbes, but the molecular machinery running the one-celled creatures remains intact.

''Essentially all the enzymatic processes have come to a standstill,'' he explained. ''It's the kind of packaging that would be ideal for long-term preservation. It's really no more sophisticated than the process we use today for many freeze-dried processes.''

As for concerns that - like the unintended consequences portrayed in Jurassic Park of reviving long-extinct life forms - reintroducing prehistoric microbes could unleash a deadly new pathogen, Cano said such fears are ''misinformed.'' Microbes formed 4 billion years ago on Earth, and for 3 billion years were the only life forms. In that light, ''45-million-year-old organisms are not that old,'' Cano said. ''The chances of isolating a pathogenic organism from modern samples are as great, if not greater . . . than from amber.''

The only concerns Hackett had about the ancient Saccharomyces was what kind of beer it would produce. Very few wild strains of yeast, such as this one, will even ferment well, much less produce good-tasting beer, he said. It usually takes workhorse yeasts cultivated to thrive in the tough conditions of a fermentation tank to produce a drinkable brew.

To his amazement, the prehistoric yeast yielded a delicious golden beer. ''The brew hash smelled like ginger and banana and clove,'' Hackett said. ''It was just remarkable.''

Dan Tensfeldt, a customer at the brewery, said he ''fell in love with the beer'' at first taste. "It's a really rich, really fulfilling beer.''

The unique beer's connection to ancient times is less important to him, other than its novelty. ''I know that it comes from really, really, really, really, really old yeast, which Peter is very proud of.''

But it's the brew it yields that most impresses Hackett.

''Frankly, I'm a lot more performance-based in my admiration of the single-cell organism,'' he said. ''It's what it does in that fermentation that makes a difference to me. And that's where I was very pleasantly surprised.''

ANCIENT ALE

Beer fermented from 45-million-year-old yeast found in amber was first publicly served in 1997 at the casting party for ''The Lost World:Jurassic Park,'' from an experimental batch made by the yeast's discoverer, Raul Cano of Cal Poly.

First commercial use: In 2006, when the brewmaster at Stumptown Brewery in Guerneville, Calif., made a batch of what would be called Fossil Fuels beer (about 6 percent alcohol content). More Fossil Fuels beer will be available in late August at Kelley Bros. Brewing Co. in Manteca, Calif., after the brewery replenishes its depleted stock. They'll make a pale ale and a wheat beer. In the fall, Kelley Bros. will begin making kegs of Fossil Fuels beer available for distribution. Stumptown Brewery will continue using the yeast to develop beer recipes, although at this point isn't serving it.

Visit www.kelleybrewing.com for more information, or call 209-825-1727.