But a new book by two University of Washington scientists contends that, contrary to popular thought, we just might be alone and Earth might be unique, if not in the universe at least in this celestial neighborhood.

In "Rare Earth," published this month by Copernicus Books/Springer, paleontologist Peter Ward and astronomer Donald Brownlee examine the remarkable confluence of conditions and events that deposited life-forming chemicals on Earth, allowed simple life to gain a foothold and then protected the planet sufficiently and created just the right environmental factors for advanced life to slowly evolve.

"It seems like something a lot of people don't want to hear, yet nearly everyone who works in these areas has remarked at one time or another how unusual the Earth is," said Brownlee, an expert on comets, the space bodies that might have delivered the first organic chemicals and life-sustaining water to Earth.

In fact, he and Ward, whose extensive research on the fossil record has provided key insights into prehistoric mass extinctions, frequently discuss the Earth's unusual character with students in their astronomy and geological sciences classes.

The scientists don't argue that life is rare. In fact, recent evidence showing simple microbial life can survive extreme conditions on Earth is an indicator that such life also might be widespread in the galaxy and the universe.

"But you need to have a vast amount of time to let evolution ramp up to animals, and we think there are only a small number of planets where that could happen," said Ward.

The key, he said, is having near equilibrium in such things as temperature and water content over enormous time spans.

Microbes have shown they can live in some of the harshest Earth environments imaginable, while advanced plant and animal life requires a delicate balance of conditions.

"For 90 percent of the age of this planet, life was slime at the bottom of the ocean," Brownlee said. But that life was given a one-in-a-million opportunity to gradually evolve to the complexity it enjoys today.

"The underlying theme of the book is that the Earth is a very charmed planet," he said. "We know of no other body that is even remotely like Earth."

Factors that made advanced life possible include the Earth's having:

• The proper distance from the sun to allow development of habitat for complex life and ensure that water remains liquid, not vapor or ice.
• The proper mass to retain atmosphere and ocean.
• Plate tectonics, which act as a sort of atmospheric thermostat, build land masses and enhance biotic diversity.
• A neighbor the size of Jupiter, not too close and not too far away, that can use its gravity to protect the planet from too many life-extinguishing collisions with comets and asteroids.
• A stable orbit unperturbed by giant planets.
• A large moon at the right distance to stabilize tilt, thus ensuring seasonal climate fluctuations that are not too severe.
• Enough carbon to support development of life but not so much to allow for runaway greenhouse conditions.

The "charmed" conditions on Earth won't always be present. Someday, some way, evolution on Earth will end. That could be when the sun gets so hot that life can no longer survive, when ultimately the ocean boils and surface rocks melt.

"There will be a time when there will be no record of life ever having existed on Earth," Brownlee said.

He and Ward acknowledge that their assumptions about how uncommon advanced life might be in the universe are based on observed conditions that allowed evolution on Earth. But this is the only place in which advanced life is known to have occurred, and it is one of only a handful of places in the solar system where even microbial life is suspected, making this planet the ultimate laboratory on advanced life.

A key condition for life on Earth is the presence of carbon, because of its unique properties.

"Probably all life is based on carbon," Brownlee said. While he concedes the possibility that life has evolved elsewhere based on an element such as silicon, he remains skeptical of that theory.

"Many things are possible. You can never imagine everything the universe can do. But we know it didn't happen here," Brownlee said. "If things have to obey physical and chemical laws, then there really aren't a lot of options in nature."