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.