Are we alone in the universe?

For some of us, it is an article of faith; for others, it's simple arithmetic: with hundreds of billions of stars in our galaxy, billions of which are circled by planets capable of supporting life, there simply must be intelligent beings elsewhere in the Milky Way. Throw in the countless other galaxies, and it goes almost without saying that the universe abounds with intelligent species capable of building civilizations, right? Not so fast.

In Alone in the Universe, acclaimed science writer and astrophysicist John Gribbin builds a convincing case for the uniqueness of intelligent life on Earth. Asserting that a ''habitable'' planet need not be inhabited by intelligent beings, he cites a wealth of recent scientific findings to suggest that the incredible diversity of life on Earth resulted from a chain of events so unlikely as to be unrepeatable in a galaxy the size of the Milky Way.

The most significant of these events was the impact of a Mars-size object with Earth soon after our planet formed. It was this unimaginable impact, Gribbin argues, that changed almost everything about our planet. It gave us a moon, and thus tides; altered the tilt of Earth in its orbit around the sun; and set the scene for continents to drift.

A novel feature of Gribbin's argument is the suggestion that another catastrophic event occurred in our solar system six hundred million years ago. An enormous super-comet collided with Venus, scattering ice balls and dust grains across the inner solar system. A side effect of this activity triggered a freezing of Earth into a ''snowball'' state.

The most profound transformation then occurred among the microscopic, single-celled organisms that had populated Earth virtually unchanged for three billion years. Suddenly, as Earth thawed, complex multicelled organisms appeared, including the first complex sea animals, and life began moving onto land.

This sudden profusion of life, known as the Cambrian Explosion, marked the effective beginning of rapid evolution on Earth¿but it took a disaster of cosmic proportions to set it off. Had it not happened, Gribbin argues, there would be no intelligent life here. What are the chances that such an improbable chain of events could occur twice in the same galaxy? Zero, says Gribbin.

Is there an upside to Alone in the Universe? For one thing, Gribbin says, Earth and human beings are special, after all. We are no longer insignificant specks in the cosmos but the unique products of an extraordinary set of circumstances that have as yet occurred nowhere else in our galaxy, and possibly not in any galaxy. As such, we are the only witnesses with an understanding of the origin and nature of the universe, and our home is the only ''intelligent'' planet. Gribbin ends his discourse with an impassioned plea for action against climate change and to restore the ailing ecological systems of a planet like no other.