The author's conclusion: Earth is the sole abode of intelligent life in the galaxy, the product of a profoundly improbable sequence of cosmic, geologic and climatic events—some thoroughly documented, some inferable from fragmentary evidence—that allowed our planet to become a unique refuge where life could develop to its full potential.I agree with this. Between science fiction stories and astronomy news, most people probably have the view that the scientific evidence favors the increased likelihood of sentient life on other planets. But there is a lot of evidence to the contrary, and I believe that it is likely that we are the only intelligent life in the Milky Way galaxy.
Chief among these, paradoxically, was a near-cataclysmic planetary collision during Earth's infancy, which gave birth to the moon. Such encounters were relatively common in the harum-scarum chaos of an early solar system that teemed with veering planets and asteroids. In its suicidal blow against our world, the Mars-size impactor generated enough heat to liquefy both itself and Earth's exterior. Its dense, metallic core plunged inward to join our planet's existing metallic center, while the rest swept up part of the fiery terrestrial shell to form the moon.
One consequence of Earth's tumultuous youth was the thinning of its rocky crust. This has provided the planet with a lively tectonic existence, complete with vapor-spewing volcanoes, continents that divide and drift, and an ecologically advantageous global-temperature-regulation system. Earth's swollen metallic core remained liquid; its constant churning gives rise to electrical currents that generate a far-flung magnetic cocoon that shields us from dangerous solar particles. (The creation of Eden is far more complex than one might have heard.)
Another fortuitous coincidence on Mr. Gribbin's checklist is the moon's large size relative to Earth, a ratio unique in the solar system. Without such a gravitational partner to restrain the disrupting tugs of the sun and Jupiter, our planet might suffer paroxysms of axis-tilting. (Try to run a civilization when your once-temperate hemisphere suddenly heels over to an Arctic orientation.) Mr. Gribbin outlines how a series of climate-altering Ice Ages and tectonic shifts benefited human ancestors roaming the grasslands of East Africa. ...
Mr. Gribbin admits the possibility —even probability—that elementary life forms have arisen elsewhere in the galaxy. But the object of his scientific and statistical scrutiny is intelligent extraterrestrial life. While he cannot prove a galaxy-wide absence of other civilizations, he presents an array of modern, research-based evidence that renders that conclusion eminently reasonable.
There are many other galaxies, and this leaves the possibility of sentient life in other galaxies. But even those science fiction stories do not suggest communication with other galaxies.