For some years scientists have been looking to the skies for the solution to the extinction of the dinosaurs--and of 70 percent of all living species--at the end of the Cretaceous period 65 million years ago. Allaby, an ecologist, and Lovelock, a physicist, explain why older, gradualist theories of climatic change, other habitat change, and competition from mammals and birds could not account for the sudden mass extinctions. As for the extraterrestrial candidates, they argue that a dust cloud from a supernova explosion, if sufficiently dense to account for the extinctions, would have had to come from an explosion so close that the entire planet would have been burned to a cinder. They point to high concentrations of irridium, osmium, and other rare heavy metals found at the geological boundary between Cretaceous and Tertiary rock layers as evidence for collision with a body from space. The prevailing ""dirty snowball"" model makes it unlikely that a comet could deliver the requisite amounts of osmium and iridium, and so ""we are left with meteorites and asteroids."" They favor an iron meteorite but, conceding uncertainty, accept the more general term ""planetesimal."" Their scenario begins with the object crashing into the North Atlantic, the sea blowing up, a sky-wide fireball giving way to darkness, and seismic waves penetrating the earth followed by sound waves and tidal waves. Locally, no living thing would survive the impact; and worldwide the period of prolonged darkness, from particulate matter ejected by the impact and the volcanic activity it triggered, would combine with a ""cocktail of poisons"" to effect the rapid demise of existing large animals--leaving niches for the mammals to fill when the earth returned to normal. More perfunctorily argued are the final chapters in which the authors insist that our own power to destroy the world or even ourselves is ""quite imaginary"": the world and humanity could easily recover from a nuclear war; and we should develop our missile technology in order to deflect the next, long-overdue planetesimal collision. Till then, the presentation is considered and convincing.