Introduction When I was thirteen, my science class got in a school bus and rode across the river into Pennsylvania. The bus pulled over on the side of lonely road. From the shoulder rose a small exposed outcrop, crowned by trees. We were each given a small, strong hammer and instructions to smash the stony slope. Some kids scrambled up the outcrop, where they could hammer in the cool shade. I preferred to keep my feet on asphalt, and so I toiled in the strong sun. I could hear the pinging hammers above me, and sometimes a little chip of rock landed lightly on my head. I was too distracted to care, because my hammer was revealing just the sort of thing we had all come here for: a series of small black ribs emerging from the rock.

As the ribs came into view, I aimed my blows to avoid smashing them, landing my hammer a few inches away on all sides. Eventually, I fractured the rock and the black ribs tumbled out. Leaning my hammer on my left shoulder, I caught it in the palm of my right hand. The sight of that ribbed rock is still with me, and will probably never leave. Our teacher had explained to us all a few days before what this was, and now I tried to join his words to my vision. This was not a rock--or, at any rate, not just a rock. It was a rock in the shape of a once-living thing. The thing in my hand was an animal known as a trilobite.

The ribs formed a flexible shield across its back. Its eyes were like a pair of speckled jewels. Trilobites became extinct 250 million years ago, but before then they lived in beetle-like abundance. The trilobite in my hand had crawled across sea floors, and then it died and was buried under a slowly thickening layer of mud. Instead of decaying away, becoming a meal for bacteria and scavenging animals, my trilobite had been transformed through a mineralogical alchemy into a stony replica. And it had remained buried in place ever since, through hundreds of millions of years of Earth''s history, as dinosaurs emerged on land and then, evolving feathers, took to the air; as the sediments that were its tomb vaulted from the ocean and became dry land, rose higher into serrated mountains that eroded down into gentle hills, which were overrun millions of years later by little hairless bipedal apes swinging hammers. This was not my first time in the presence of fossils. I had spent plenty of time as a boy standing before the fossils of tyrannosaurs and mastodons in museums.

But in those fossil halls, I couldn''t be viscerally sure that those fossils had actually been delivered from the Earth. On that hot day standing by that Pennsylvania outcrop, looking at this little cousin of horseshoe crabs I had liberated from the Earth, I knew for sure. Extinction and Evolution brings back that late summer day to me. Perhaps it''s because trilobites have meant so much to Niles Eldredge. He didn''t just dig one out of the ground as I did: he''s inspected countless trilobites over the course of many years. And in that time, he has been able to do more than just imagine the life of an extinct animal. He could envision the evolution of trilobites over millions of years, through countless generations. The adventures of a single trilobite--its mating, its laying of eggs, its eventual demise as a predator''s meal, perhaps, or the victim of a virus--becomes a tiny tile in a grand mosaic.

Only by stepping back and looking at the entire mosaic could Eldredge begin to see patterns in the stasis and pulses of trilobite evolution. Trilobites have much company in the pages of this book. Each image of a fossil has its own particular anatomical beauty, from whorled snail shells to toothy crocodile snouts. But the genius of the theory of evolution is that it joins nature in all its disparity into a single flow. Genes obey the same logic whether they are carried by bacteria or bears. Mutations give rise to variations, and natural selection and other processes can assemble them into new forms of life. New ecosystems can take shape, which then turn back and guide the transformation of the genes in their constituent species. The fact that a species could become aware of billions of years of evolution is astonishing, and we can count ourselves lucky that that species is our own.

Carl Zimmer.