In this book we are going to ask you to imagine a series of events so violent, they dwarf the most vicious crimes that humans have committed against humans and the most horrible natural catastrophes that have taken place on Earth. Even the puniest of the three big bangs, the crashes of comets onto the Earth many millions of years ago, released a destructive energy greater than would be unleashed by simultaneous detonation of every nuclear weapon ever made. In fact, its energy would surpass such a nuclear holocaust by more than ten thousand times. The challenge to the reader of this book is to come to grips with these horrendous events. For only by understanding them can we really understand our origins.

In school we learn of biological evolution: how species competed with other species, often violently, until the weaker ones were eliminated. But in just the last decade, new discoveries have challenged this understanding of biological evolution. More remarkably, scientists have recently begun to understand our physical evolution, to the point that we can now speak in a meaningful way about the origins not only of our countries and our cells but of our very atoms. Perhaps most surprising, we are even beginning to understand the origins of the universe itself, which according to current theory involves the creation not of matter alone, but of space itself and maybe even of time.

The physical creation of the world we know is dominated by violence -- violence on a scale so far beyond human dimensions that some consider it impossible to imagine. In recent years we have begun to recognize that the violence of nature is key to answering the otherwise impossibly difficult question: How did we get here? Fascinating to both children and adults, this question lies at the core of myths and religions in primitive and advanced civilizations alike. Perhaps your parents told you, "We were created by God." Even if this answer were literally true, it would not be entirely satisfactory, because it doesn't explain how God created us.

Scientists have often underestimated the role of large-scale sudden violence in nature, for the simple reason that such events are so rare, we have little experience with them. Their very infrequency means that they are not part of our imagination. We are accustomed to thinking of evolution, for example, as gradual. The evolutionary changes that Darwin saw were slow, like the colors of surviving butterfly species that are altered to match their changing environments. But more recently, two outstanding paleontologists, great fans of Charles Darwin, have argued that the theory of evolution may require a major overhaul. Stephen Jay Gould and David Raup, renowned experts on early life and evolution, argue that the great changes in species may have been driven less by day-to-day competition than by extremely rare and violent events.

Our normal vocabulary is inadequate to name such destructive events. For lack of a better phrase, we use the term originally applied to a particular cosmological theory: big bang. (Fred Hoyle originally coined the phrase to make fun of the new theory of his friend George Gamow.) For the results of such events, however, we do have a name: We call them mass extinctions, for most life-forms on earth are completely destroyed by such events.

This book is about three big bangs. The first -- closest to the human scale -- is the big bang that took place 65 million years ago. One day without warning, a comet, or possibly an asteroid, smashed into the Earth, forever changing life on our planet. The impact blasted out a giant crater in the present-day Yucatan, in Mexico. The atmosphere, the oceans, forests, and jungles were totally disrupted in ways that scientists are now busy unscrambling. Dinosaurs and most other life- forms vanished, including most existing mammals. But some mammals, our ancestors, survived and went on to flourish. There have been many such biological catastrophes, but the one best understood, thanks to a series of remarkable discoveries over the last fifteen years, is this Cretaceous- Tertiary catastrophe.

To astrophysicists, the impact of a comet on Earth is a puny event -- tiny compared with that of a star that erupted 5 billion years ago, a cataclysm even more important to our evolution in the sense of physical rather than biological evolution. Where biologists ask "How did life arise and become what it is today?" physicists ask how the matter we are made of was created, how it changed over the aeons, and what forms it takes now.

When the first stars formed, you could not have found the atoms that now make up your body, but you could have discovered their precursors buried deep inside the stars. Many of these atoms were totally unrecognizable. The iron of your present-day blood, for example, was not yet iron; mostly it existed as hydrogen and helium. Unformed also were the carbon, nitrogen, and oxygen in your organic molecules. In the next few billion years, the nuclear forge of a star would cook hydrogen and helium, through thermonuclear fusion, to create new atoms. But these atoms were still buried in the star. In the second big bang, these atoms were created and ejected into space.

This big bang preceded the extinction of the dinosaurs by about 5- 10 billion years. With little warning, the star erupted, blasting the new atoms over a region of space hundreds of light-years in size. It was a supernova, and without it no life would have appeared in our part of space, for none of the elements that make life possible would have been present. From the ashes of this supernova a second star eventually coalesced, a star that someday would be named the Sun by the two-legged creatures whose bodies were formed from the atoms forged inside the supernova and who now inhabit the small water- cloaked planet that formed near the Sun.

Our third big bang is the Big Bang, the one you read about in the newspapers and science magazines and that long preceded the other two. It was the ultimate explosion that involved all the energy of the universe, the explosion that can never be exceeded, the violent event that dwarfs all other violent events. Although the great scientist George Gamow originally conceived this Big Bang as the event in which all the elements of the universe were created, we now know that most of them, except for hydrogen and helium, were created much later, inside stars.

The story of the Big Bang has evolved in ways that few people could have foreseen more than forty-five years ago, when the idea was first set forth. We now understand the Big Bang as the event in which hydrogen and helium were created from more primitive particles -- and something much more fundamental and mysterious as well. Here is a concept even more jarring to our minds than the creation of all matter: The mindwrenching idea that makes the Big Bang so fascinating is that it represented not only the creation of matter within space but the creation of space itself. And since the Big Bang was the creation of space, then by our current understanding of relativity theory it may also have been the creation of time.

These great catastrophes played a role in our physical and biological evolution that is only now being recognized. Scientists overlooked them for a long time, we think, because catastrophes are rare events, very far from our usual experience. They learned to explain continuous change with the mathematics of Newton and his successors. But now in the late twentieth century, with all other explanations exhausted, scientists finally stretch their minds to calculate the unimaginable. Catastrophe is at the forefront of science today because it is a mystery left over from the triumphs of science through the mid-twentieth century. (Chaos is another such mystery.) Much more difficult to understand than Newtonian mathematics, catastrophe has been left for us to unravel now.

The authors of this book have had the wonderful privilege of being able to research each of the three big bangs. ("Our research has been one catastrophe after another," we sometimes joke.) Although the three big bangs may not seem to have much to do with each other, they do. The strongest link tying them together is their deep shared relevance to the roots of all life on Earth. When we study the comet impact, the supernova, and the Big Bang, we are studying our common history, our truly ancient history. What has drawn us to all of these events is a deep desire to find out where we came from.