In the Beginning and Before : SCIENCE : THE TIME BEFORE HUMAN HISTORY: 5 Million Years of Human Impact,<i> By Colin Tudge (Scribner: $27.50; 366 pp.)</i>

“I want to tell the true history of human beings, beginning at the beginning, which, in truth, was around 5 million years ago,” writes Colin Tudge at the start of “The Time Before Human History.” Conventional history, he says, generally begins at the end, with the development of organized agriculture, the creation of cities and the development of the first large, complex civilizations about 10,000 years ago. Tudge, a British journalist and science writer, wants to place this familiar recent history “into perspective, by showing what went before that and why our own history began in the first place.”

In fact, the sweep of the book is far grander still: It begins in the primeval sludge with the creation of oxygen in the Earth’s atmosphere, treats the formation of the oceans and continents and then moves through the entire history of evolution from the microorganisms to the current dilemmas facing modern man. Indeed, the book’s subtitle, “5 million years of human impact,” is somewhat misleading: The hominids do not make their entrance until halfway through the text.

Tudge is covering all this ground with a clear political purpose: to contribute to the current debate on issues such as global warming, population control and the limits of economic growth by reminding us of the complex roles played by atmosphere, climate and other species in the life of our planet.

Tudge describes the extraordinary concatenation of circumstances--the Earth’s size and position in our solar system, the particular chemical composition of the atmosphere and the ozone layer--that made life possible. These conditions are by no means “natural” or inevitable, Tudge writes, and the Earth’s climate has changed violently over time, creating both periods of “global warming” and epochal ice ages, altering the shape of continents and oceans and affecting the course of evolution.

By stressing the incredibly long history of life on Earth, he is urging greater humility on the part of human beings. Homo sapiens has been around for just 40,000 years, he writes, while other species have survived for millions of years. By any measure, the dinosaurs were extraordinarily successful and were probably wiped out not because of some fundamental inadequacy but because of truly dramatic climatic change.

He discusses the way in which the shifting of the continental plates has influenced the emergence and disappearance of numerous species. The linking of North and South America, combined with cooler temperatures, led to a mass migration of mammals toward the South, wiping out many local species. The fact that tiny lemurs, but not predator monkeys, were stranded on Madagascar when it broke off from Africa explains why they have survived there but not on the continent. Recent fossil evidence shows that the number of extinct species vastly outnumbers those still in existence, a useful perspective on the current discussion of biodiversity.

Tudge’s great strength as a generalist is his ability to move comfortably across a wide range of scientific disciplines, from astronomy and physics to geology, zoology and anthropology. But at times, he seems to get bogged down in the internal debates of a particular field, which may not have much bearing on his larger point. It was not necessary, for example, to replay the whole history of plate tectonic theory in geology to make us understand that the movement of land masses has affected the evolution of many species. Similarly, he devotes a long and turgid central chapter to discussing virtually every taxonomic classification of mammal, from the Monotremata (duck-billed platypus) to the Pongidae (chimps, gorillas and orangutans). After a while, one feels like a gatekeeper at Noah’s Ark, counting the animals two by two.

The considerable ambition of “The Time Before Human History”--what the author himself describes as its “vauntingly presumptuous purpose”--is simultaneously its great strength and weakness. The interest of the book lies in its willingness to tackle the big questions of life on Earth, but one sometimes feels that the author has simply tried to do too much, losing the overall thread of his story by delving into so many different scientific fields. Ironically, even though Tudge is a writer by trade, “The Time Before History” often makes for choppier and more difficult reading than the work of some professional scientists (Steven Jay Gould, Freeman Dyson or Lewis Thomas) who have written for a general audience. But if one sticks to Tudge’s book, it does yield considerable rewards.

He familiarizes the reader with some of the principal questions in a whole host of fields. He discusses the various theories about why the dinosaurs became extinct, about the nature of “punctuated equilibrium” in biological evolution, about how the various kinds of hominids may have evolved, about the origin and extinction of the Neanderthal.

Perhaps the most novel and provocative section of the book deals with the possibility of “human overkill”: In this theory, put forward by some biologists, the spread of human beings across the globe caused the mass extinction of an exceptionally large number of animal species. Although this is not a universally accepted idea, it does appear that a dramatic decrease of numerous mammals coincided with the arrival of humankind on various continents. While modern humans are placing enormous pressures on the environment, the “noble savages” of the past may have already had an equally profound impact. This does not lessen the potential tragedy in our current predicament, namely that hundreds of beautiful and important species may soon disappear if the patterns of human development remain unchanged.

Although he is relying on the research of others, Tudge navigates skillfully through an extraordinary number of the natural sciences with what appears (to a layman’s eye) as balance and common sense and represents this wealth of knowledge in a framework that forces us to rethink some of our assumptions about the permanence and immutability of human life on Earth.