In the beginning was the gene

We have become accustomed to the idea that successive generations will rewrite history according to their own lights; but rewriting religion is less familiar a concept. Yet we now live in a secular time, and the book of Genesis is being rewritten accordingly. The new chapters are appearing, in of all places, the popular science shelves of our bookshops.

The physicists started it, with titles along the lines of The Physics of Immortality, God and the New Physics, and At Home in the Universe. All of them are variants on that oldest and most potent opening to any narrative: "In the beginning...''

But just as in the religious life, so in the secular imagination: it is not enough, psychologically, to discuss the origins of the cosmos. God moved on quickly from creating heaven and earth to living creatures and, by verse 26 of Genesis's first chapter, he had created humanity. Now the biologists are catching up fast on the physicists to reinterpret the creation story in a modern idiom.

James Shreeve, in The Neanderthal Enigma (Viking, pounds 18), gives us a haunting tale of the Neanderthals as prelapsarian inhabitants of a real Eden, whose expulsion from the Garden was so complete that they perished from the book of life some 35,000 years ago, just as we modern humans consolidated our dominion over the world. It is a story that has already received literary attention: William Golding's novel The Inheritors focuses on one incident of ineffable sadness - the theft by modern humans of a Neanderthal family's baby - to illuminate the ambiguity of our notions of good and evil. Were the Neanderthals without sin, and were we moderns sinfully responsible for their extirpation - in a sort of Paleolithic ethnic cleansing?

Shreeve certainly seems to think of them as sinless Noble Savages: "I picture two Neanderthals sitting side by side, their intimacy so exact that their interior voices cross and coalesce like two streams merging into a river, their waters indistinguishable." The Neanderthals' lives would be poorer and yet richer than our own: "There would be no sciences, arts, media... On the other hand, there would be none of the heated, sustained hatred and aggression of war, no oppression of one folk by another, no contamination of the one earth by all."

Imagery of the Noble Savage appears again in The Wisdom of Bones by Alan Walker and Pat Shipman (Weidenfeld, pounds 18.99) in the shape of "The Nariokotome Boy" - the most perfect specimen yet found of Homo erectus, the "missing link" between ape-like creatures and the lineage which led, eventually, to Neanderthals and to humans. The boy died some 1.5 million years ago, aged between 11 and 13. The first of his bones was discovered in 1984 by Kamoya Kimeu, a brilliant Kenyan fossil finder who had once worked for Louis and Mary Leakey at the famous Olduvai Gorge in the 1960s. By 1984, he had become foreman to an expedition at Nariokotome, Kenya, led by Walker, one of the world's leading paleoanthropologists.

The stunning revelation from the boy's skeleton is that, although he looked human rather than ape-like - he was tall, long-limbed and narrow- hipped - his brain was grossly underdeveloped by our standards. He walked upright, as the erectus component of his name signifies, but in modern human terms he had a one-year-old's brain in a 13-year old's body. Something essential to being fully human was missing and, over a period of many years, Walker painstakingly established that it could only be language. The Nariokotome Boy lacked not just the capacity to speak, but to think.

Part of the excellence of these two books lies in the way they show science in the making. Walker's book, co-written with his wife Pat Shipman, details the sheer physical labour involved in shifting tonnes of rock and earth to uncover fragmentary scraps of bone. Once the fragments are prised from the soil, a different sort of labour ensues as researchers struggle to piece them together like some frustrating jigsaw puzzle. Only after all this has been done it is possible to address the intellectual puzzle of what it all means.

Both books are good, too, on the human passions and human failings of science and scientists. The Dutchman Eugene Dubois discovered the first evidence of the missing link in Java towards the end of the 19th century, but no one would believe him, and Dubois's passionate intensity turned almost to paranoia. In the late 20th century, passions fly between Chris Stringer, of the Natural History Museum in London, and Milford Wolpoff, of the University of Michigan, as Shreeve's book shows. He presents a fascinating insight into the intense personal animus that can develop between scientists as they passionately debate the origins of modern humans.

The point at issue is the "Out of Africa" hypothesis. No one now disputes that Homo erectus was the transition between ape-like creatures and the human lineage, and that Homo erectus spread out of Africa to populate the entire globe. What is at issue is how Homo erectus evolved into Homo sapiens. Did it happen in Africa all over again, but this time a few hundred thousand years ago, rather than a few million?

According to this idea, promulgated by Chris Stringer, anatomically modern humans originated in Africa and migrated out to populate the entire globe. Wolpoff, however, believes that the transition proceeded in parallel right across the planet. Both sides have defended their view not just by appeal to "objective" evidence, but also by partisan showmanship far removed from the lay understanding of how science is conducted. It is a strength of Shreeve's book that it displays this human side of science rather well.

For the truth is that there is room for doubt, opinion and polemic in science - in evolutionary biology perhaps more so even than in other scientific disciplines. Like cosmology, that other surrogate religion, evolutionary biology has more in common with history than with conventional experimental science. A chemist or a physicist can go back to the laboratory to repeat and refine an experiment; a biologist cannot replay the evolution of Homo sapiens. Evolutionary biology in general does not depend on the fossil record - Darwin's "origin of species" does not rely on arguments deriving from fossils - but it does require the mute testimony of the rocks and the fossils, just as the historian requires written historical records. Each fossil has to be placed in context and milked for maximum information. Although astonishing information can be gleaned from the slightest specimens, there are gaps in the fossil record as there are in the historical record. These gaps offer ample space for personal interpretation - and therefore for dispute and contention.

Oddly, virtually none of this appears in Richard Dawkins's account of evolution, Climbing Mount Improbable (Viking, pounds 20). While the others record disputes and debates, Dawkins opts for a detached, empyrean tone. Like the Pope on Catholic dogma, Professor Dawkins decrees ex cathedra (with overtones of infallibility) that biology is as he describes it. Yet this tone is a paradox, for evolutionary biology has its disputes and interpretations too, and his book is in part a polemic.

Darwinian evolution has been established beyond all reasonable doubt. The "Ultra-Darwinists", of which Dawkins is a high- priest, and the "Naturalists", of which that other great prose stylist of science, Stephen J Gould, is a leading light, dispute the technical details of its operation. Natural selection forms a unifying principle to which Richard Dawkins clings with almost religious fervour in the belief that it explains everything: to the Naturalists, natural selection is absolutely necessary - but not sufficient - for us to understand the diversity of the living world today. Gould considers it a matter of historical contingency that humans have ended up on the planet. Start off the evolution of life on earth again and you would not necessarily recreate Homo sapiens. The meteorite which destroyed the dinosaurs 60 million years ago on Earth might, in some parallel universe, just have missed.

These books are narratives about how life comes about on this planet and, ultimately, about humanity's place in the biological scheme of things. It may seem perverse to think that the problems of individuals living in the technological societies of the late 20th century can be illuminated by looking at creatures such as the Neanderthals, who did not know how to sow grain and harvest crops, or at Homo erectus, whose brain was too small to enable it to use language. Indeed it is perverse, but it seems that if human beings are to have a sense of who they are today, and what they might make of themselves, they will turn inexorably to look whence they have come.