Concrete views over the Garden of Eden

Palaeoanthropologists are, for the most part, gentle people - perfectly agreeable most of the time, but hardly likely to light a fire in, or under anybody. Of course, there are exceptions, and the African early hominid record in particular has been dominated by the strong, forceful personalities needed to attract attention and so funding, and with the drive to overcome the logistic difficulties of fieldwork. By contrast, the study of later human evolution has been a comparatively genteel, consensual affair. Until now, that is. And when palaeoanthropologists squabble, just like the little girl with the curl, they can be really horrid.

About ten years ago ideas about modern human origins crystallised into two alternative hypotheses: regional continuity sees modern humans gradually evolving over the last one-and-a-half million years on an Old World-wide basis from Homo erectus, the first pan-continental hominid. On this interpretation later fossil forms such as the Neanderthals and ourselves are all part of a single species, and their genes therefore contributed to our gene pool. The alternative, replacement theory is that modern humans originated by speciation in one region, subsequently expanding and supplanting Neanderthals and other archaics with little or no interbreeding, so that these groups became extinct. This view is also known as the Garden of Eden or Out of Africa model because of the body of evidence favouring that continent as the origin, but the essentials are the same, irrespective of locale.

Both interpretations have their energetic protagonists, and over the past decade a good deal has been written for and against each one. Fossil evidence has featured prominently but not exclusively in the debate, and genetic data now occupy a good deal of attention. Battle honours have shifted to and fro: in the late 1980s the replacement model received a major boost from Mitochondrial Eve, only to see her credibility undermined in the early 1990s. Bemused observers have noted a shift in the tenor of argument in recent years: dialogue has been replaced by duologue, and what started out as alternative theories of later human evolution have now taken on the edge of competing ideological crusades. Each side appears to pay scant attention to the other's arguments, and there is precious little agreement on even the common body of data needing interpretation. Almost 90 per cent of the evidence cited to support either view differs between the two camps, and there has been talk of a "paradigm crisis" in this area of palaeoanthropology.

In this volume Chris Stringer, one of the senior figures responsible for developing the replacement model, and Robin McKie, science editor of the Observer, describe the history of the theory, the supporting evidence, its major conclusions and implications. The title was inspired by Robert Ardrey's African Genesis, and there are other similarities with that book in the opacity of the chapter headings, the rather meandering structure, the personalisation of the account and strong vein of romanticism, and the style with its at times irritating stringing of nouns as adjectives, and occasional lapses into rather desperate attempts at street cred such as "the projectile Neanderthal hooter" (ie, nose). But there are also delights: one is the illustration of the splendidly eccentric Grover Krantz wearing his home-made Homo erectus brow ridges. Six months' experience convinced him that they were ideal for keeping the sun and long hair out of his eyes, so effectively debunking all those theories that identify Upper Palaeolithic designer sunglasses and permanent waves as key elements of human evolutionary success.

The text gives a clear, complete and up-to-date survey of the evidence supporting the replacement theory. To their credit the authors have omitted any trace of the bitter exchanges that have crept into the conference halls and professional journals - exchanges in which, from my perspective at least, Stringer appears to have been much more sinned against than sinning. The account is, however, inevitably one-sided and, consciously or not, the impression given of multi-regionalism is of an otiose theory clung to out of sheer perversity by a small band of blinkered diehards. This is unfortunate, for while the bulk of evidence favours replacement there are also gaps and inconsistencies, and there are many findings compatible with continuity. And the example of Mitochondrial Eve shows how suddenly the picture can change.

In fact, much of the support for the African origin and replacement model comes from the findings of molecular genetics, and the authors give a remarkably clear and accessible account of this difficult topic, but with one important omission. While the possible action of natural selection is considered at various points throughout the book, it is completely missing from treatment of the genetic evidence. Of course, neutrality is widely assumed in many molecular studies, and selection poses real problems for reconstructing replacement via the genetic data, but selective influences have been proposed to account for both mitochondrial and nuclear DNA diversity. Call me old-fashioned, but I would have liked to see some mention of selection as a possible influence here, even if only to reject it for the purposes of the argument. Ignoring it completely is misleading for the reader.

The later part of the book includes a stimulating and wide-ranging survey of behavioural and cognitive changes in human evolution that I greatly enjoyed, although the anti-racism, anti-genetic determinism sections, laudable enough in themselves, sit rather uncomfortably with the rest of these chapters. The final pages review research into recent human origins and end with a victory proclamation for the Out of Africa hypothesis.

The book is well produced, although the illustrations are often somewhat small and some are unmistakably derived from rather elderly Natural History Museum displays. The text, in contrast, is undeniably current. All in all, this is a clear, accessible and stimulating account of one side of a major controversy in human evolution. But I am less confident than the authors that their theory is the last word on the subject.

Alan Bilsborough is professor of anthropology, Durham University.