The story of human origins is a story about seafaring folk. This may seem a strange claim, given that the first boats appeared five million years after our earliest ancestors. The human story is surely one of landlubbers, riding the waves of climatic change, including repeated ice ages, rather than voyaging to new lands. This, of course, is what the evidence overwhelmingly tells us.

The hominids, of which we are the latest variety, split from the last common ancestor in Africa south of the Sahara around five million years ago. The date comes from comparing our genetic fingerprints with our closest relative, the chimpanzee, and estimating the speed of divergence. The oldest hard fossil evidence comes from the Aramis region in Ethiopia where remains of a badly crushed skeleton dated to 4.4 million years ago were found in 1992. Tim White and Desmond Clark from the University of California at Berkeley initially named this and other finds as Australopithecus ramidus in 1994, but a year later reclassified them as a new genus, Ardipithecus ramidus. Ramid means root, so originally this fossil's Latin name could be translated as the earliest Australopithecine or southern ape, even though it comes from the northern hemisphere. Ardi comes from the Afar word for ground or floor. Here then is the "basement" ancestor from which the ramifying pattern of hominid evolution developed.

And this is why human origins is a story of seafaring folk, involving some colours being well and truly nailed to the mast with six-inch Latin names. Palaeoanthropologists, rather than being treasure hunters or scientific Sherlock Holmeses able to squeeze, literally, blood from stones, are instead charting a course in largely unknown waters. They have principles of navigation, as supplied by evolutionary theory and comparative anatomy, but it is difficult for them to take accurate bearings. Moreover, their data vary enormously in the time scales they cover. The oldest stone tools, also from Ethiopia, 2.5 million years old, took perhaps a few minutes to make; yet that tradition of manufacture lasted for almost a million years. The average life expectancy of an Australopithecine was about 20 years, as large samples from caves in southern Africa where they were first named has shown. However, many of the subspecies afarensis, robustus, boisei and africanus were around for at least a million years. Ecological time, as measured by the environment which surrounds an organism and to which it must adapt, must be compared with evolutionary or geological time, in which the effects of these small-scale, immediate contexts for selection and change are immured.

Because of these disparities in scale, not to mention the variable quality of the data itself, the methods of the palaeoanthropologist can best be described as similar to sailing a yacht into the wind, tacking from one scale to another, building up an argument in a zig-zag fashion rather than steering a dead-ahead course. As a result, books on human origins take a number of forms. There are those that chart the known rocks in the water - the hominid fossils, their ages and affinities - and then sail a course around them. Then there are others which present a personal account of what it is like to be on deck as the waves crash and new shores loom out of the mist. Finally, there are books devoted mostly to the theory of navigation and the shape of the world.

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Donald Johanson, working in collaboration with several co-authors, has written in all three styles. What initially propelled his craft through the water was his remarkable discovery in 1974 of a partial but very small female skeleton dated to 3.2 million years ago. A. L. 288-1, Australopithecus afarensis, was a ground-breaking discovery for studies of upright walking and sexual dimorphism in early hominids. Known as Lucy, because "Lucy-in-the-Sky-with-Diamonds" was playing in camp that day, she stars in two of Johanson's books which, with co-authors James Shreeve and Maitland Edey, present the view from the deck. In idle moments I have sometimes wondered whether, had Johanson happened to find a large male skeleton that day, and the Beatles's track "Bungalow Bill" had been the camp-fire song, this would have changed the course of human origins research. Bill would have suited these afarensis fossils very nicely, since they have nothing up top.

Lucy's third book, written with Blake Edgar and richly illustrated with David Brill's magnificent photographs, lacks this personal touch and instead charts the rocks in the water. It is a book of two halves with the first section devoted to presenting central issues of palaeoanthropology and the second to encountering the evidence. Forty-eight central issues are discussed under 13 major headings. They range from dating fossils, drawing the family tree, to cannibalism and questions of human extinction. The headings range from "Evidence" to "Migration" and from "Customs" to "Imponderables". Archaeology takes a back seat to the fossil remains. As a result the context of discovery and the reconstruction of past life-styles from the preserved positions of stone tools to huts, hearths, animals and bodies, is lost. The impression from both text and photos is that stone tools provide an adequate measure of what was going on inside those skulls.

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I found this section informative in parts but plodding. The title of the book is not really used as a framework for the choice of issues to be discussed. Neither is it used to present a time line for the tempo and mode of hominid evolution. The closest we get are the gushing sleeve notes where we are informed this is one of the greatest stories ever told by the fusion of two great writing talents. I would judge such talents by the ability to take difficult issues and present them simply. Moreover, I would expect the case to be made for the wider relevance of one branch of knowledge to central issues not just in palaeoanthropology but in wider contemporary concerns. By definition this must lead towards controversial matters. The authors flirt with controversy by including a section on race and the problem of consciousness. The treatment of both, however, is too short to provide a satisfying view of how palaeoanthropology has contributed to these major questions. Rather, it seems that such issues are beyond a discipline which is primarily presented as a charting expedition.

Two further areas of controversy which might have backed up their claim that primary research requires intellectual rigour concern fundamentalism and human skeletal remains. The former is discussed under the heading "Why is palaeoanthropology so contentious?" Sadly they misquote Ashley Montagu, scourge of fundamentalists, having him say that "creationism has certainty with evidence''. If that is the case, then we should abandon ship now. The second area that courts controversy is the treatment of modern human remains. To illustrate the short section on race, full-page colour photos with frontal and side views of seven modern skulls are presented. The point is graphically made by removing the soft tissues, skin colour, eye shape and hair, that geographical diversity is still there, albeit revolving around a modern skull form. What comes as a surprise, however, are the large, hand-lettered find numbers which stand out like tattoos on the skulls. This is unfortunate because in one case the person's name is also written on the skull along with the date of her death, 1900, while in another, number 3662, we are told that the person's teeth are stained because he ate betel nuts. It is interesting that the photos of the early hominid skulls have no visible numbers, indeed they even have names like Lucy and Mrs Ples, while people who have died in this century and been collected for science are treated as specimens. Undeniably, as the authors write, "hominid fossils are glamorous and bring scientific and popular success to those who find them", because in our culture the physical remains of bodies are powerful symbols. But with that power also come responsibilities, as physical anthropologists have repeatedly discovered when they trumpet science over sensitivity in reburial issues. Indeed the Pleistocene age skulls from Kow swamp in Australia that are shown in part two have been returned to local Aboriginal communities and reburied.

Which brings me to the second section, encountering the evidence. This part of the book is more successful. In the first place it justifies the large format because nearly all the skulls and limb bones can be presented as life-size colour photos. Those infuriating scales are done away with and we can appreciate the relative size and scale of the material. Second, they have provided us with some new rocks in the sea of ignorance, particularly in the shape of an early jaw of early Homo which is 2.3 million years old and was found in 1994 on Johanson's Ethiopian expedition. This photographic essay backed up by clearly written comment on each of the 53 fossils from Ardipithecus to Homo sapiens is a tremendous gallery of hominid evolution. It is nicely complemented by the opening sequence of colour plates where we go from the tiny skull of Australopithecus africanus at 2.5 million years ago to a huge skull of an early modern human from Israel some 90,000 years ago. Only one reconstruction of these skulls is presented, John Gurche's afarensis with slightly receding hairline. This was a bold decision: more such reconstructions would normally be expected in a book that aims at a wide readership.

What we have instead is the presentation of fossil skulls as art objects. The black background and expertly lit faces are both artistically dramatic and scientifically informative. So striking are the images that as you gaze into their empty orbits, and wonder at the sights they must have seen, the temptation is to conclude that discovery and presentation is enough, that the fossils tell their own story once they have been positioned on the time chart. Fortunately they have mostly been photographed with their mouths firmly shut. It is the job of the palaeoanthropologist to prise them open - and that can only be done by setting a new course.

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Clive Gamble is professor of archaeology, University of Southampton.