Martin Ince talks to Pat Shipman and Alan Walker, the winners of the Rhone-Poulenc science book prize, whose revelations about a 1.5-million-year old skeleton added much to our knowledge of the 'missing link' between man and ape

The one thing everyone thinks they know about the story of man's evolution is that the bones of the "missing link" between apes and humans have never been found. Alan Walker, however, says that they have. He argues that his discovery, in Kenya in 1984, of a nearly complete skeleton of the species that filled that gap casts new light on much that we thought we knew about the steps by which man developed from apes.

As Walker and Pat Shipman (his wife and co-author) tell it in their book The Wisdom of Bones, the Nariokotome skeleton - of a boy who died 1.5 million years ago aged 11 - gives us "most of the answers" about Homo erectus, the hominid whose discovery filled the gap for which the term "missing link" was devised. Including the answer to the question - did Homo erectus have a capacity for language? Contrary to received wisdom, Walker found that he did not.

The Nariokotome boy, so called from the place where his fossil skeleton was found, is unique partly because his body did not suffer the fate of most edible items left lying about in the African plain. "If you died out there today, there would be nothing left by the evening except your fillings," says Walker. Most hominid bones that appear in Kenya have predator toothmarks on them, but the boy has none. Despite this, quite a lot can be gleaned about how he probably died. It is known that after death he fell into shallow water where his head bobbed in the waves, and that his bones washed ashore when his body rotted. His jaw, which was in the throes of teething, shows signs of an abscess which could be an indication of septicemia: because there are no injuries, the odds are that he died of some infectious disease.

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Walker and Shipman call The Wisdom of Bones, which last week won the 1997 Rhone-Poulenc science prize, "the book of the book," since it is in part a popularisation of a massive monograph published by Harvard University Press, the profits of which have gone to further research in Kenya. But the most significant aspect of the skeleton is not in the detail but in the big picture. He was about five feet three tall, and was heading (on the basis of detailed work on humans, chimpanzees and other species) to be about six feet tall as an adult.

Anyone who has looked at medieval suits of armour in museums thinks that people have been getting steadily bigger, but Shipman notes the masses of evidence that points the other way. In 1900, the height requirement for British soldiers was lowered from five feet three inches to five feet in response to the dwindling supply of qualified applicants. The industrial revolution led to a particular shrinkage among the British population, but Walker points out that a more general height reduction followed the introduction of agriculture. "Agriculture has been invented several times," he says, "and we know a lot about the effects. You get far higher population density, but because the diet is monotonous and prone to pests and other hazards you get people who are smaller."

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The large stature of the Nariokotome boy, since confirmed by reexamination of other Homo erectus bones, is more than a curiosity. It is a gross physiological difference between erectus and his predecessors. Walker says that such a change is highly informative. "A person that looks like this is a predator," he says. Probably he ate mainly animals like today's gazelles and impalas. This tells us that he had weapons, and that he lived in a complex society, since people on their own cannot catch fast-moving animals, and dogs had yet to be domesticated.

Predators are larger than herbivores, but their population densities are far less. On the basis of case lore of herbivores going carnivore, Walker predicted that the growth in body size of erectus would be matched by a spread in its habitat - predators need a lot of space. For some years, he says, the prediction languished, but it has now come good with finds from China, Georgia in the former USSR, Indonesia and elsewhere. The switch to the predatory lifestyle led to hominids spreading beyond Africa as far afield as northern Europe.

Walker is dismissive of the idea, made popular by concerns about global warming, that the spread of human ancestors owed a lot to climate change. As the earth warmed, it has been argued, the area they could inhabit expanded. But he points out that chimpanzees, gorillas and others lived through the same climate change without moving to Java. In any case, Shipman says, there is a lot of loose talk about habitat change which does not allow for the ways species actually operate. She says: "Usually there is a refuge, perhaps quite close by, where the species can survive." The rainforests occupied by the great apes have over time shrunk almost to nothing and then expanded again, and the range the apes occupied has altered with them.

Winning the Rhone-Poulenc prize vindicates a set of interlinked life decisions that have turned Walker and Shipman into a formidable teaching, research and writing machine. They have moved to Pennsylvania, where he is now distinguished professor of anthropology and biology at Pennsylvania State University. Shipman also has a part-time post there.

Walker moved to Pennsylvania from the medical school at Johns Hopkins University in Baltimore, a top US research institution. He left partly to get out of a job teaching anatomy. "I had been cutting up bodies for a living for 30 years, and turning people into something that looks like Shredded Wheat was still emotionally problematic for me," he says.

The move allowed him to teach and work full-time on fossil hominids, and to leave a grim inner-city life (the medical school spends $7 million a year on security) in favour of a rural locale in which black bears digging through the bins are the biggest hazard. He was encouraged by a MacArthur Foundation "Genius Award" paying $60,000 a year for five years with no fixed duties. Shipman's writing career has also benefited. As well as this book and a host of articles, she is finishing a book on the origins of flight. And they plan a book on the split between chimpanzees and hominids, using molecular biological evidence as well as fossils to illustrate the separation.

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The experts involved in finding out about Nariokotome are legion, starting with Kamoya Kimeu, who found a bit of the boy's cranial vault in 1984. Kimeu is one of the "hominid gang," six men who between them have found parts of dozens of our ancestors. Also involved were Walker's collaborators from the Leakey dynasty of Kenyan anthropologists. Walker originally worked with Richard Leakey, whose energies now go mainly into Kenyan politics, collaborates with Meave Leakey, Richard's wife, head of palaeontology at the Kenya National Museums.

In his hominid work, Walker is the most visible member of a team whose net spreads to Australia, home of the team that dates Walker's samples, to US experts on tooth growth and other subjects, and to Britain. Ann MacLarnon of Roehampton Institute, London joined the team even though she had previously worked only on living primates. She had shown that the size of the holes in the discs that make up a primate spine is related closely to the size of the spinal cord, the bundle of nerves that connects all parts of the body to the brain. Humans have a big spinal cord, especially in the region where the nerves run that control the upper body. Nariokotome boy has smaller holes, like those of monkeys and apes. The reason, Walker and MacLarnon eventually agreed, is that he had far less control over the rib cage and abdomen than we do. This in turn means that he could never have managed the breathing control we take for granted to enable us to speak.

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The message of the spinal column holes matches the signals about Nariokotome boy coming from a much more obvious site, his skull. Measuring brain volumes in the hope that they will tell us about human or animal intelligence is a controversial business, but in this case the answers were consistent with other evidence.

It turned out that the boy's brain was 880cc in size. Walker points out with regret that teenagers then and now have as much brain as they are ever going to have - at most, his brain would have expanded to perhaps 900cc had he lived to adulthood, a figure consistent with other Homo erectus fossils. By contrast, modern man has over 1300cc of brain.

The boy thus had far less brain as a percentage of his body (a low encephalisation quotient, in the jargon) than we do, and indeed had only about as much as Homo habilis, his evolutionary predecessor. His encephalisation lies about halfway between ours and that of the chimpanzees.

A big brain is a fundamental part of being human. Our brains are so over-sized by comparison with those of our primate relatives that they will not fit down the birth canal when we emerge from the womb. Hence the soft and holed skull which allows the brain to go on growing after birth, an attribute which had already developed when Nariokotome boy was alive.

The boy's small brain reveals that although he might have looked like a modern 11-year-old, he could never have acted like one. He lived in an era well before primates started using symbols, a step that seems to date back 100,000 years at most. There are circular stone markings about 50,000 years old in Australia, while the more sophisticated Lascaux cave paintings date back about ,000 years.

Walker says that animals which do not use symbols probably do not use language as we know it, in which the past and the future, or things that are absent or imaginary, can be discussed in the same way as things that are present and tangible. In Walker's view, that is exactly the difference between an 11-year-old of today and the boy who fell in the lake at Nariokotome 1.5 million years ago. A time traveller from today might think he was about to meet a modern youth: but as Nariokotome boy turned to look, his eyes would be as cunning and blank as those of a lion.

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The Wisdom of Bones, Alan Walker and Pat Shipman, Phoenix, Pounds 7.99