Richard Leakey tells Andrew Robinson how a young bone peddler grew up to become a respected palaeoanthropologist and politician. "You say I'm a scientist and a conservationist and a politician. I'm actually none of those things. I'm just me, dabbling in all these things." Richard Leakey paused and then added, "I have no investment in terms of training that most scientists have to protect."

Despite being a palaeoanthropologist and conservationist of world renown, and a leading political opposition figure in his native Kenya, Leakey has a point. Now in his early fifties, he left school when he turned 16 and took no formal scientific degrees. He became a conservationist abruptly in 1989, when Kenya's president appointed him - without prior consultation - director of the Wildlife Conservation and Management Department. In 1994, after being forced out of the same job by vested interests and horrified at the corrupt state of the country, he felt an inner compulsion to help found a new political party, Safina.

When he quit school, in December 1960, there was a severe drought affecting in Kenya. Tens of thousands of animals perished, and the plains around Nairobi were littered with carcasses. The normal scavengers could not cope; the vultures pecked out only the eyes and left the rest untouched, pristine. The young Leakey saw his opportunity. He borrowed money to buy an old Land-Rover and set off to collect dead animals. "I boiled the soft tissues off them in an old oil drum, cleaned and dismantled the skeletons, and shipped them to museums and universities the world over, for a satisfying profit." In so doing he got to know comparative anatomy intimately because he had to label each bone in order that the skeleton could be reassembled abroad. In palaeoanthropology, identification of, say, a skull, often proceeds from mere fragments of fossilised bone. "So, although I didn't know it at the time, my brief, early career as a bone peddler provided me with a solid foundation for my later, lengthy career as a palaeoanthropologist."

Today, that drought provokes him to think about long-term effects rather than short-term gains. What does such a catastrophic die-off tell us about mass extinctions in the geological record? It puzzles Leakey that nowhere in his travels did he ever see an accumulation of bone that would in any way compare to the massive bone beds he excavated with his famous parents Louis and Mary Leakey. His wife has recently come across such a bone bed in her own dig, made of monkey teeth, hominid teeth, bits of jaw, bits of skull and every conceivable type of bone. "What is it that did this?" Leakey wonders. "Why have we no parallels?"

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Decades of exposure, both to the remains of our earliest ancestors around the shores of Lake Turkana in northern Kenya and to the constant flux in the ecology of contemporary animals, have led Leakey to see extinction as a creative force in evolution, "as part of life's flow". To him, unlike many evolutionary theorists such as Richard Dawkins, "the randomness of extinction seems much more persuasive than the randomness of mutation." Darwin, basing his gradualist view of evolution on an extremely incomplete fossil record, denied the existence of mass extinction of species. Were he alive today, faced by the extraordinary fossil evidence discovered since his death, "Would he not have placed as much emphasis upon the process of extinction as mutation?" Leakey asks. He thinks he would. Extinction, according to Leakey, "is a norm" - not an abnormality. What is certain is that Darwin would have been underwhelmed by our century's progress in quantifying living species. We have a better idea of the number of stars in our galaxy than of species on earth. "I can guarantee that if you go into my garden in Nairobi and spend a Sunday, you will find species that have never been collected. They wouldn't be grasshoppers and moths, but you might get midges and if you get down to fungi, heaven knows what you might find."

Leakey believes we have an ethical duty - as well as an economic imperative - to understand this biodiversity much better than we do, and to preserve rather than destroy it as we are presently doing on a vast scale, particularly in the tropics. He is a passionate advocate of the view that humans are part of this biodiversity, not separate from it. "There is no justification on the basis of our evolutionary record for remaining separate and taking the high ground. I find it offensive and disturbing still to find people saying that there are the animals - and there is us. We are not a separate category, we are animals. I don't think you can say we are the only species that is self-aware, which you'd have to do as a justification for our being totally separate."

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One species, more than any other, has deepened this conviction: the elephant. Leakey's celebrated fight to save Kenya's elephants from likely extinction by poachers in pursuit of ivory was at the centre of his unexpected government appointment and subsequent stormy removal from office. "I have been more disturbed by elephants than by any other species," he remarks. For elephants are clearly not related in any evolutionary way to humans (unlike, say, chimpanzees) - and yet elephants do a lot of things humans can identify with. For instance, they sometimes intercede on behalf of other species, by driving lions off buffalo or covering sleeping drunks with branches. Leakey recalled for me an incident during the last month in which an elephant, a matriarch, got thoroughly stuck in a mud wallow at Amboseli National Park in southern Kenya. "The warden said, shoot it - someone else said, no let's pull it out. So they got about 30 guys from the nearby village and took them into the park with tractors and planks. For three days they worked on this elephant, and they got her out. A bit shaky, very disturbed. And somebody panicked and ran and tripped in front of the elephant. She carefully stepped over this person and walked away. Now that affects me."

Leakey, a man of real courage, was moved. He went on: "Those are probably not ethical values, but they're something. There's clearly a brain in the elephant that's doing very complex things. I'm finding myself in this whole debate saying one moment I'm a scientist, I can't accept any of this nonsense - but at the next moment, as a human being I am affected by this nonsense." As a politician, he may just get the chance to apply both standards of judgement in the service of the country he loves.

Leakey and Lewin's The Sixth Extinction is reviewed overleaf