Donald Johanson, the flamboyant fossil-hunter who shot to fame after his discovery of a three million-year-old skeleton in 1974, tells Lucy Hodges about his plans to excavate in Eritrea

Why does paleoanthropology seem to attract swashbuckling types with big egos and vaunting ambition? Is it that you need that kind of pizzazz to raise money for frequent trips to the Rift Valley and the self-belief to think you will find the oldest human fossil ever? Or does the subject simply fire the imagination of restless people who in former times would have been buccaneers or explorers?

Certainly Donald Johanson has all those qualities. He is the discoverer of Lucy, a partial skeleton he stumbled across in Hadar, Ethiopia, in 1974, and the oldest such find at that date. Named after the Beatles' song "Lucy in the Sky with Diamonds'', the fossil, with its apelike skull and upright walking posture, catapulted him to instant stardom.

It also began a controversial career, leading to a rift with Richard Leakey, another flamboyant character in the turbulent world of fossil-hunters, and a more recent falling-out with members of the millionaire Getty family who used to bankroll his Institute of Human Origins in Berkeley, California, but withdrew their money on the grounds that, in their view, Johanson seemed more interested in high-profile TV work than in low-profile science. Since then Johanson has been re-thinking his career. He may be setting up shop shortly at Arizona State University and will continue to root around in Africa in his quest for the origins of man.

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Johanson did not start out studying human origins. At the University of Illinois, he did chemistry. "After a while I realised I was sitting in a room with 300 students. We were all solving the same problems. I was more interested in a field where there was still a lot of opportunity and room for discovery. So I switched to anthropology."

In 1970 he moved to the University of Chicago to do a PhD on chimpanzee teeth. He soon teamed up with someone who was digging in Africa. "This was part of my plan, to get somewhere with someone who had an active field project that I might be able to get on." He flew out to Omo in southern Ethiopia and realised that he was doing what he wanted to do. "I decided I wanted to work in Africa. Those were some of the happiest times for me."

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"Anybody involved in the hunt for human origins at the alpha level must harbour the hope that they too might make a discovery," he says. Early on in Africa he met a group of French scientists and was invited to Paris where he bumped into a geologist called Maurice Taieb who was working in north-eastern Ethiopia. Taieb showed him pictures of fossils - pigs, elephants, gazelles, rhino. Johanson realised these fossils were probably more than three million years old, so he leapt at the chance to visit Taieb's site. "This was the ultimate opportunity. I did everything I could to scrabble together some money.'' What he gleaned from that trip persuaded him to return and look more systematically. The next year, 1973, while surveying the Hadar badlands, he saw something sticking out of the ground. Thinking it was a hippo rib, he kicked it. What he found was a tiny hominoid fossil of a knee joint, the oldest evidence at the time of human upright walking. On the basis of that discovery Johanson was able to drum up funding for another expedition to the same region the following year. He invited the Leakeys to visit his campsite in north-eastern Ethiopia and they had a wonderful evening under the stars, debating their favourite subject and laying bets on what they would find.

The next day, a Sunday, Johanson decided to do some paperwork. But one of his students wanted help locating the site of an important pig skull for an aerial survey. The area had been well trodden and footprints were still evident. "We walked around this ridge and looked out," says Johanson. "It was about noon and getting hot and all I'm thinking about is going back for a swim in the river. We're walking along, going back to the Landrover, chatting to one another and I look back over my right shoulder and see an elbow bone on the ground. I look at it and say 'Tom, look at that bone'. He replies, 'Yeah, it's a monkey.' I kneel down and say, 'This is a hominoid. This is not a monkey bone'.'' All around were pieces of cranium.

The rest, as they say, is history. Johanson had found Lucy. Looking up the slope, they saw ribs sticking out, the crest of the pelvis and a piece of jaw, all of it 3.5 million years old and eroding out of the hillside. The pair raced back to camp and that night had a great celebration at which "Lucy in the Sky with Diamonds" was played. It seemed a good idea to call their skeleton find Lucy.

"It launched my career in an unbelievable manner,'' says Johanson, who was only 30 at the time. He and his students had found the first upright creature who was more than three million years old. In the flesh she was bipedal, walked upright, but was only 3.5 feet tall which suggests she probably spent quite a bit of time in the trees escaping a lion or hyena. She may even have slept in the trees. But her skull was apelike, with a projecting face and small brain, and she couldn't talk.

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In short, she didn't fit in to the hominoid family tree. More discoveries followed in that area and further south in Africa. Johanson decided she was a new species and called her Australopithecus afarensis. "Lucy has led the way for the field and of course has been an incredible guide for me during my entire academic career," he enthuses. "She has become in many ways the touchstone by which other discoveries are compared." Not only has she fired up researchers, she has captured the public imagination. "She is the mother of mankind isn't she?" he asks. "That's how kids look at it."

But the Lucy discovery also led him into trouble with the Leakey family because it connected humans firmly to apes - and not that long ago. According to Johanson and many others, we most likely shared a common ancestor in the last six to eight million years. The Leakeys, under the influence of the late Louis Leakey, believe human origins to be older. Richard Leakey and Johanson have not spoken to one another for a very long time.

For Johanson the evidence speaks for itself. He is trying to find out more about the evolutionary record through new excavation in Eritrea. "This is going to turn out to be one of the next important fossil places in the Rift Valley," he says. Aged 53, he claims, not very convincingly, that he is not so interested in making the big discovery any longer.

Instead, he says, he would like to see other people get opportunities to do fieldwork. "It is one of the things you want to do when you're young," says Johanson. "I'm more interested in what the fossil record can tell us about how we did become human, what sort of worlds we lived in, what were the reasons why we changed and developed large brains. It wasn't tools solely. It was to do with an increase in the social milieu in which hominids lived."

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From Lucy to Language, by Donald Johanson and Blake Edgar, Weidenfeld and Nicolson, ?25.