Human Evolution: An exclusive interview with the man who discovered the oldest child in the world

Dr Zeresenay named her Selam and calls her his daughter. But she was born 3.3 million years ago

She was three years old when she died. Flood waters tore through the forest, separating mother from child. Guttural cries of alarm echoed in the lush canopy. Possibly. There was nobody there to record her death. The body sank to the bottom of the water, out of sight of predators, and was covered over by stones and sand. The riverbed turned to rock eventually, and so did her bones.

The child lay buried for a very long time. More than three million years, until a Sunday afternoon in December six years ago when strangers came looking for bodies. By then the forest had long disappeared. The hillside was a dry, rocky, hostile place. The heat was ferocious. A boot stirred the dust. Its owner looked down and saw a portion of bone, half-buried.

"A cheekbone was sticking out of the sand," says the young Ethiopian who found her, Dr Zeresenay Alemseged (Zeray, as his friends call him). He was looking because that has been his life's work, driven by an obsession with finding the remains of all our ancient ancestors in the country of his birth. On that day, after several years of trying, he did it. The find was stunning. Details of it, emerging only now, challenge long-held beliefs about the way human beings evolved.

For the first time, the scientist holding up an extraordinary find from Africa is an African. But whatever the impact he has made across the world, for the 37-year-old this is still deeply personal. "I have a son who is nine months old," he told me, speaking in Addis Ababa. "I also have a daughter who is 3.3 million years old. I am living in both epochs."

Except she is not his daughter, and could never have been. That is the point. This child is a hominin, an ape closely related to humans. Hers is the oldest and most complete infant skeleton ever found, a refugee from the time when our ancestors had just begun to stand on their two hind limbs.

Zeray has spent more than five years separating her bones from the cement-like sandstone "grain by grain". He named her Selam, which means "peace" in several Ethiopian languages. She is in pieces now, in a laboratory at the National Museum in Addis, close to the bungalow in which his real family lives.

Selam is also on the cover of National Geographic magazine, as a computer reconstruction that gives her a friendly expression and a relatively hairless face. It is guesswork, but the image is one reason why this tiny ape-child from prehistory has struck a chord in cynical modern human hearts. The other is the unearthing of her hyoid, the delicate bone that holds open the throat and which suggests the noises that Selam made as she was held in her mother's arms (or was parted from them by surging waters) were more human than ape.

"This has been my life," said Dr Zeresenay, who trained in Paris but returned to search the Afar badlands as soon as he could. "All my efforts, energies and time have been devoted towards finding, analysing and describing this specimen and what it represents." So what does Selam represent? "She has many significances. The completeness of this find means it contains many new and previously unknown elements, and they raise many questions."

For scientists like Bernard Wood, a paleoanthropologist at George Washington University in the US, Selam is "a veritable mine of information about a crucial stage in human evolutionary history". The chief surprise is that her shoulder blades and arms still look like those of a gorilla. There is now hot debate over whether they are just useless evolutionary baggage or a sign that 3.3 million years ago Selam and her family were still swinging.

If they were, then the most popular theory of why humans stood up is challenged, says Desmond Morris, the zoologist who studies human behaviour. This theory says they did it because they had learned to use their hands for making tools and weapons but still needed to get around. "If Selam is significant and not an oddity, that means bipedalism came first. So there must have been a different reason for it."

But he warns against drawing too many conclusions. "People end up basing their idea of an entire species on a little girl's skeleton. There's a man in the Guinness World Records who is eight feet tall. If you found him, and only him, in three million years' time what would you think we had been?"

The question of who we once were has led many people to Axum, the town where Zeray was born in 1969. It was once the seat of the Axumite Kingdom, a power listed by one contemporary as ranking alongside those of Rome, Persia and China. So the young boy grew up watching Western archaeologists, fortune hunters and tourists rifling through his town. "The Axumite ruins were just like a kitchen in disorder on an ordinary day," he once said. History and the present lay side by side in the dust.

Zeray was five years old when the ageing Emperor Haile Selassie was deposed by Marxist members of the military in 1974. That was also the year a paleoanthropologist called Donald Johanson saw a bony elbow sticking out of the gravel in the Afar region. He had found the partial skeleton of an adult female from 3.2m years earlier. Nicknamed Lucy after a Beatles song, she was the oldest hominin ever found at the time. Lucy was the first to be identified as Australopithecus afarensis, a missing link between apes and humans.

Zeray was 15 and at high school in Addis when Western eyes turned to Ethiopia again in 1984, during the famine. He graduated in geology there and was sent to France on a scholarship. But he came back at the age of 30, badgering the ministry for a permit to start searching his own parcel of land. The chosen spot was the inhospitable Dikika, two hours' drive from the nearest village. The name means nipple in the local language, and comes from the hill being shaped like a breast.

The landscape has a wild beauty, but it can be deadly. The heat can kill, but there are also lions and snakes, and mosquitoes that carry malaria. Flash floods still sweep people away. The bandits have a reputation for scuttling out of the heat haze like scorpions.

For a long time the team found nothing of great significance. They were close to giving up on the afternoon of 10 December 2000 but kept going, moving slowly, their heads bowed to scan the sand and rocks. A shout was stifled by the dead air. Someone had seen a flash of bone. Zeray got down on his knees and breathlessly but carefully removed all he could of the debris and dry grass. The cheek was attached to a skull. He recognised the smooth brow as that of a hominin, but had to do many more tests before he could be sure it was A. afarensis, the same species as Lucy: his dream find.

There were many other bones too, invisible in a hard ball of sandstone. "We brought the initial block here to the museum because there is no laboratory on the hillside," said Zeray with huge understatement. "It is a place with many challenges."

The bones were extricated using dental drills that direct precise blasts of compressed air into the rock. "This causes vibrations in the sand and cleans it from the fossil without you having to touch the bone at all," he said. Cleaning the tiny ribs and vertebrae with meticulous care took thousands of hours. The true scale of the find was not revealed until a paper was published in the journal Nature last Thursday.

"This is a stunning discovery," said Martin Meredith, author of The State of Africa, "but it does not change the picture of human evolution. What we're doing now is filling in the pieces." For him and many other observers, one of the most significant things about Selam is the identity of the man who discovered her. Desmond Morris agrees. "It is wonderful news," he said. "There is still a colonial and imperial flavour to anthropology - virtually all the discoveries are made by Europeans and Americans who go out to these remote places with the help of the locals, who also do the digging. So the fact that Selam was discovered by an African leading a team in his own backyard is brilliant."

Dr Zeresenay is now attached to the Max Planck Institute of Leipzig in Germany. He will have to get used to acclaim. But it will not keep him from his "daughter", who is still partly encased in sand in the laboratory in Addis. And soon he will be out in the crippling heat again, on the hillside. The bones of her long-lost relatives are calling.