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Too many diamonds, not enough fossils. That was Brigitte Senut鈥榮 verdict on a Namibian diamond mine she once visited. In 2000, she discovered Orrorin tugenensis, the first human ancestor known to have walked upright, in the Tugen Hills of Kenya. She told Laura Spinney about fossil-hunting in Africa.

What is the big question you鈥檙e trying to answer?

How and when did humans diverge from the African great apes? I鈥檓 interested not only in the physical characteristics of those early ancestors, but also in how they lived, what environments they inhabited. That鈥檚 why I鈥檓 as excited by the discovery of a pig, a snail or a rodent as by a primate. It鈥檚 also why I work with geologists, sedimentologists and palaeobotanists, among others.

When did you know that the bones you were important?

Immediately. The head worker at the site, Kiptalam Cheboi, found two fragments of a jaw. Other members of the team then uncovered two femurs and a humerus. We went on to study the femurs in more detail, but we could already see from their morphology that they belonged to a bipedal hominid. And from the geology of the site, we knew that it was 6 million years old. That put back the origins of bipedalism by about 3 million years, since the oldest biped known up to that point was the Ethiopian australopithecine Lucy.

The find was controversial. Why?

According to the dominant paradigm at the time, there were no hominids or human ancestors on Earth before the Pliocene, the geological epoch that began 5.5 million years ago. As soon as I laid eyes on Orrorin I knew our problems were only just beginning. Sure enough, some people said we had unearthed chimpanzee remains. Everyone now acknowledges that Orrorin is of the human family, but there is still a debate over the relationship between Orrorin and Lucy. Are they both direct ancestors of modern humans, or did the australopithecines branch off at some point?

Do you consider Orrorin to be the notorious 鈥渕issing link鈥�?

No! The missing-link concept implies that a great ape of the modern kind is the common ancestor, but the ancestor of Orrorin bore no resemblance to modern chimpanzees.

What does the name mean?

鈥淭he original being鈥�, in the Tugen language of Kenya. The Tugen people have long devoted songs and dances to this mythical creature.

What did Orrorin look like?

A young adult was between 1.10 and 1.37 metres tall, that is, slightly taller than Lucy. Like Lucy, it climbed trees as well as walking on two feet. But while Lucy had a small skeleton and large teeth, Orrorin had small teeth and a relatively large skeleton. It seems unlikely to me that a microdont such as Orrorin gave rise to a macrodont like Lucy, which in turn gave rise to the microdonts that were later hominids, but others disagree.

Do you know how the first individual whose remains you found died?

Some of the bones were covered in a fine layer of sodium carbonate, which made me think initially that it may have ventured onto the fragile crust of a hot soda lake- the likes of which you still find in the Rift Valley today 鈥� fallen through it and become trapped. However, one of the bones, a femur, also has tooth marks on it, and the top part is missing, as if the leg had been ripped away from the torso at its fleshiest part. That鈥檚 how a leopard tackles its prey. I think a leopard-like animal killed Orrorin, then carried its carcass up into a tree. From time to time, its bones dropped into the lake below. That鈥檚 just one possible scenario, but it fits the facts.

How well does the molecular biologist鈥檚 view of human evolution fit with the palaeontologist鈥檚?

They are complementary. We both agree that the African great apes are our closest relatives, but the dates of the divergence put forward by the molecular biologists are a bit recent with respect to the story the fossils tell. The study of DNA is relatively new in this context, and it doesn鈥檛 yet take full account of the genetic variability between species and subspecies of ape. However, it鈥檚 a field that鈥檚 evolving fast.

You must have had some close shaves, having worked in the field for nearly 25 years.

Once, in Uganda, our car got stuck in a hole and we found ourselves being shot at. This was just before the coup d鈥櫭﹖at of June 1985. Suddenly we were surrounded by 20 young warriors who rifled our car and touched my hair and clothes. They were stark naked apart from the feathers in their hair, their cartridge belts and Kalashnikovs. My Swahili was pretty basic back then, but luckily my colleague spoke it, and so did their leader. The negotiations ended peacefully enough with them helping us to rescue the car and wishing us a good trip, but I was furious.

The next morning, I woke to find a white-bearded old man standing in front of my tent, leaning on his stick, trembling. He was the warriors鈥� chief, and he had walked through the night to come and apologise to me. He gave me a pot of honey and held my hand for a long time. I forget if there was a translator, or what language we communicated in. But I will never forget his face or his dignity, his anger and his embarrassment.

Working in a war zone didn鈥檛 faze you?

It wasn鈥檛 exactly a war, and if Uganda wasn鈥檛 easy to operate in, we managed. Having said that, there were times when I was frightened. In the late 1980s and 90s the roadblocks were often manned by child soldiers. They were armed, they were children 鈥� there was no reasoning with them. There was also the risk of ambush by the Lord鈥檚 Resistance Army, though we were fortunate enough to be spared that ordeal.

Are there any difficulties in being a woman?

No, although I was almost sold in marriage once. That was in Karamoja, in north-east Uganda. The chief of a subdistrict there told a colleague of mine that I walked well and he wanted to marry me. He was offering a dowry of two camels, 50 cows, five ducks and five bundles of wood.

鈥淚 was almost sold in marriage once in north-east Uganda鈥�

Do you enjoy fieldwork?

I鈥檓 passionate about it, and I think it鈥檚 essential for a palaeontologist. On a human level, I have had wonderful experiences in the field. The peace and solitude of the Namibian desert are almost necessary to me now. When I鈥檓 there, I feel as if my brain has been washed clean. I have written through the night while camping in the dunes.

When is your next trip?

As soon as the rains are over, in April or May, I鈥檒l go to Uganda and Namibia. We have only explored a fraction of the area that potentially holds the secret of our existence. We know peanuts compared with what there is to know.

So Africa is in your blood.

And in yours. Your ancestors were African, don鈥檛 forget.