Too many diamonds, not enough fossils. That was Brigitte Senut's 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're trying to answer?

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

When did you know that the bones you unearthed in 2000 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 "missing link"?

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?

"The original being", 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 - 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's 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's just one possible scenario, but it fits the facts.

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

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't yet take full account of the genetic variability between species and subspecies of ape. However, it's a field that's 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'état 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.

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