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When Kiptalam Cheboi of the Community Museums of Kenya stumbled across a jaw bone in Kenya's Tugen Hills he knew that who it belonged to was no ordinary ape. Soon, Martin Pickford and Brigitte Senut found a leg bone and then what appeared to be an arm bone of a new creature the team named Orrorin tugenensis.

Examining a fossilized molar

Dr. Martin Pickford examines a molar belonging to Orrorin.

Martin and Brigitte have been studying fossils for 30 years and there was no doubt in their minds that these were the bones of a hominid -- an early ancestor of humankind. The bones had elements of human bones, while also being strangely apelike.

The bones came from geological strata believed to be around six million years old, a time thought by scientists to be close to when a group of apes split away, forging the path that would eventually lead to modern humans.

But how could they be sure? What signs does a scientist look for when they believe they have found one of our earliest ancestors?

First they have to be sure of the age. The geological strata of the Tugen hills was previously dated to before or around six million years of age, but the team wanted a more accurate date for the discovery. Their colleagues from the University of Shimane in Japan analysed the rock in which Orrorin was found. The results are conclusive. The fossils are between 5.8 and 6.1 million years old.

But how can they be sure Orrorin was not just an ancient ape ancestor? The defining feature of the human race is that we walk on two feet, so this is what we look for in the evidence of our earliest ancestors.

The femur of Orrorin shows the classic marks of a bipedal creature; its form suggests that it supported the weight of an upright walker. Computer Tomography, or CT, scans (the same high-tech scans used to diagnose illness in humans, show that the inside of the femur bone bears a pattern of bone density that could only be the result of an upright walking stance.

Orrorin shows signs that it is an animal that walks on two feet rather than four, yet it comes from a staggering 6 million years ago. This leads to a conundrum.

Scientists have long thought that the split of humans from apes occurred when parts of eastern Africa lost their trees as a result of climate change. It is thought that the need to cross open land to find food led to some apes learning to walk upright.

But Orrorin comes from a time when East Africa was still heavily forested. So how could our earliest ancestors have learned to walk on two feet, and why?

Robin Crompton, of the University of Liverpool, has a solution. He believes that orangutans can show us how we learned to walk -- by balancing and walking within the branches of the trees at times upright on two feet.

This could explain how Orrorin and Orrorin's ancestors learned to walk six or more million years ago in a forested environment. They would actually stride along in the trees, as well as walk from tree to tree. This idea and the discovery of Orrorin is revolutionising our understanding of how we came to be.

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Because of Orrorin's advanced appearance (its surprisingly human-like teeth and femur) Martin and Brigitte present an even bolder claim: Orrorin is more closely related to modern-day humans than Lucy, who at 3.2 million years of age, was heralded as the earliest known human ancestor. By comparing Lucy's bones to those of Orrorin's, Martin and Brigitte believe that Lucy was a side-branch that died off, while Orrorin is a more likely candidate to be a human ancestor.

In just this way, the Neanderthals once shared the planet with modern humans. We do know conclusively that the evolutionary tree was bushy, with some hominid lines surviving, while others faded away.

Scientists continue to debate whether Lucy really was a side-branch. As Martin says, only time will tell if he is right or not. Meanwhile, the team from the Community Museums of Kenya is still finding more pieces of the creature that is rocking the study of human origins.

Read Lucy McDowell's Notes from the Field >
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