Hominids started walking on two legs six million years ago, shortly after diverging from chimpanzees, according to a study of the inner structure of a fossilised thighbone. The finding puts upright posture at the base of the human family tree.The evolution of upright posture is a key issue in anthropology. Together with large brain size, it marks the dividing line between humans and the great apes.Researchers know that upright posture evolved first because the skeleton of famed Australopithecine, Lucy, has a small braincase but modern ankles. Yet with few known fossils older than about four million years, the details of how and when upright posture evolved have been hazy. Over the past few years, however, two important new finds have begun to fill in the gaps. The older animal is Sahelanthropus tchadensis, which lived in Chad six to seven million years ago, but only its hominid cranium was found.

More complete is the chimpanzee-sized Orrorin tugenensis, as 20 fossils from at least five individuals have been found in Kenya. These are about six million years old.A team led by Brigitte Senut of the French Museum of Natural History in Paris initially classed Orrorin as a hominid, on the human side of the split from chimpanzees. This classification was based on skeletal features, but other anthropologists remained sceptical.

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