Our ancestors were such an upright lot

Parents now have two more reasons for telling their children not to slouch when they walk: our ancestors never did and it uses twice as much energy as walking upright. Early humans walked upright as soon as they left the trees and never slouched or dragged their knuckles, as artists often depict them. The findings, announced yesterday, overturn previous theories on evolution.

Our ancestors must have learnt to stand on two feet while still in the trees - "probably to gather fruit or for foraging," said Robin Crompton, of the University of Liverpool. Homo sapiens could not have survived on the ground by slouching, because it would use too much energy: "You either walk erect, or you're extinct." His work also pushes back the time when human ancestors learnt to walk by "a couple of million years", to as much as six million years ago.

Dr Crompton built a computer model of the body and programmed it to "walk" on two legs. His team looked at forces exerted on the ground by the foot during walking and compared them with those exerted by a real human in tests. This led to a "very, very accurate" computer model which can also show the forces that muscles put on the joints of the legs.

The scientists substituted into the model the proportions of "Lucy", the oldest known human ancestor, whose 3.6-million-year-old skeleton was found in the Rift Valley, Africa, in the 1980s. They then tried to make it walk like a chimpanzee, with a slouch, and then upright like a human. "When we asked the model to walk like a chimpanzee, it fell over repeatedly, showing that Lucy's proportions are totally incompatible with the way real chimpanzees walk," said Dr Crompton. "It is thus impossible that Lucy could have walked like this." But when they told the Lucy model to walk upright, it succeeded.

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