Bipedal animals – those that walk upright – have a center of gravity close to their hips. Their upper leg bones fit onto the end of the lower leg bone – on humans, the hinge-like joint created there is called the knee. The result is an energy-efficient posture that allows the bones – not just muscles – to help support the weight of a bipedal animal.

The first such animal might have been Eudibamus cursoris – a lizard that flourished briefly about 290 million years ago. It might have run on two feet as a means of escaping larger carnivorous predators. Eudibamus probably died out 80 million years ago – before there were dinosaurs that could walk on two legs.

Scientists think the first mammal that walked upright was the ape-like australopithecines – believed by scientists to be an ancestor of early humans – which lived about five million years ago in Africa. Fossils of our human ancestors suggest that the ability to walk upright was an important development. It freed the arms to be used for other purposes – long before a large, complex brain set hominids apart from other creatures.