Christine Tardieu is an evolutionary biologist and a palaeontologist specialising in functional and bI'mechanical morphology. A research director at the National Centre for Scientific Research (CNRS), she works in the Laboratory for Comparative Anatomy at the National Museum of Natural History, in Paris. Are there any human beings who never stand upright and remain quadrupeds? The answer is yes, at least in the case of wolf-children. But is this a myth or a biological reality? Some children, who are neither 'wolf-children' nor 'wied children', also remain quadrupeds. Why? What do they lack? Such questions can be reworded more scientifically: How did humans become bipeds? What changes occurred during the evolutionary process to adapt the human skeleton to bipedalI'm? Natural selection may have altered the human skeleton for upright movement, yet at birth the human skeleton differs from what it will become in adulthood. A significant and omnipresent factor intervenes here: gravity. What role does it play in attaining an upright position and in walking? How does the infant skeleton learn to deal with the laws of equilibrI'm? And why does this apprenticeship sometimes fail to occur? - Wolf-children, wied children, cases of quadruped families in Turkey: these amazing stories show that the equilibrI'm of bipedalI'm does not always 'just happen'.- Palaeontology, anatomy, morphology: a multidisciplinary approach to explain the foundations of permanent bipedalI'm, a condition that humans are the sole mammals to have adopted.