The nervous system moves the limbs using a small number of elementary commands. Like the phonetic units used in speech, also the elementary locomotor commands can be combined together in a flexible manner to generate a rich repertoire of behaviors, such as walking and running at different speeds, walking forwards or backwards, or walking with variable loads. The commands correspond to basic patterns of activation of alpha-motoneurons which briefly excite the muscles. The commands are generated sequentially in time during each step by a network of neurons located in the spinal cord. The spinal networks (Central Pattern Generators) are normally influenced by signals descending from the brain and by sensory signals. Recent studies showed that two of the basic activation patterns are present at birth allowing flexion-extension of the limbs during newborn stepping. Two more patterns are first revealed in toddlers. With development, all four patterns are progressively tuned to exploit limb and body biomechanics. We also compared development of locomotor patterns in humans with that in other vertebrates. In newborn rats, we found two patterns essentially identical to those of human newborns, while in adult rats, cats, macaques, and guinea fowls we found four patterns closely resembling those of human children. These results suggest that locomotion of several animal species is built starting from common elements, perhaps related to ancestral neural networks.