Bipedal beings on land motivate themselves by two legs. Through evolution, many animals have come to be bipedal, the closest example being humans. Bipedals are found among many branches of animal life, not just mammals: Ostriches along with many other birds are bipedal, as are kangaroos. Bipedals are almost exclusively land living, as sea animals rarely have use for legs unless they live on the bottom of the sea, where a larger number of legs is favoured.
Bipedalism is mainly advantageous for freeing up the front limbs (as in humans and birds) and for fast locomotion, and in fact a few creatures like cockroaches will rear up on their hindlegs when moving quickly. Until recently, bipedal robots have been very difficult to construct. Increasing computer power, however, has enabled two legged robots in recent time.