How will robots of the future get around? Some say tank treads. Some say legs and feet. But nobody knows for sure, and that's why researchers at Cornell University designed a computer program to figure it out. The software simulates evolution. Robots begin as blocks of muscle, tissue, and bone, then natural selection kicks in: The fastest bots in each generation have offspring and are more likely to move on to the next round. The slower ones die out. Here are five of the most memorable variations from 175,000 generations.