Until now, biologists have had just one model of the evolutionary process to study - life on Earth. But now an amazing computer model can compress billions of years of evolution into just a few hours. The 'Artificial Life' program was developed by Karl Sims at the Thinking Machines Corporation in Massachusetts. The program starts with very simple 'creatures'. When the creatures reproduce, random mutations arise in their 'genes' which may enable them to achieve an objective (in this case being able to capture a green cube). By reaching the objective they are more likely to survive and reproduce this passing on their genes to the next generation. After about 100 generations, creatures evolve which have limbs, move around with ease, and even show aggressive behaviour to prevent their rivals getting close to the target cube. Even though the computer model is fairly simple, it will allow biologists to experiment with evolutionary theory.