The great apes live in tropical rainforests. It is thought that human evolution started when a group of apes (now called the australopithecines) began to live more in the savannah. A savannah is more open, with trees, shrubs and grass. This group started walking on two legs. They began to use their hands to carry things. Life in the open was different, and there was a big advantage in having better brains. Their brains grew larger, and they began to make simple tools. This process began at least 5 million years ago. We have fossils of two or three different groups of walking apes, and one was the ancestor of humans.