# Progressive Summary # Structured Notes # Rough Notes The first human ancestors, *homo habilis*, roamed the African savannah about 1.7 million years ago. They eventually learned to make tools in order to tenderize meat, which made digestion easier. The energy saved was used to build bigger brains. Around 800,000 years ago, *homo erectus* began cooking food over fire. This released even more calories. Our large intestine, which helped us break down fiber, started shrinking, and this saved even more energy. The brains of *erectus* were 50 percent larger than those of *habilis*. The enlarged brain needed more space, which was taken from our sinuses, mouths and airways. Muscles in the center of the face loosened and the jaw got thinner and weaker. The face shortened and the mouth shrank, and our snouts were replaced by noses. By this time, *erectus* looked very similar to modern-day humans. > The more we cooked, the more soft, calorie-rich food we consumed, the larger our brains grew and the tighter our airways became. The larynx is a valve that shuttles food into the stomach and prevents it from going into the lungs. Like other animals, we initially evolved to have a high larynx, since this allowed unwanted objects to be expelled more quickly. When humans developed speech, our larynx lowered to give more space at the back of the mouth for vocalizations. This made choking easier, and we are the only animal species that can easily choke on food and die. # Quotes