In this chapter, Lieberman explains how the patterns of rapid climate change affected the evolution of Homo erectus. During the time when Homo erectus was migrating out of Africa, the Earth was undergoing cycles of ice age to rapid heating, with each cycle lasting 100,000 years. This greatly affected Europe, which had cycles of glaciers forming and melting. Even in Africa, which was not affected by glaciers, had wet and dry periods. Despite the challenging time, Homo erectus not only survived, but thrived. During the time that Homo erectus existed, brain size nearly doubled. Around 500,000 years ago, Homo erectus learned to make spear points, and learned to use fire regularly 400,000 years ago. Lieberman calls the discovery of fire a transformative advance. Cooked food allows absorption of more nutrients and kills pathogens. Fire also allowed Homo erectus to keep warm and fend off predators. However, Lieberman focused on Homo erectus’ increased brain size, which doubled over the Ice Age. With most other primates, as body size increases, brain size gets comparatively smaller. However, Homo erectus does not fit this trend at all and is within the size range for modern human brain size. But Lieberman emphasizes that larger brains come with sizable costs. Despite being only 2% of modern humans’ weight, the brain uses up 20-25% of the energy budget, or 280-420 calories a day. These cost are only amplified for mothers caring for children. Additionally, large brains require extra protection from concussions and make childbirth extremely difficult. One additional adaption necessary for large brains is the ability to store large quantities of fat. The brain must receive a constant supply of sugar from the blood, and even short interruptions can have devastating effects. Fat is the most efficient way of storing energy, and Homo erectus evolved to be able to store large amounts of fat efficiently. Lieberman believes that one of the main advantages of large brains is the increased ability to cooperate with each other. Additionally, large brains are helpful to remember and predict the location of food, or to track animals while hunting.