This chapter starts with a question: How did humans become so ill adapted to life in the trees, as well as feeble, slow, and awkward? The catalyst for this was likely bipedalism, when our ancestors left the trees to walk on two feet. The split between chimps (our closest relative) and humans is estimated to have occurred 8-5 million years ago, but no fossils of the last common ancestor of chimps and humans has ever been found. Lieberman lists proposed early hominins, such as Sahelanthropus, Orrorin, and Ardipithecus ramidus. One common trait between all of these fossils is the ape-like features, including small brains, browridges, big front teeth, and a protruding snout. Lieberman argues that these organisms were all hominins because the defining trait of hominins is adaptations to walking upright. Lieberman also makes the point that occasional bipedalism is not uncommon, in fact, apes and many other mammals can walk on two legs. Only the habitual bipedalism of humans is unusual. He explains the important traits that allow humans to walk upright efficiently, including orientation of the hips, S-shaped spine, and the arch of the foot. Early hominins acquired these traits while also retaining some distinctly ape-like traits such as opposable big thumbs, curved toes, and slightly tilted ankles. Lieberman makes the argument that these first hominins were occasional bipeds, who were able to walk efficiently, but also able to climb trees. However, because adaptations cause tradeoffs, they were likely less efficient at climbing trees than chimps, and less efficient at walking than humans. Now the question is why did bipedalism evolve? Lieberman suggests that climate change was the trigger for the evolution of bipedalism. As the climate got colder, Africa started drying out and our ancestors may have had to travel further to find food. Lieberman has found that apes spend four times more energy than humans do while walking, which shows that bipedalism may be a more efficient way to walk long distances. Lieberman does acknowledge that more research in this area is needed to find more evidence supporting this hypothesis. Either way, bipedalism was a key change that allowed for more changes, such as tool making, but it also caused major problems. A couple of problems include back pain during pregnancy, loss of speed and agility, and loss of the ability to climb trees. But somehow, the advantages of bipedalism outweighed the disadvantages.