This chapter deals with how the Australopiths partially weaned us off fruit. Chimps eat enormous quantities of fruit, and sometimes resort to lower quality food such as leaves and stems. Lieberman argues that the second major transformation in the history of the human body is the trend toward dietary diversity. The Australopiths lived between 4-1 million years ago and were obligate bipeds. However, they still had ape-like traits such as small brains, long snouts, and browridges. One difference from apes was that their diet had shifted from fruit to tubers, seeds, and plant stems. The evidence for this are the large teeth and jaw, forward placed cheekbones, and large chewing muscles. In fact, when the first Australopith was discovered, he was nicknamed “Nutcracker Man” because of his extremely large jaw. Lieberman argues that the ancestors of Australopiths were forced to eat lower quality food because of the gradual cooling of the planet, which caused Africa to dry out. These foods are termed “fallback foods” and are only eaten when necessary. Evolution favors those who are able to eat fallback foods when normal foods are unavailable, because it is often a matter of life or death. Lieberman quips that evolution logic dictates that sometimes, “you are what you’d rather not eat”. Lieberman also explains that there is some evidence that Australopiths started digging for food for roots, tubers, and bulbs, which collectively are called underground storage organs (USOs). Lieberman suggests that USOs became an important source of calories for Australopiths, because they are more starchy and energy rich than many fruits. However, USOs are extremely fibrous and tough, meaning that they were very difficult to chew. Lieberman theorized that Australopiths would eat them by wadging – which means chewing for a long time to extract nutrients and then spitting out the pulp. This is reflected in the molars of Australopiths, which were about the size of a thumbnail, two times the size of chimpanzee molars. Lieberman argues that this adaptation is still present in humans today, because our molars are still larger than chimpanzee molars. Additionally, foraging for USOs requires more walking, so efficient walking must have been very important. Although bipedalism prevented Australopiths from galloping, bipedalism allowed for efficient walking and freed the hands from digging or carrying food. Lieberman argues that Australopiths were a key intermediate stage in human evolution, and without the adaptations that they developed, we would not be here today.
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