This chapter deals with the question of why Homo sapiens are the only Homo species left. Homo sapiens evolved in Africa about 200,000 years ago, and started to migrate out of Africa between 100,000 and 80,000 years ago. However, modern humans descended from a relatively small group of only 14,000 individuals. This resulted in a current population that is very genetically similar. Additionally, from fragments of DNA extracted from Neanderthal fossils, it was discovered that 2-5% of the human genome is from Neanderthals. As Homo sapiens moved out of Africa, they settled in every continent except for Antarctica. And at the same time, Homo erectus and Neanderthal populations went extinct. What makes Homo sapiens different? Lieberman argues that Homo sapiens were better able to invent tools and think creatively. He also argues that while Neanderthals certainly had language, the short and retracted face of Homo sapiens would be better able to communicate clearly and rapidly. However, Lieberman explains that the adaptation of clearer speech comes with a disadvantage. To gain this advantage, the larnyx in humans is dropped a couple centimeters, which allows for clearer speech, but also creates the risk of choking. It is unknown if the extinction of other species is a result of a lower birth rate or competition with Homo sapiens, but clearly, Homo sapiens had an evolutionary advantage. Lieberman argues that this advantage could have been cultural evolution, meaning the advancement of technology and clothing. If this technology could have helped them survive in harsh conditions, it would keep the population alive long enough to select for beneficial adaptations. Finally, Lieberman argues that humans are adapted to be adaptable. We have the capabilities to communicate, cooperate, invent, and think, and this allows humans to adapt to different environments. Lieberman concludes that it is our cultural achievements such as farming and writing that have allowed humans to thrive.
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.
In this chapter, Lieberman argues that during the gradual cooling of the climate, natural selection favored hunting and gathering. Lieberman argues that the first hunter-gatherers were from the genus Homo, either Homo habilis or Homo erectus. The advantage of hunting and gathering is that plants are abundant, easy to find, and won’t run away. The disadvantages are that plants often contain indigestible fiber and have a low nutrient density. The plant based diet could have been supplemented with meat to increase the amount of calories consumed. Stone tools have been discovered, indicating that meat was an important part of the diet. Additionally, these tools could have been used to process food, because it’s difficult to chew tough plants and meat. Lieberman also explains that hunting and gathering resulted in a division of labor and food sharing. In fact, Lieberman argues that food sharing extended past immediate family to the whole community.
Compared to Australopithecus, Homo erectus had much longer legs, and Lieberman argues that this is because longer steps are more efficient. This is evidence for the Homo erectus hunter-gatherer lifestyle, because hunter-gatherers must walk long distances to find food. Lieberman argues that the evolution of a large external nose in Homo erectus helped conserve water when Homo erectus was walking during the middle of the day. Lieberman argues that Homo erectus was skilled at endurance running, and used the strategy of persistence hunting. The basic premise of persistence hunting is that humans are able to cool down by sweating, but prey animals are only able to cool down by sweating. Persistence hunting works if the hunters run constantly after an animal, giving it no time to cool down. Eventually the animal will collapse from heat exhaustion, and won’t be able to run away anymore. It’s unknown when sweat glands first evolved, but Lieberman believes that it evolved in the genus Homo or was expanded on after it emerged in Australopiths. Lieberman believes that the increase in calories from adding meat (and the additional calories) to the diet made it possible for larger brains to evolve.
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.
More information about Australopiths!
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.