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ALL EVIDENCE IN CHAPTER 1
”Understanding the Early Human Past” People throughout the world have developed systems of classification that help them understand things: earth and sky; seen and unseen; animal, vegetable, and mineral; past, present, and future. Among these systems of classification was one invented in eighteenth-century Europe that divided all living things on earth into groups. Each of these divisions — such as that between plants and animals — is further subdivided into smaller and smaller groups, such as class, order, family, tribe, and genus. The final important division is the species, which is generally defined as a group of organisms that can interbreed with one another and produce fertile offspring of both sexes.
In their natural state, members of a species resemble one another, but over time they can become increasingly dissimilar. (Think of Chihuahuas and Great Danes, both members of the same species.) Ever since humans began shaping the world around them, this process has often been the result of human action. But in the long era before humans, the increasing dissimilarity resulted, in the opinion of most scientists, from the process of natural selection. Small variations within one species enabled some individuals to acquire more food and better living conditions and made them more successful in breeding, thus allowing them to pass their genetic material on to the next generation. When a number of individuals within a species became distinct enough that they could no longer interbreed successfully with others, they became a new species. Species also become extinct, particularly during periods of mass extinctions such as the one that killed the dinosaurs about 65 million years ago. Natural processes of species formation and extinction continue, although today changes in the biosphere — the living matter in the world — result far more from human action than from natural selection.
The scientists who developed this system of organizing the world placed humans within it, using the same means of classification that they used for all other living things. Humans are in the animal kingdom, the order of Primates, the family Hominidae, the tribe Hominini, and the genus Homo. Like all classifications, this was originally based on externally visible phenomena: humans were placed in the Primates order because, like other primates, they have hands that can grasp, eyes facing forward to allow better depth perception, and relatively large brains. Humans were initially the only animals classified as hominids, but in the past few decades this family has grown to include chimpanzees, gorillas, and orangutans because they share so much genetic material with humans. Over 98 percent of human DNA is the same as that of chimpanzees, which indicates to most scientists that humans and chimpanzees share a common ancestor. That common ancestor probably lived between 5 million and 7 million years ago. Humans and their close extinct relatives are now placed in a smaller group within the hominid family, the tribe of hominins.
Homo Sapiens, “Thinking Humans”
Homo erectus was remarkably adaptable, but one of its descendants proved still more so: Homo sapiens (HOH-moh SAY-pee-enz) (“thinking humans”), who evolved in East Africa between 300,000 and 200,000 years ago. Further small evolutionary changes afterwards led to our own subspecies, Homo sapiens sapiens (which literally translates as “thinking thinking humans”), generally referred to now as Anatomically Modern Humans (AMH).
What distinguished Homo sapiens from earlier hominins was a bigger brain, in particular a bigger forebrain, the site of conscious thought. The ability to think reflectively allowed for the creation of symbolic language, that is, for language that follows certain rules and that can refer to things or states of being that are not necessarily present. Greater intelligence allowed Homo sapiens to better understand and manipulate the world around them, and symbolic language allowed this understanding to be communicated within a group and passed from one generation to the next. Through spoken language Homo sapiens began to develop collective explanations for the world around them that we would now call religion, science, and philosophy. Spoken language also enabled Homo sapiens to organize socially into larger groups, thus further enhancing their ability to affect the natural world.
The advantages of a larger brain seem evident to us, so we may not think to ask why hominins evolved this way. Large brains also bring disadvantages, however. They take more energy to run than other parts of the body, which means that large-brained animals have to eat more than small-brained ones. Large brains also create particular problems for bipedal mammals because the narrow pelvic structure that works best for upright walking makes giving birth to a large-headed infant difficult and painful.
The question of why hominins developed ever-larger brains might best be answered by looking at how paleontologists think it happened. As Homo habilis, Homo erectus, and Homo sapiens made and used tools, the individuals whose mental and physical abilities allowed them to do so best were able to obtain more food and were more likely to mate and have children who survived. Thus bigger brains led to better tools, but, in a feedback loop, the challenges of using and inventing better tools also created selective pressure that led to bigger brains.
The same thing may have happened with symbolic language and thought. A slightly bigger brain allowed for more complex thought and better language skills. These thinking and speaking skills enabled individuals to better attract mates and fend off rivals, which meant a greater likelihood of passing on the enhanced brain to the next generation. As we know from contemporary research on the brain, learning language promotes the development of specific areas of the brain.
The growth in brain size and complexity may also have been linked to social organization. Individuals who had better social skills were more likely to mate than those who did not — this has been observed in chimpanzees and, of course, in modern humans — and thus to pass on their genetic material. Social skills were particularly important for females because the combination of bipedalism and growing brain size led to selective pressure for hominin infants to be born at an even earlier stage in their development than other primate infants were. Thus the period when human infants are dependent on others is very long, and mothers with good social networks to assist them were more likely to have infants who survived. Humans are unique in the duration and complexity of their care for children. Cooperative child rearing, along with the development of social skills and the adaptability this encouraged, may have been an impetus to brain growth.
All these factors operated together in processes that promoted bigger and better brains. In the Paleolithic period, Homo sapiens’ brains invented highly specialized tools made out of a variety of materials: barbed fishhooks and harpoons, snares and traps for catching small animals, bone needles for sewing clothing, awls for punching holes in leather, nets for catching fish, sharpened flint pieces bound to wooden or bone handles for hunting or cutting, and slings for carrying infants. By 25,000 years ago, and perhaps earlier, humans in some parts of the world were weaving cloth, nets, and baskets out of bark, rushes, grasses, and other natural materials, and by 17,000 years ago they were using bows and atlatls (AHT-lah-tuhlz) — notched throwing sticks made of bone, wood, or antler — to launch arrows and barbs with flint points bound to wooden shafts. The archaeological evidence for increasingly sophisticated language and social organization is less direct than that for tool use, but it is hard to imagine how humans could have made the tools they did — or would have chosen to decorate so many of them — without both of these.