Homo erectus was remarkably adaptable, but another hominid proved still more so: Homo sapiens (“thinking humans”). A few scientists think that Homo sapiens evolved from Homo erectus in a number of places in Afroeurasia, but the majority think that, like hominid evolution from earlier primates, this occurred only in East Africa. The evidence is partly archaeological, but also genetic. One type of DNA, called mitochondrial DNA, indicates that modern humans are so similar genetically that they cannot have been evolving for the last 1 million or 2 million years. This evidence suggests that the evolution of Homo sapiens has instead taken place for only about 250,000 years. Because there is greater human genetic variety today in Africa than in other parts of the world, the evidence also suggests that Homo sapiens have lived there the longest, so that Africa is where they first emerged.

Although there is some debate about where and when Homo sapiens emerged, there is little debate about what distinguished these humans from earlier hominids: 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 hominids evolved this way. Large brains also bring disadvantages, however. They take more energy to run than other parts of the body, so that large-brained animals have to eat more than small-brained ones. Large brains create particular problems for bipedal mammals, for 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 hominids 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 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.

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 hominid infants to be born at an even earlier stage in their development than other primate infants. 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.

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 that replaced the more general-purpose stone tools made by Homo erectus: 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 and baskets, 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.