The Handy Psychology Answer Book. Lisa J. Cohen

chimpanzee, although our bodies are not much bigger than theirs. Secondly, the brain has become much more complex. It is less the size of an animal’s brain that determines its intelligence than the complexity of its neuronal networks. Billions of neurons create trillions of connections. Brain size and brain complexity, however, do tend to go together.

      How has the human cortex grown compared to other animals?

      Another way that the human brain has changed across evolution is the growth of the cortex. The neocortex, the six-layer tissue that forms the outer layer of the human brain, is found only in mammals. Primitive cortices are evident in smaller mammals, such as hares, opossums, and armadillos, and more developed cortices are evident in higher-order mammals, such as elephants, dogs, and dolphins. In humans, the cortex wraps around the entire brain. The cortex allows for much more sophisticated processing of sensory information (e.g. sight, sound, touch). It also allows for more varied and flexible behavioral responses to internal and external stimulation.

      Do the different brain regions ever perform redundant functions?

      In some cases, the cortex and subcortical areas provide redundant or overlapping functions. For example, both the frontal lobe and the basal ganglia regulate motor behavior. But the behavior regulated by the basal ganglia is relatively crude and inflexible. Although fast and efficient, it is not easily adapted to changing conditions. Behavior regulated by the frontal lobe, on the other hand, is much more nuanced, flexible, and responsive to changing conditions. The frontal lobe is often slower, however, and consumes more energy than the basal ganglia. Thus we are happy to rely on our basal ganglia when walking down the sidewalk, but would prefer to employ our frontal lobe when performing surgery or defusing a bomb.

      Has the frontal lobe grown across human evolution?

      The greatest change in brain structure across human evolution relates to the frontal lobe. Frontal lobes are tiny in many smaller mammals, such as tree shrews and hedgehogs. In higher-order mammals, like cats and dogs, they are still smaller than in humans and also considerably less convoluted. As mentioned above, the convolutions (wrinkles) on the cortex provide more surface area for dendrites to expand. Relatedly, our species has the most complex and sophisticated cognitive capacities on earth. That is not to say that other animals do not use some form of thought—chimpanzees use tools to solve problems and gorillas can be taught the rudiments of language. Nonetheless, as far as we know, no other species really comes close to us with regard to intelligence.

      Comparisons of human, elephant, gorilla, macaque (a kind of monkey), dolphin, dog, cat, and mouse. Frontal lobes are proportionately larger and more convoluted (wrinkled) in species with greater intelligence.

      How has the olfactory bulb changed across human evolution?

      One of the most striking differences between the brains of humans and other mammals involves the size of the olfactory bulb, which is the part of the brain involved with smell. In many mammals, the olfactory bulb is a major portion of the entire brain. In fact it is present in even the most primitive vertebrates, such as fish. In humans it is a tiny little orb sandwiched between our limbic system and the bottom of our frontal lobe. This contrast illustrates our reduced reliance on the sense of smell in favor of vision, hearing, and analytic thinking, all functions supported by the cortex.

      THE BRAIN SINCE EARLY HOMINIDS

      When did human beings evolve from early hominids?

      About six million years ago, our ancestors and chimpanzees diverged from a common ancestor. The Australopithecus genus was one of the earliest forms of hominids. The Homo genus followed, with several species, such as Homo habilis, Homo erectus, and Homo neanderthalensis (Neanderthals) preceding or even overlapping with modern humans. Modern humans (Homo sapiens) evolved about 200,000 years ago.

      How do we compare our brains with those of extinct species?

      Because soft tissue decomposes quickly we cannot expect hominid brains to survive over the hundreds of thousands and even millions of years of evolution. Therefore, paleontologists must work with skeletal remains, using skulls and other bone fragments to draw inferences about the biology and behavior of early hominids. However, tools, animal bones, fossilized seeds, and even cave paintings (in the case of early modern humans) have been found alongside hominid skeletal remains, providing intriguing clues about the mental capacity of our predecessors.

      How has brain size changed from early hominids?

      Hominid skulls show a steady increase in cubic centimeters across evolution. Estimating from skull size and shape, Homo habilis had a brain size of 600 to 700 cubic centimeters and Homo erectus about 900 to 1,000 cubic centimeters. Homo sapiens (modern humans) have a brain size of about 1,400 cubic centimeters. Concurrent with the increase in brain size, paleontologists find an increase in the complexity of tools found with hominid remains. Larger brains apparently translated into more sophisticated tool use. Additionally, larger-brained hominids were adapted to more varied and/or harsher climates.

      Is there evidence of language in hominid skulls?

      Indentations on the inside of Homo habilis skulls (Homo habilis lived about two million years ago) suggest an enlarged area around the location of Broca’s area, a central region for speech production in modern humans. While we cannot know if this area was connected to speech in Homo habilis brains, we can suggest that at least a precursor to modern language regions of the brain was present at a very early point in hominid evolution.

      Has the frontal lobe increased?

      Along with the increase in skull size, the shape of the skull also suggests enlargement of the brain and of the frontal lobe in particular. Australopithecine skulls do not look much different from ape skulls. There is a prominent jaw, a small sloping forehead and a relatively small brain casing. Modern humans, in contrast, have flatter faces, very steep foreheads, and small jaws. Our foreheads, which lie just in front of the frontal lobe, cover about 50 percent of our faces. Likewise, our brain casing is greatly enlarged relative to the rest of our skull.

      What do we know about Neanderthals?

      The Neanderthals (Homo neanderthalensis) were a species of the Homo genus that lived from about 200,000-300,000 years ago until they became extinct around 30,000 years ago. Neanderthals were not the ancestors of modern humans but rather shared a common ancestor with our species. In that way they were more like our cousins. The ancestors of Neanderthals and modern humans broke off from a common ancestor about 400,000 years ago. As modern humans (Homo sapiens) date back to about 200,000 years ago, Neanderthals overlapped with modern humans for about 170,000 years, although for most of that time (about 140,000 years), the two species lived on different continents: Neanderthals in Europe and Asia and modern humans in Africa. In fact, modern genetic studies tell us that our two species interbred to some extent and that many modern humans carry a small amount of Neanderthal genes. Evidence suggests the interbreeding took place from 60,000 to 30,000 years ago across Europe and Asia.

      Neanderthal skeletons were first recognized as belonging to an earlier species in the nineteenth century. These skeletons had shorter, stockier bodies than those of modern humans, with heavier brows and larger jaws. Their brain casings were actually larger than ours. Originally it was thought that Neanderthals were much more primitive than modern humans—not much different from apes. However, advances in the science of paleontology as well as the new ability to decode the Neanderthal genome tells us much more about these fascinating relatives of ours. They ranged across Europe and Asia and ate a wide variety of foods, suggesting they could flexibly adapt to very different environments. They used sophisticated stone tools, hunted with wooden spears, and even buried their dead. While we have not yet established whether they used language or were