Far from being a single, elegant machine, the human brain is instead a system comprised of many distinct components that have been added at different times during the course of evolution. The human species has retained what worked best in earlier versions of the brain, then added bits and pieces to get us to our present state through evolution.

To understand the central nervous system, it is helpful to consider two aspects of its development. Prenatal development (growth from conception to birth) reveals how the nervous system develops and changes within each member of a species. Evolutionary development reveals how the nervous system in humans evolved and adapted from other species.

The nervous system is the first major bodily system to take form in an embryo (Moore, 1977). It begins to develop within the 3rd week after fertilization, when the embryo is still in the shape of a sphere. Initially, a ridge forms on one side of the sphere and then builds up at its edges to become a deep groove. The ridges fold together and fuse to enclose the groove, forming a structure called the neural tube. The tail end of the neural tube will remain a tube, and as the embryo grows larger, this tube forms the basis of the spinal cord. The tube expands at the opposite end, so that by the 4th week, the three basic levels of the brain are visible. During the 5th week, the forebrain and hindbrain further differentiate into subdivisions. During the 7th week and later, the forebrain expands considerably to form the cerebral hemispheres.

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As the embryonic brain continues to grow, each subdivision folds onto the next one and begins to form the structures easily visible in the adult brain (see FIGURE 3.18). The hindbrain forms the cerebellum and medulla, the midbrain forms the tectum and the tegmentum, and the forebrain subdivides further, separating the thalamus and hypothalamus from the cerebral hemispheres. In about half the time it takes you to complete a 15-week semester, the basic structures of the brain are in place and rapidly developing.

The central nervous system evolved from the very simple one found in simple animals to the elaborate nervous system in humans today. Even the simplest animals have sensory neurons and motor neurons for responding to the environment (Shepherd, 1988). For example, single-celled protozoa have molecules in their cell membrane that are sensitive to food in the water. Those molecules trigger the movement of tiny threads called cilia, which help propel the protozoa toward the food source. The first neurons appeared in simple invertebrates, such as jellyfish; the sensory neurons in the jellyfish’s tentacles can feel the touch of a potentially dangerous predator, which prompts the jellyfish to swim to safety. The first central nervous system worthy of the name, though, appeared in flatworms. The flatworm has a collection of neurons in the head—a simple kind of brain—that includes sensory neurons for vision and taste and motor neurons that control feeding behavior.

During the course of evolution, a major split in the organization of the nervous system occurred between invertebrate animals (those without a spinal column) and vertebrate animals (those with a spinal column). In all vertebrates, the central nervous system is organized into a hierarchy: The lower levels of the brain and spinal cord execute simpler functions, while the higher levels of the nervous system perform more complex functions. As you saw earlier, in humans, reflexes are accomplished in the spinal cord. At the next level, the midbrain executes the more complex task of orienting toward an important stimulus in the environment. Finally, a more complex task, such as imagining what your life will be like 20 years from now, is performed in the forebrain (Addis, Wong, & Schacter, 2007; Schacter et al., 2012; Szpunar, Watson, & McDermott, 2007).

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The forebrain undergoes further evolutionary advances in vertebrates. In lower vertebrate species such as amphibians (frogs and newts), the forebrain consists only of small clusters of neurons at the end of the neural tube. In higher vertebrates, including reptiles, birds, and mammals, the forebrain is much larger, and it evolves in two different patterns. Reptiles and birds have almost no cerebral cortex. By contrast, mammals have a highly developed cerebral cortex, which develops multiple areas that serve a broad range of higher mental functions. This forebrain development has reached its peak—so far—in humans (FIGURE 3.19). This refinement allows for some remarkable, uniquely human abilities: self-awareness, sophisticated language use, abstract reasoning, and imagining, among others.

Is it genetics (nature) or the environment (nurture) that reigns supreme in directing a person’s behavior? The emerging picture from current research is that both nature and nurture play a role in directing behavior, and the focus has shifted to examining the interaction of the two rather than the absolute contributions of either alone (Gottesman & Hanson, 2005; Rutter & Silberg, 2002; Zhang & Meaney, 2010).

What Are Genes?

A gene is the major unit of hereditary transmission. Historically, the term gene has been used to refer to two distinct but related concepts. Genes are sections on a strand of DNA (deoxyribonucleic acid) that are organized into large threads called chromosomes, strands of DNA wound around each other in a double-helix configuration (see FIGURE 3.20). Chromosomes come in pairs, and humans have 23 pairs each. These pairs of chromosomes are similar but not identical: You inherit one of each pair from your father and one from your mother.

As a species, we share about 99% of the same DNA (and almost as much with other apes), but there is a portion of DNA that varies across individuals. Children share more of this variable portion of DNA with their parents than with more distant relatives or with nonrelatives: Such children share half their genes with each parent, a quarter of their genes with their grandparents, an eighth of their genes with cousins, and so on. The probability of sharing genes is called degree of relatedness. The most genetically related people are monozygotic twins (also called identical twins), who develop from the splitting of a single fertilized egg and therefore share 100% of their genes. Dizygotic twins (fraternal twins) develop from two separate fertilized eggs and share 50% of their genes, the same as any two siblings born separately.

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Many researchers have tried to determine the relative influence of genetics on behavior. One way to do this is to compare a trait shown by monozygotic twins with that same trait among dizygotic twins. This type of research usually enlists twins who were raised in the same household, so that the impact of their environment (their socioeconomic status, access to education, parental child-rearing practices, environmental stressors) remains relatively constant. Finding that monozygotic twins have a higher presence of a specific trait suggests a genetic influence (Boomsma, Busjahn, & Peltonen, 2002).

As an example, the likelihood that the dizygotic twin of a person who has schizophrenia (a mental disorder we’ll discuss in greater detail in the Psychological Disorders chapter) will also develop schizophrenia is 27%. However, this statistic rises to 50% for monozygotic twins. That sounds frighteningly high … until you realize that 50% of monozygotic twins of people with schizophrenia will not develop the disorder. That means that environmental influences must play a role too. In short, genetics can contribute to the development, likelihood, or onset of a variety of traits. But a more complete picture of genetic influences on behavior must always take the environmental context into consideration. Genes express themselves within an environment, not in isolation.

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A Role for Epigenetics

The idea that genes are expressed within an environment is central to an important and rapidly growing area of research known as epigenetics: environmental influences that determine whether or not genes are expressed, or the degree to which they are expressed, without altering the basic DNA sequences that constitute the genes themselves. To understand how epigenetic influences work, think about DNA as analogous to a script for a play or a movie. The biologist Nessa Carey (2012) offers the example of Shakespeare’s Romeo and Juliet, which was made into a movie in 1936 starring Leslie Howard and Norma Shearer, and in 1996 starring Leonardo DiCaprio and Claire Danes. Shakespeare’s script formed the basis of both films, but the directors of the two films used the script in different ways, and the actors in the two films gave different performances. Thus, the final products were different from one another, even though Shakespeare’s original script still exists.

Similarly, epigenetics provides a way for the environment to influence gene expression—without altering the underlying DNA code. Epigenetics are now known to play a role in several important functions, including learning and memory (Bredy et al., 2007; Day & Sweatt, 2011; Levenson & Sweatt, 2005), and responses to stress (Zhang & Meaney, 2010). For example, subjective levels of stress in a sample of 92 Canadian adults, as well as physiological signs of stress, were correlated with epigenetic markers in the subjects’ DNA (Lam et al., 2012). Other studies have reported a link between gene expression and early life experience, with differences observed between individuals who grew up in relatively affluent households versus those who grew up in poverty, even when controlling for such factors as current socioeconomic status (Borghol et al., 2012; Lam et al., 2012). These observations fit well with an important line of research described in the Hot Science box, showing that gene expression plays a key role in the long-lasting effects of early experiences for both rats and humans. In short, genes set the range of possible characteristics that can be expressed, but the environment and experience—through mechanisms such as epigenetics—determine which of those possibilities actually occur.

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