The Handy Psychology Answer Book. Lisa J. Cohen

thought, the complexity of their tools suggests they had some form of complex communication. Scientists are not clear why Neanderthals became extinct or to what extent modern humans were a contributing factor to their disappearance.

      What does pedomorphy mean?

      Pedomorphy refers to an evolutionary process in which adult animals maintain the traits of juveniles. One fairly easy way for genetic mutations to produce physical changes in the animal is to adjust the timing of maturation. No new physical structures or behaviors need to be introduced; the animal simply maintains its youthful traits instead of shedding them when it reaches maturity. There is evidence that many advances in human evolution involve pedomorphy. For example, we are one of the few mammals that retain a high level of playfulness throughout adulthood. Secondly, the shape of our skull mirrors that of juvenile apes. The skulls of juvenile chimpanzees look more like the skulls of adult humans than the skulls of adult chimpanzees. Adult chimpanzees have small sloping foreheads, prominent jaws, and more horizontally aligned faces. Adult humans, on the other hand, have high foreheads, small jaws, and flat, vertically aligned faces, similar to juveniles of both species. Of note, juvenile chimpanzees have a larger brain to skull ratio than do adult chimpanzees. In humans, this favorable ratio is retained into adulthood.

      What does “phylogeny recapitulates ontogeny” mean?

      This is a nineteenth century idea put forward by a German zoologist named Ernst Haeckel (1834–1919) that development in childhood exactly parallels evolution. Phylogeny refers to evolution and ontogeny to development across the life span. Haeckel believed that every stage in the development of a human embryo exactly parallels the stages of human evolution. Besides having a faulty view of human evolution (e.g., there was no cow stage in human evolution), this theory oversimplified the processes of embryonic development. While Haeckel’s specific theory has been discredited, he was correct in pointing out important parallels between evolution and maturation. Careful understanding of our development across the life span does offer some clues as to our evolutionary history.

      THE BRAIN IN UTERO

      How does the brain develop in utero?

      All vertebrates start life the same way. In their earliest stages of embryonic development, a flat plate is formed composed of three layers, the ectoderm, mesoderm, and endoderm. Cells in certain sections divide more quickly than cells in adjacent areas, causing the layers of cells to buckle and fold. In this way, curves and bends are formed and the different parts of the body begin to take shape. The outer layer of the flat plate is called the ectoderm and it is this layer of cells that will curl into the neural tube, out of which the brain and spinal cord will develop.

      After an initial period of furious cell division, some cells are created that are post-mitotic, that is, they stop dividing. These cells begin the fascinating process of migrating to their final destination. They do so by means of molecular and cellular signposts that guide their progress. After the cells arrive at their proper place, neuronal connections must be established. Axons are then sent out to travel across broad swaths of territory to create synaptic connections with other cells. Their journey throughout the brain is also directed by chemical signals that point them toward their destination. The establishment of specific synaptic connections between neurons is partially controlled by genetic factors during pregnancy. However, the refinement of these synaptic connections takes place largely after birth and is highly dependent on experience.

      Diagram of the three bulges at the head of the neural tube that will develop into the hindbrain, midbrain, and forebrain.

      What is the neural tube?

      The neural tube is a long tubular structure that develops from the outer layer of the initial tube of embryonic cells. At the head of the neural plate, three bulges form distinct sections. These are called the forebrain, the midbrain, and the hindbrain, also known as the prosencephalon, the mesencephalon, and the rhombencephalon. The table below lists the parts of the neural tube and the brain regions that develop out of each section of the neural tube.

      The Neural Tube and Its Corresponding Brain Regions

      What does the hindbrain become?

      The hindbrain divides into the metencephalon and the myelencephalon. These two sections subsequently develop into the cerebellum and pons on one hand and the medulla oblongata on the other. The pons and medulla oblongata are both part of the brain stem. Together with the cerebellum, they form the reptilian brain of Paul MacLean’s triune model.

      What does the midbrain become?

      The midbrain divides into the tectum and the cerebral peduncle. These deep brain structures lie just above the brain stem. In primitive vertebrates such as amphibians, fish, and reptiles, the tectum serves as the main visual processing center in the brain. In primates, its function is more restricted as the majority of visual processing is done in the neocortex. The primate tectum helps to control eye movement. The cerebral peduncle includes several brain areas that contain the neurons that produce important neurotransmitters. For example, the substantia nigra is the origin of a major tract of dopamine neurons. We are now learning that the midbrain played a central role in the evolution of early emotional systems.

      What does the forebrain become?

      The forebrain develops into the evolutionarily newest parts of the brain and those most closely involved with complex mental life. The forebrain divides into the diencephalon and the telencephalon. In MacLean’s model, the forebrain includes both the paleomammalian and the neomammalian brains.

      What does the diencephalon turn into?

      The diencephalon divides into the thalamus, the hypothalamus, and several related regions. The thalamus is involved with sensation. It is the relay station between the sense organs and their corresponding cortical regions. Olfaction (smell) is the only sense modality that does not run through the thalamus as it is processed directly by the olfactory bulb. The hypothalamus links the brain to the autonomic nervous system and serves a critical role in emotion, connecting the mental aspects of emotion with the body’s physical response.

      What develops from the telencephalon?

      The telencephalon contains the most advanced parts of the brain. Although present in all vertebrates, it is most developed in birds and mammals. In humans the telencephalon develops into the cerebral cortex, the limbic system, the basal ganglia, and important white matter regions. The cerebral cortex includes the four lobes of the neocortex as well as those areas on the inside surface of the cortex that directly contact the subcortical regions. These include the cingulate gyrus, hippocampal, and parahippocampal regions, as well as the insula, which is sandwiched between the temporal, frontal, and parietal cortices. The basal ganglia, amygdala, and septum also develop out of the telencephalon. Finally, cerebral white matter is made up of bundles of axons that travel across large sections of the brain. Important cerebral white matter structures that develop out of the telencephalon include the anterior commissure, the internal capsule, and the corpus callosum.

      What does it mean to say that the brain is plastic?

      Plasticity of the brain refers to the brain’s ability to change with experience. As the very development of our brain is dependent upon our experience, we can say that the human brain is very plastic. In fact, brain development in humans is more experience-dependent than in any other species, reflecting the central role our capacity to learn has played in human evolution.

      What impact does brain injury have during fetal development?

      Because