Evolving Brains, Emerging Gods
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Living in one place allowed the dead to be buried next to the living; consequently, ancestor worship became increasingly important and elaborate. As populations increased, hierarchies of the ancestors inevitably emerged. At some point, probably between 10,000 and 7,000 years ago, a few very important ancestors crossed an invisible line and conceptually became regarded as gods (chapter 6).
By 6,500 years ago, when the first written records became available, gods had become numerous. Initially, their responsibilities focused on sacred issues of life and death. However, political leaders soon recognized the usefulness of the gods and increasingly assigned them secular duties as well, such as administering justice and waging war. By 2,500 years ago, religion and politics were supporting each other, as the major religions and civilizations became organized (chapter 7). In the final chapter, the utility of a brain evolution theory of the gods will be compared with other theories that have been proposed (chapter 8). The utility of any theory should be assessed by its ability to explain the known facts.
The evolutionary theory of gods proposed in this book is not original. Rather, it is an updated version of a theory first proposed, fittingly, by the father of evolutionary theory, Charles Darwin. As a young man, Darwin had held traditional Christian beliefs and had even considered entering the ministry. During his five-year voyage on the Beagle, he later recalled, he had been “heartily laughed at by several of the officers … for quoting the Bible.” When Darwin returned to England and started developing his theory of natural selection, it occurred to him that religious belief might also be a consequence of brain evolution. In his personal notebook Darwin wrote that he had “thought much about religion,” and in his typical telegraphic writing style, he speculated that “thought (or desires more properly) being hereditary” might be “a secretion of the brain.” If this were true, he continued, “it is difficult to imagine it [belief in God] anything but structure of brain hereditary … love of deity effect of organization.” Thus, thoughts, desires, and “love of deity,” he speculated, were all products of our brain organization.5
Darwin, only twenty-nine at the time, was not about to express such thoughts publicly. He was aware that his emerging theories of natural selection were sharply at variance with the Christian belief that man had been made in the image of God; his reluctance to offend the religious establishment as well as his pious wife was a major reason why he did not publish his theories of natural selection for another twenty years.
Just as Darwin’s views on natural selection were shaped by the animals he had encountered on his voyage around the world, so his views on gods were shaped by the people he had encountered. He had met native people in South America, in New Zealand, in Australia, in Tasmania, and on myriad islands across the Atlantic and Pacific Oceans and had been impressed by their many gods. In The Descent of Man, he noted that “a belief in all-pervading spiritual agencies seems to be universal,” and this “belief in spiritual agencies would easily pass into the belief in the existence of one or more gods.” Foreshadowing theories of brain development, Darwin added that such beliefs only occur after a “considerable advance in the reasoning powers of man, and from a still greater advance in his faculties of imagination, curiosity, and wonder.” Darwin likened “the feeling of religious devotion” in humans to “the deep love of a dog for his master” and cited a writer who claimed that “a dog looks on his master as on a god.”6
In later years, Darwin’s theories led him to a complete disbelief in God. In his autobiography, he wrote: “Disbelief crept over me at a very slow rate but was at last complete. The rate was so slow that I felt no distress, and have never since doubted even for a single second that my conclusion was correct.” As with many people, the problem of evil contributed to Darwin’s ultimate loss of faith. He was especially troubled by the death of his favorite daughter at age ten from what was probably tuberculosis. Darwin also asked how a supposedly omnipotent and omniscient God could allow “the sufferings of millions of the lower animals throughout almost endless time.” To a friend he wrote: “I cannot see, as plainly as others do, evidence of design and beneficence on all sides of us. There seems to me too much misery in the world.” Ultimately, Darwin even failed to perceive a deity in the process of creation, finding “no more design in the variability of organic beings and in the action of natural selection, than in the course which way the wind blows.”7
THE HUMAN BRAIN
In order to assess an evolutionary theory for the emergence of gods, it is necessary to understand something about the human brain. This will be briefly summarized in this chapter, with more details provided in the notes and appendixes. The brain is a wondrous organ, thought to comprise 100 billion neurons and 1,000 billion glial cells. If you decide to give away your brain cells, there will be enough to give 16 neurons and 160 glial cells to every person on earth. Each neuron is connected to at least 500 other neurons, resulting in a total of 100,000 miles of nerve fibers in each brain; if laid end to end, these nerve fibers could circle the earth 4 times. The nerve fibers are covered with myelin, a light-colored substance; because it is light in color, the nerve fiber connecting tracts are referred to as “white matter.” The neurons, glial cells, and connecting tracts together create infinitely complex brain networks, making the human brain the most complex object known in the universe. British neurologist Macdonald Critchley described it as “the divine banquet of the brain … a feast with dishes that remain elusive in their blending, and with sauces whose ingredients are even now a secret.”8
FIGURE 0.1 The four lobes of the brain.
Topographically, the human brain is divided into two hemispheres, each of which has four major lobes: frontal, temporal, parietal, and occipital (figure 0.1). It is further subdivided into 52 separate areas based on the organization of brain cells as seen under a microscope. The original division of brain areas was done in 1909 by Korbinian Brodmann, a German anatomist; it has been modified several times over the years, but the brain areas are still referred to as Brodmann areas, usually abbreviated BA plus a number, for example, BA 4. The Brodmann numbering system will be used in this book for readers interested in the localization of brain functions. Figure 0.2 shows the Brodmann areas.9
Neuroimaging studies and postmortem brain studies have shown which human brain areas evolved first and which evolved more recently, as detailed in appendix A. The brain areas that evolved most recently are often referred to as “terminal areas,” so named by Paul Emil Flechsig, a German researcher. Importantly, these most recently evolved brain areas are the same areas that are associated with most of the cognitive skills that make us uniquely human. Neuroimaging studies have also determined the order in which the white matter tracts, which connect the brain areas, evolved. Four white matter tracts that have evolved most recently connect the brain areas that have evolved most recently, the same brain areas that are associated with the cognitive skills discussed in this book. As will be detailed in subsequent chapters, what is known about brain evolution and what is known about the acquisition of specific cognitive skills fit together remarkably well.
FIGURE 0.2 Brodmann brain areas.
The importance of the brain’s connecting fibers in making us uniquely human also suggests that there is no single “god part” of the brain. Like almost all human higher cognitive functions, thoughts about gods are the product of a network of multiple brain areas. Such networks have been described as “grids of connectivity” that “allow a very large number of computational options to be associated with specific cognitive processes.” These networks have also been referred to as “modules” or “cognitive domains.” Thus, even language, which has traditionally been thought to be localized in two brain areas (Broca’s and Wernicke’s areas), is now known to be part of a network involving at least five other areas. Therefore, there is no “god part of the brain,” but there is a network that controls thoughts about gods and religious beliefs. This is the network of the numinous, the same network that con
trols the cognitive skills that make us uniquely human.10
THE NATURE OF THE EVIDENCE
Since the proposed evolutionary theory of this book depends on an understanding of how the brain evolved, it is reasonable to ask how we know what we know. What is the nature of the evidence? Information regarding hominin brain evolution comes from five major research areas: studies of hominin skulls; studies of ancient artifacts; studies of postmortem brains from humans and primates; studies of brain imaging of living humans and primates; and studies of child development.
Hominin skulls have been an important source of information on human brain evolution. It would of course be preferable to have the brains themselves, but following death, the brain is one of the first organs to deteriorate, liquefying within hours if the temperature is warm. We therefore have no brains from ancient hominins to examine. Imagine how much we could learn if we had preserved brains from Homo habilis, Homo erectus, Homo neanderthalensis, and early Homo sapiens to place side by side, to compare with the brain of modern Homo sapiens and then to dissect each brain in minute detail.
Alas, we do not. What we do have, however, are skulls that held those brains. Like Hamlet standing in the churchyard with the skull of “poor Yorick,” we can use the skulls to speculate on past behaviors that were the products of the brains within. As the developing brain grows during fetal and infant life, the pliable bones of the skull mold themselves to the shape of the brain. Skulls are thus like ancient footprints left in volcanic ash that subsequently hardened; we no longer have the feet to examine, but we do have the shape of the feet and even some details of the toes.
Skulls that have been well preserved can provide us with considerable data. The volume of the brain is, of course, relatively easy to calculate. The overall shape of the brain is also evident, including whether the two halves are symmetric, as they were early in hominin evolution but not later. By examining the shape, we can also make informed guesses regarding the relative size, and thus importance, of the frontal, parietal, temporal, and occipital areas. In early hominin brains, the occipital area was prominent, but in later brains other areas became more developed. The inner lining of skulls includes grooves for the major arteries and veins, and on the floor of the skull are concave impressions for the cerebellum and the underside of the frontal lobes. On especially well-preserved skulls, it is even possible to find impressions of individual brain ridges, or gyri. Overall, having the skull is a distant second choice to having the brain to examine, but when combined with other evidence of our ancestors’ behavior, the skull nonetheless may yield substantial useful information.
Ancient artifacts are a second important source of clues regarding the cognitive abilities and behavior of earlier hominins, and thus the evolution of the brain. The finding of improved tools made by Homo habilis two million years ago suggests higher intelligence and improved cognitive function in general. As previously noted, the finding of shells fashioned for self- ornamentation and used by early Homo sapiens approximately 100,000 years ago suggests that they had acquired an ability to think about what others were thinking about them. The finding of food, tools, weapons, jewelry, and other supplies buried with dead bodies by modern Homo sapiens approximately 27,000 years ago suggests that they had acquired an ability to think about a possible life after death.
THE DATING OF SKULLS AND ARTIFACTS
Ancient skulls and artifacts are useful for understanding human evolution, however, only insofar as they can be dated with reasonable accuracy. For the period up to about 40,000 years ago, radiocarbon dating has been commonly used. Carbon is present in all living things, and an isotope, carbon-14, decays at a predictable rate. By measuring the amount of carbon-14 remaining in a sample of hair, bone, wood, charcoal, or other organic matter, it is possible to calculate a probable date with a margin of error of approximately 10 percent. Thus, a burial that is radiocarbon dated to 30,000 years ago probably took place between 27,000 and 33,000 years ago. A limitation of radiocarbon dating is that the amount of carbon-14 in the atmosphere has varied over time, depending on solar activity and the earth’s magnetic field, so various methods have been developed to correct for this source of error. Because of such limitations, radioactive thorium and uranium are now increasingly being used as an alternative dating method.
For years earlier than 40,000 years ago, dating is much less precise. Various methods have been used, including a system measuring the decay of potassium to radioactive argon (potassium-argon dating), a system measuring the buildup of electrons due to radioactive damage (electron spin resonance dating), and a system based on DNA mutations. All three systems have very wide margins of error, and the earlier the event being dated, the wider the margin of error. DNA mutations, for example, have been used to estimate when species split, such as the ancestors of chimpanzees from the earliest hominins. It was recently discovered that DNA mutations occur more slowly than previously assumed. Thus, the chimpanzee-hominin split, which was thought to have occurred four to seven million years ago, may actually have occurred eight to 10 million years ago. And the migration of early Homo sapiens out of Africa, commonly dated to about 60,000 years ago, may instead have occurred 120,000 years ago. Thus, all dates discussed in this book before 40,000 years ago should be assumed to have wide margins of error.
J. Hellstrom, “Absolute Dating of Cave Art,” Science 336 (2012): 1387–1388; A. Gibbon, “Turning Back the Clock: Slowing the Pace of Prehistory,” Science 338 (2012): 189–191.
A third major research resource for learning about brain evolution is postmortem brains from humans and primates. It is generally accepted that brain areas that evolved early during the evolution of Homo sapiens also mature early in the development of an individual; similarly, areas that evolved later mature later. As summarized in one study of this phenomenon, “phylogenetically older cortical areas mature earlier than the newer cortical regions.” For example, brain areas associated with specific muscle functions, such as the movement of arms, lips, and tongue, were among the earliest areas to evolve and are also among the earliest areas to mature, thus enabling a newborn to grasp and suckle from its mother’s breast.11 Three methods of assessing the relative maturation of brain areas are summarized in appendix A.
In addition to providing information on brain areas that developed more recently in evolution, postmortem brains of humans can also be compared with the postmortem brains of chimpanzees and other primates. Such comparison studies reveal which hominin brain areas have increased or decreased in size over the course of evolution, the relative degree of connectivity of various brain areas, whether there are unusual cell types specific to hominins, the anatomical spacing of the cells, and whether there are differences in the chemical composition of such things as neurotransmitters and proteins.
Regarding the size of specific areas, it is generally assumed in brain development that the size of a specific brain area correlates with the importance of the function served by that area. This principle has been summarized as follows: “The mass of neural tissue controlling a particular function is appropriate to the amount of information processing involved in performing the function.” Thus, a bat, which relies on sound, has a large auditory cortex; a monkey, which relies on vision, has a large visual cortex; a rat, which relies on smell, has a large olfactory cortex; and a desert mouse, which relies on memory to recall where it has hidden seeds, has a highly developed memory area (hippocampus). Studying the relative size of specific human brain areas in comparison with those of chimpanzees can therefore help to identify which human areas are most important and have evolved more recently.12
In addition to studying hominin skulls, artifacts, and human postmortem brains, a fourth approach to understanding how brains evolved is to study living brains using recently developed imaging techniques. Such techniques include magnetic resonance imaging (MRI) and its functional component (fMRI) as well as diffusion tensor imaging (DTI), which is especially useful for assessing connections between brain areas. MRI st
udies of living humans and chimpanzees have highlighted structural brain differences between them and thus complemented the postmortem studies. MRI studies of children have also been used to assess which brain areas mature early and which ones later. The results have been remarkably consistent and show that “phylogenetically older brain areas mature earlier than newer ones.” The combination of the postmortem studies and the MRI studies indicates which brain areas developed most recently during the course of hominin brain evolution.13
Functional MRI (fMRI) studies can also be used to link specific brain functions to specific brain areas or networks. For example, an individual can be asked to think about what another person is thinking while an fMRI measures which brain areas are activated. This then links the process of thinking about another person to the activity of specific brain areas. Since we know which brain areas developed more recently in evolving hominin brains, the fMRI studies give us information regarding the functions of the more recently developed brain areas.
The use of diffusion tensor imaging (DTI), which has recently become available, has enabled us for the first time to visualize the brain’s white matter connecting tracts in living individuals. To date, over 15 separate connecting tracts have been identified, and by doing DTI studies on children and young adults, it is possible to assess the degree of maturation of each tract at different ages. Some connecting tracts are mature shortly after birth; an example is the corpus callosum, the large tract connecting the two hemispheres. This tract has also been linked to intelligence and found to have been especially large in a postmortem study of the brain of Albert Einstein. Another white matter tract that matures shortly after birth is the inferior longitudinal fasciculus, which connects the prefrontal brain area with the occipital lobe and visual cortex at the back of the brain. By contrast, four other white matter connecting tracts are among the very last to mature, and these connect the parts of the brain that are crucial for becoming modern Homo sapiens. These four connecting tracts are the superior longitudinal fasciculus, arcuate fasciculus, uncinate fasciculus, and cingulum; they are shown in figure 0.3 and will be discussed in subsequent chapters.14