Series: The Human Life Cycle and the Family (1) Human History from the Perspective of Brain Evolution

I am a psychologist—my specialties are neuropsychology and clinical psychology—and I have long been interested in how the humanities, social sciences, and natural sciences each conceive of the human being. Referring to insights that range from the humanities to the natural sciences reflects my own inclinations, but it also fits the nature of psychology itself: the discipline began with a philosophical concern for individual cognition, then branched in one direction toward the study of social relationships and in another toward areas that intersect with brain science and computer science. In this series, I will explore the characteristics of human beings and families while surveying this wide scholarly terrain.

• Neurophysiologist John C. Eccles
• The Modern Human Limbic System
• Evolving Toward Greater Sensitivity to Positive Affect
• The Emergence of the Genus Homo

Neurophysiologist John C. Eccles

Therefore, to begin the discussion in this series, I would like to present a diagram from a certain academic book. This diagram was very impressive to me, and I have already mentioned it several times in papers and other publications. However, until now I have mainly referred to only a part of the meaning of the diagram, so this time I would like to carefully examine the author's arguments surrounding the diagram. The title of the book is

"Evolution of the Brain," and the diagram in question is used in Chapter 5. The original title of the book is "Evolution of the Brain: Creation of the Self," which shows that the author's interest lies in the creation of self-awareness along with the evolution of the brain. The original book was published in 1989. The author is the neurophysiologist John C.C. Eccles, a Nobel laureate in Physiology or Medicine. Here, I will quote from the Japanese translation by Masao Ito (1990). This book comprehensively discusses human evolution from the author's perspective, drawing on insights from various fields, including medicine, physiology, paleontology, archaeology, philosophy, and psychology. Translator Ito describes it as a culmination of the author's thoughts. Eccles himself has stated that "almost 70 years of my life has been spent preparing to write this book." What I will be discussing here is only a small part of it, specifically the considerations in Chapter 5. Regarding anthropological insights into evolution, I will refer to Eccles' arguments along with the writings of Yamagiwa Juichi, a leading Japanese primatologist, as appropriate. Incidentally,

Eccles believes that the naturalistic explanation of the evolution of human self-consciousness through Darwinian evolutionary theory has its limitations. For this reason, he expresses dissatisfaction that "nothing is said about the evolution of mentality" in the major works of some of the most prominent biologists of the 20th century, such as E. Meyer, J. Monod, and E. Wilson. And so, at nearly 90 years old, a neuroscientist who had become "obsessed" with the question of the evolutionary origins of humankind, wrote "The Evolution of the Brain," a book in which he directly examined the evolution of the human brain and mind. The book consists of 13 chapters, of which the last three are dedicated to discussing the problem of human consciousness. While the content of these chapters is also very interesting, here we would like to first consider the characteristics of the human life cycle, and therefore look at Eccles's claims related to that discussion. This is the section shown in the diagram mentioned at the beginning.

The Modern Human Limbic System

The diagram in question is used in the latter half of Chapter 5, "The Evolutionary Relationship Between the Limbic System and the Reproductive and Emotional Systems." This chapter discusses the evolution of the human limbic system. Before discussing the diagram, I would like to briefly touch upon the author's considerations in the first half of Chapter 5. Section 1 of Chapter 5 is "Some Anatomical Considerations," Section 2 is "The Limbic System and Emotional Expression," and Section 3 is "The Limbic System and the Pharmacology of the Hypothalamus." Regarding the limbic

system, neuroanatomy textbooks state the following: "It refers to the limbic lobe and its associated subcortical structures" (Yamadori, 1996). The limbic lobe is a cortex, but it includes the hippocampus, olfactory brain, basal forebrain, and cingulate gyrus. Subcortical structures include the amygdala, septal nuclei, superior and inferior thalamus, various thalamic nuclei, and others. Furthermore, it has been pointed out that the functions of the limbic system, which includes these structures, are referred to as the visceral brain, are involved in the formation of emotions, maintain homeostasis, and play a role in controlling emotions and sexual behavior (Figure 1).

Figure 1. Structure of the Limbic System

Image: PIXTA

Evolving Toward Greater Sensitivity to Positive Affect

In Chapter 5, Eccles describes the evolution of the limbic system with this structure and function. In Section 4, "Size Indices of Limbic System Components in Primate Evolution," he points out that the septal nucleus and amygdala, important components of the limbic system in humans, are larger than those of other great apes. This is based on a comparison using size indices that take body weight into account. He notes that

this enlargement is particularly noticeable in the basolateral part of the amygdala. The medial cortex is developmentally older, while the basolateral part is developmentally newer and is interconnected with the sensory association areas of the neocortex (Yamadori, 1996). For example, comparing chimpanzees and humans, with insectivores set at 1.00, the medial part of the amygdala is 1.11 in chimpanzees and 2.52 in humans, while the lateral part is 2.28 in chimpanzees and 6.02 in humans (Table 1). In other words, the lateral part of the amygdala is noticeably larger in humans. The same is true for the septum (nucleus).

Regarding this, Eccles states that in the evolution of the human limbic system, "there is a tendency to increase these components (septum and basolateral) related to pleasant and enjoyable experiences, while the components related to aggression and anger (medial cortex) remain underdeveloped."

This is an interesting observation. In other words, it is possible that, in the process of evolution, humans have become more sensitive to stimuli that evoke positive emotions than to stimuli that evoke negative emotions. If such an evolutionary tendency is recognized when trying to understand human nature, then perhaps we can explore coexistence and cooperation more than conflict.

Table 1. Comparative Size Indices for the Amygdala and Septum

Source: Eccles, JC (translated by 伊藤正男) (1990) "Table 5.1 Evolution of the amygdala and septum"

The Emergence of the Genus Homo

Eccles’s analysis compares present-day primates with humans, but in the actual fossil record, an increase in cranial capacity becomes evident a little over two million years ago（山極, 2008）. Australopithecus garhi, a species on the line leading to the genus Homo, had a brain volume of about 450 cc, comparable to that of great apes. As Yamagiwa notes, “our ancestors, after diverging from the common ancestor with chimpanzees, soon began walking upright and survived for over four million years with ape-like brains (山極, 2008).” Even so, tool marks on nearby animal bones suggest that garhi already used stone tools. Crucially, “only about 100,000 years later, Homo habilis appears with a brain volume exceeding 600 cc,” he writes. Stone tools were likely used to cut meat and break bones. Nutritional improvements afforded by meat eating are thought to have been indispensable for the expansion of the brain, an organ with exceptionally high energy demands. With this enlargement came the emergence of the genus Homo.

References

• Eccles, J. C.（1989）．Evolution of the Brain: Creation of the Self. Routledge. (エックルス, J, C. 伊藤正男（訳）（1990）．脳の進化　東京大学出版会
• 山鳥　崇（編著）（1996）．実用神経解剖学　金原出版
• 山極寿一（2008）．人類進化論－霊長類学からの展開　裳華房

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