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Our brain can be seen as possessing a tripartite structure that consists of the reptilian, mammalian, and neomammalian or higher brains (Hirth, 2010; MacLean, 1973). The reptilian brain is, in evolutionary terms, the most ancient part of the brain and one that we largely share with lizards and similar species. The mammalian brain refers to structures of the brain, such as the amygdala, hippocampus, and thalamus, that play a central role in fear, aggression, hunger, thirst, and sex. The core functions of the mammalian brain revolve around basic physiological survival needs. Finally, the higher brain consists of the neocortex or outer layer of the cortex that we commonly associate with our most advanced brain functions, including reasoning, planning, personality, and language. The neocortex comprises a much larger portion of the brain in humans than in other species. Although this tripartite model is overly simplistic, it is useful as a quick sketch of the evolutionary origins and structure of the brain.

One of the most interesting questions regarding the human brain is how it evolved to its current size and complexity. In order to answer this, scientists have tried to identify the conditions and pressures that led to the appearance of the larger brain in our primate ancestors. A number of theories have been offered to explain this evolutionary advance, but I’ll discuss two types of explanations: ecological and social (R. Dunbar, 1998; Gamble, Gowlett, & Dunbar, 2014). Ecological theories hold that the larger, more intelligent brain is a consequence of features of the nonsocial environment, such as climate or geography. One theory is that as primates increasingly relied on fruit for sustenance, they needed to be able to maintain mental maps of the locations of various fruits and keep track of when they were likely to ripen (R. Dunbar, 2011; Harvey, Clutton-Brock, & Mace, 1980). Such detailed and extensive mapping would have required considerable cognitive ability, and those who were better able to remember where food was located—presumably those with slightly larger brains—would have been more likely to survive and reproduce.

HUMAN EVOLUTION Conceptual image depicting the stages in the evolution of humans. At lower center is an amphibian creature rising from the water. These evolved into the first land vertebrates 360 million years ago. Clockwise from this are proconsul (23–15 million years ago) depicted as an African ape with both primitive and advanced features; Australopithecus afarensis (4–2.5 million years ago), which displayed a bipedal, upright gait walking on two legs; Homo erectus (1.5 million years ago) used fire and wooden tools; and Homo sapiens or modern man, who appeared 300,000–200,000 years ago).

Christian Jegou Publiphoto Diffusion / Science Source.

An alternative ecological hypothesis states that greater intellectual capacities were necessary to be able to extract the most nutritious components from food sources, such as nuts and seeds (Parker & Gibson, 1977; Walker, Burger, Wagner, & Von Rueden, 2006). Thus the more intelligent individuals would able to maintain their own health and strength as well as those of their mates and offspring, and consequently, their gene pool was more likely to be passed along. The premise of both these theories is that nonsocial pressures were the primarily drivers of the increase in brain size.

In contrast, the social brain hypothesis holds that greater intelligence was required to monitor increasingly complex social networks (R. Dunbar, 1998, 2011; Humphrey, 1976; Stevens & King, 2013). That is, with larger communities comes more frequent interaction with greater numbers of people, and in order to successfully negotiate all of the social relationships among members of a community, an individual needed to be able to predict the consequences of his or her behavior and the behavior of others. Furthermore, larger communities required more hierarchy, alliances, and cooperation and the need to keep track of them all. In other words, successful navigation of these complex social networks required greater social intelligence (Humphrey, 1976).

Dunbar (1993, 1998; R. I. M. Dunbar, 2014) provided an empirical test of these competing models by seeing how well variations in diet or community size predicted social intelligence. As an index of social intelligence, Dunbar used what he called the neocortex ratio, which is the quotient of the neocortex volume divided by the volume of the rest of the brain. Higher neocortex ratios reflect enhanced ability for complex reasoning, planning, and greater social intelligence. By plotting the neocortex ratios of various nonhuman primate groups against appropriate comparison measures—percent of fruit in the diet or group size—it became clear that trends in group size fit much better with the data and therefore more adequately explained the origin of our sophisticated brains (see Figure 2.2). In other words, there is a positive correlation between neocortex ratio and group size, but no correlation between the ratio and fruit consumption. The human brain, as you can see, is truly social, both in terms of how it processes information and in the way that it evolved (Mercer, 2013; Vugt & Kameda, 2014).

Figure 2.2 Origin of the Social Brain

Source: Gamble, C., Gowlett, J., & Dunbar, R. (2014). Thinking big: How the evolution of social life shaped the human mind. London: Thames & Hudson.

Neocortex Ratio: Quotient of the neocortex volume divided by the volume of the rest of the brain