Big Brains And Intelligence

The large human brain evolved relatively late in hominin evolution, once Homo erectus arrived on the scene. However, because the human brain is seen as the champion of human evolution, we will consider it first. The irrepressible curiosity surrounding human brain evolution is perpetuated by the very matter that is so puzzling. However, there is nothing inside the human skull that is unique (Figure 5.1). Only the relative sizes of the anatomical regions within the brain and the number of neurons and the nature of their networks are unique to humans. Primates in general have large brains compared to most mammals. In fact, Neanderthals even had bigger brains than we do, so it is the wiring of our brains (the number and nature of the neural networks), not the size of our brains, which sets humans apart.

There is a specific type of "spindle" neuron in the human brain that is only known to exist in the brains of great apes and cetaceans (an order of mammals that includes dolphins and whales). Like most of the ocean floor, there is much uncharted territory in the brain. It is still not exactly known what spindle neurons do and how (or if) they correlate to higher cognitive abilities. However, it is probably no coincidence that spindle neurons are shared by the groups that share complex social patterns, intricate communications skills, coalition formation, cooperation, cultural transmission, and tool use.

Brain size is correlated with intelligence at the species level. Humans have the largest brains for their body size, with cetaceans and great apes falling close behind, and these species are considered the most intelligent animals. Higher levels of mammalian intelligence are characterized by flexible problem solving in complex scenarios, incorporating

Parietal lobe

Parietal lobe

Occipital lobe



Occipital lobe


Figure 5.1 Cross-sectional slices through the brains of a human and a chimpanzee show their similarities in content but their differences in relative sizes of regions. Illustration by Jeff Dixon, based on CT scans provided by Kristina Aldridge.

Parietal lobe

Occipital lobe


Figure 5.1 Cross-sectional slices through the brains of a human and a chimpanzee show their similarities in content but their differences in relative sizes of regions. Illustration by Jeff Dixon, based on CT scans provided by Kristina Aldridge.

novel solutions into existing behaviors, and curiosity. Human intelligence is distinguished by, for instance, our propensity for abstract and symbolic thought. The enlarged frontal lobe ofhumans in relation to the great apes is credited with advanced emotions, awareness, and memory. Strokes to parts of the frontal lobe can affect moral decision-making processes. The prefrontal region in particular is slightly increased in great apes and humans and contains centers for forethought and planning.

Much of what is known of ape intelligence comes from the laboratory where the animals are introduced to situations they do not experience in the wild. Chimpanzees will spontaneously draw and paint in captivity. At the Kyoto University in Japan, a chimpanzee named "Ai" learned over 100 visual symbols, including lexigrams, Japanese Kanji characters, letters of the alphabet, Arabic numerals, and also learned to understand some human speech and gestures. Many other chimpanzees and gorillas in captivity have learned to understand a huge vocabulary of English words in the context of complex sentence structures. In the wild, apes make and use tools. Chimpanzees are even known to save a tool to use later which requires forethought.

Within species, brain sizes vary and the correlation between brain size and intelligence does not hold. A person with a brain size on the lower end of the spectrum of variation is not necessarily any less intelligent than a person with the largest brain in the world. Measuring intelligence is also problematic. For example, intelligence quotient (IQ) tests applied to humans can only measure specific aspects of higher brain function, not the whole phenomenon. Physical intelligence, like the massive and complex neural coordination it takes to throw a baseball at 96 miles per hour into a catcher's mitt, is completely ignored by standard tests of intelligence.

Without being able to easily measure intelligence levels to compare between species, the measurable size of the brain is used as a proxy for amount of intelligence. And instead of using basic brain size, the size of the neocortex is compared, which is the region for spatial reasoning and sensory perception, and it is made up of the outer surface of the brain. Specifically, the ratio of the neocortex size to the size of the rest of the brain correlates to problem solving and flexibility or "intelligence" in primates. Robin Dunbar and colleagues have shown that neocortex size is related to social group size in primates (presumably because complex socialization requires exceptional intelligence).

The fossil record shows that the hominin brain clearly increases in size with time, but so does body size. Since brain size and body size are correlated across mammal species, values of brain size alone indicate nothing without taking into account body size. The encephalization quotient (EQ) of a species is the ratio of its brain size to its expected brain size based on the known correlation between brain size and body size in mammals. EQ offers a way of comparing brain sizes between vastly different-sized species (like whales and humans) and for tracking brain size increase in hominins (Table 5.1). A significant increase in brain size relative to body size occurred in H. erectus and continued thereafter until the emergence of modern humans.

Several different factors could have contributed to the selection for a larger brain in H. erectus and beyond. But first we should consider the evolution of primate intelligence. Major hypotheses for the evolution of primate intelligence are based on food procurement, food extraction, and socializing. If a food source is seasonal like fruits, a primate must

Table 5.1 Approximate Average Brain Sizes and Encephalization Quotients (EQ) of Living Hominoids and Fossil Hominins


Average CC











Australopithecus afarensis



Australopithecus africanus



Paranthropus boisei



Homo habilis



Homo ergaster/ erectus



Archaic humans



Homo neanderthalensis






mentally map sites, plan routes, and anticipate availability and quality of the resources. Such cognitive needs may also require traits like acute, stereoscopic, and color vision. Grey-cheeked mangabeys (a species of African monkey) have evolved the ability to use the weather to predict when figs will ripen. If the weather has been warm, they will return to a tree where they know figs are on the cusp of ripening. Extracting foods like hard nuts, roots, insect larvae, seed pods, and stems require higher cognitive functions as well as precise motor skills. Living in a group, like most primate species, involves competition, conflict, affiliation, cooperation, kin selection, and reciprocity, and tracking the behavior of others. Primate intelligence can and did evolve because of any of these models or combinations thereof.

Certainly these issues were at play in early hominins and all of them could explain the ratcheting up of brain size, but because human brains are larger and capable of higher cognitive functions than other primates, something else must have been at play. Brain tissue is very expensive to grow and to maintain. Developing such large brains would have required strong selection. An enhanced nutrient-rich diet, particularly rich in essential fatty acids provided by animal protein, would certainly help. It is probable that the incorporation of meat into the diet—which happened to coincide with body size and brain size increase during H. erectus times—contributed to brain size evolution. It is not concretely known why selection favored brain size increase, although it is easy to think ofreasons to do with culture, diet, socialization, reproduction, etc. The genes microcephalin and ASPM likely played roles. Microcephalin is a critical regulator of brain development and mutations in the gene cause a disorder that is localized to the brain—microcephaly, which is severe brain size reduction on the order of up to 70 percent.

Functions of the brain are often localized to either the left or right hemisphere, which is called lateralization. Language is one such lat-eralized function with centers located in the left hemisphere. In several books beginning in 1983 and continuing to the present, William Calvin has proposed that the overwhelming trend for humans to be right-handed (controlled by left hemisphere) and also to be highly dexterous, is correlated to the buildup of neurons in the left hemisphere during language development in a feedback loop where language and handedness were each increasingly specialized with selection for complex neural networks. Calvin's hypothesis illuminates the significance of the evolution of physical intelligence (e.g., dexterity and hand use), which is often overshadowed by the evolution of mental intelligence (e.g., language).

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