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This chapter looks at the social brain hypothesis. The term social brain has come to stand for the argument that the human brain, and indeed that of some other animals, is specialized for a collective form of life. One part of this argument is evolutionary: that the size and complexity of the brains of primates, including humans, are related to the size and complexity of their characteristic social groups. However, the social brain hypothesis is more than a general account of the role of brain size: for in this thesis, the capacities for sociality are neurally located in a specific set of brain regions shaped by evolution, notably the amygdala, orbital frontal cortex, and temporal cortex—regions that have the function of facilitating an understanding of what one might call the “mental life” of others.

This chapter describes and criticizes research that seeks the biological foundations of intelligence differences by studying head size, brain size, electroencephalographic indices, brain imaging, and molecular genetics.

This chapter fleshes out the “cultural drive” hypothesis proposed by eminent scientist Allan Wilson. It first considers the question of exactly how social learning could drive brain evolution when some animals managed to copy with tiny brains. Greater specification of the feedback mechanism by which cultural processes fostered the evolution of cognition was required if the argument was to be compelling. Second, the chapter looks at how many variables (e.g., diet, social complexity, latitude) had been shown to be associated with brain size in primates. In order to evaluate the hypothesis that cultural processes had played a particularly central role in the evolution of the human mind, whether social learning was a genuine cause of brain evolution must first be established. Third, the chapter argues that talk of increases in “brain size” is rather simplistic. The brain is a complex organ with extensive substructure, and with particular features and circuitry known to be important to specific biological functions. How the brain had changed over evolutionary time, and whether the observed changes in size and structure were consistent with what the cultural drive hypothesis predicted, also had to be established.

This chapter discusses issues concerning the interface between scientific knowledge and legislation for animal rights. It reviews available and up-to-date scientific evidence for awareness of self, theory of mind, complex memory, planning actions, complex communication, and intelligence in animals. The chapter considers brain size in relation to cognitive ability and social behaviour, and mentions that even smaller brains can process information very efficiently. Based on the anomalies in animal intelligence, it cautions us against ranking species according to performance on a single task or on a single set of criteria, and against attributing higher value to one set of characteristics than another.

Humans differ from chimpanzees and other apes with respect to our large brains and our abilities to use language, among other differences. This chapter discusses several candidate genes involved in brain size and linguistic ability (e.g., FOXP2) upon which natural selection appears to have acted in the human lineage. The complete genome sequences of humans, chimpanzees, and mouse (an outgroup) allow evolutionary geneticists to determine what genetic changes have occurred along the lineage that led to us. The McDonald-Kreitman tests and other tests to detect positive selection (Chapter 4) allow for the determination of which changes have been involved in adaptive evolution.

The brain consumes about 20 per cent of the total energy intake in human adults. Primates, and especially humans, have unusually large brains for body size compared with other vertebrates, and fuelling these is a significant drain on both time and energy. Larger-brained primates generally eat fruit-intense diets, but human brains are so large that a reduction in gut size is needed to free up sufficient resources to allow a larger brain to be evolved, placing further pressure on foraging. The early invention of cooking increased nutrient absorption by around 30 per cent over raw food. Increasing digestibility in this way perhaps inevitably leads to risk of obesity when food is super-abundant, as it is in post-industrial societies. However, obesity has clearly been around for a long time, as suggested by the late Palaeolithic Venus figures of Europe, so it is not a novel problem.

This chapter examines the notions that behavioural flexibility can be a useful comparative concept, that innovation frequency is an appropriate measure of behavioural flexibility, and that the reported frequency of novel behaviour is a valid indicator of the ‘innovativeness’ of a species or population. It suggests that innovation can be used to gauge species differences in behavioural flexibility, and demonstrate that innovation frequency correlates with relative brain size in primates. In the past, hypotheses regarding the ecological causes and consequences of enhanced behavioural flexibility have tended to use brain size as a proxy measure. It discusses the utility of brain size measures for these purposes, and notes how innovation frequency may provide a more direct measure of behavioural flexibility. The chapter explores the links between innovation and brain evolution, problems and solutions for comparative methods, and discusses what further data would be helpful for testing these ideas. The chapter further examines the evolutionary causes and consequences of innovative capacities and enhanced brain size in primates.

African apes and orangutans experience temporal and spatial fluctuations in fruit availability with similar behavioral consequences. Relying on the African apes as a comparative ecogeographic model, this chapter examines jaw form among Pongo pygmaeus morio, P.p. wurmbii, and P. abelii to determine if these populations differ predictably in ways that reflect their ecological profiles. Pongo p. morio is characterized by the longest lean fruiting periods and relies to the greatest extent on resistant and hard foods. These orangutans are found to exhibit the relatively most robust mandible, and thus display the relatively greatest capacity to counter large and repetitive jaw loads. Pongo abelii, which maintains a fruit-dominated diet even in times of fruit scarcity, displays the relatively least robust mandible. Orangutans are further shown to display a relationship between variance in energy intake, feeding efficacy, and relative brain size, suggesting a link among morphological divergence, behavioral ecology, and life history.

Three independent developments in the last decade or so have rekindled interest in relating brain size to cognitive ability both within and across different species. The first has to do with the rapid spread of neuroimaging technology, the second is the explosive growth in a field called behavioural genetics, and the third is a renewed interest in comparative cognition. The obvious conclusion at this stage seems to be that the range of scores on an IQ test of one sort or another is constrained, by the nature of the test, the difficulty of the items, the background and attitudes of the test taker, the time limits imposed for some subtests, and the fact that people have human brains, eyes and hands, all which limit how quickly one processes information and responds to it.

In general, an animal's brain size is in direct proportion to its body size, hence humans have bigger brains than other primates. The issue, however, is not the relativity of the brain sizes but how the anatomical areas of the brain, the cyto-architectonic areas connect with each other to carry out the specialized functions of the brain. This chapter discusses the physical differences between the human brain and the brain of chimpanzees and macaques. In addition to brain size, the neocortex size as well as the anatomy of the prefrontal and parietal cortices, and the Wernicke's and Broca's areas have also been examined to determine how the connectional fingerprints — or the connections between the functions of these anatomical areas — translate into functional fingerprints — or the response of the brain to different stimuli in different situations.

Information concerning the size of a brain structure and how this changes with normal development and disease provides vital information on the function of the structure. Modern stereology offers a series of methods for estimating size at both the macroscopical level and the microscopical level. This chapter discusses global number-weighted volume estimators (the principle of Cavalieri), local volume estimators of number-weighted mean volume, indirect estimate of number-weighted mean volume, local estimator of volume-weighted mean volume, local estimator of star volume, and estimation of volume and tissue deformation.

After summarizing the earlier chapters, which focused on the evolution of specific lineages, this chapter examines general patterns in the evolution of vertebrate nervous systems. Most conspicuous is that relative brain size and complexity increased independently in many lineages. The proportional size of individual brain regions tends to change predictably with absolute brain size (and neurogenesis timing), but the scaling rules vary across lineages. Attempts to link variation in the size of individual brain areas (or entire brains) to behavior are complicated in part because the connections, internal organization, and functions of individual brain regions also vary across phylogeny. In addition, major changes in the functional organization of vertebrate brains were caused by the emergence of novel brain regions (e.g., neocortex in mammals and area dorsalis centralis in teleosts) and novel circuits. These innovations significantly modified the “vertebrate brain Bauplan,” but their mechanistic origins and implications require further investigation.

Extensive field data are available for three (out of four) Pongo taxa: the Sumatran P. abelii and the Bornean P. pygmaeus wurmbii of west and central Kalimantan and P. p. morio of east Kalimantan and Sabah. The data show a strong west–east gradient in morphology, behavioral ecology, and life history. From west to east, relative jaw robusticity and tooth enamel thickness increase, the frequency of reliance on non-fruit fallback foods—in particular inner bark of trees—increases dramatically, female day journey length and home range size decreases, the frequency of fat mobilization (and probably deposition) increases (although this has not yet been measured in P. p. morio), brain size decreases, sensitivity to selective logging decreases, average density decreases, and interbirth interval decreases. Social organization shows a similar west–east gradient, with Sumatran orangutans exhibiting a greater degree of sociality by a number of measures, although variation within Borneo is less clear. On Borneo, there may be less developmental arrest, male long calls are slower and have fewer pulses per call, consortships tend to be shorter, and a higher proportion of matings are forced. Geographic variation in orangutan features is probably produced through a combination of plastic developmental responses, genetic differences and cultural processes. The chapter offers a new hypothesis for the adaptive significance of these differences, based on the observed reduction in mean level of fruit production and increased incidence of periods of extreme scarcity from west to east. We highlight important remaining questions.

Jean-Baptiste Bouillaud was the most ardent supporter of cortical localization in the cerebral cortex during the second quarter of the 19th century. He found support from his son-in-law Simon Alexandre Ernest Aubertin and Paul Broca. In March 1861, Broca delivered a paper on the relationship between brain size and intelligence. The more he thought about it, the less willing he was to accept the idea that all parts of the cerebral hemispheres function in the same way. Broca raised the possibility that the frontal lobes may serve other executive functions, including judgment, reflection, and abstraction. In his 1865 paper on cerebral dominance, Broca was forced to deal with some recently surfaced exceptions to the idea that the center for articulate language resides in the third frontal convolution of the left hemisphere. Broca contributed significantly to human physical anthropology and wrote important papers on the anatomy of the brain.

This chapter examines the benefits of a big brain. In humans, the brain constitutes about two percent of body weight but consumes twenty percent of total energy intake. It flies in the face of basic evolutionary theory that any animal would incur such high costs without accompanying benefits. Although lab tests of learning generally have been inconclusive, field studies in recent years have shown significant differences in social complexity, innovation, and behavioral versatility for species with high relative brain size. The rate of evolution itself may be affected by large brain size and the behavioral flexibility it affords. This chapter begins with an overview of the social brain and proceeds by discussing how brain size is related to diet, innovation, social learning, and tool use, as well as invasion success and evolution. It also considers behavioral differences in large-brained animals, with particular emphasis on flexibility and play. Finally, it describes the link between brain size and life history changes.

Intelligence has been associated with brain size and organisation. Ralph Holloway and M. C. De LaCoste-Lareymondie stated that australopithecine fossils show human rather than pongid patterns of hemispheric asymmetry, suggesting a human pattern of cerebral organization early in human evolution. From this, they speculated that human-like cognitive patterns evolved early in hominid evolution and were related to selection pressures for symbolic and visuospatial integration. They suggested that brain restructuring was an important early component in hominid evolution, despite the fact that the early hominids had brains that were clearly smaller than those of modern humans. Investigations of the learning capabilities of non-human primates has pointed to the orderly progression of the evolution of brain size within the order Primates as being vitally important to cognitive capability. There is a striking increase in the size of the brain as a whole and many of its regions.

This chapter focuses on hierarchical circuits in the brain that might make higher levels of objectivity associated with an emergent self possible, with particular emphasis on concepts as feature extraction. An animal with an encephalization quotient (EQ) above 1 has higher relative brain weight than an average animal of its body weight. This extra brain matter may be available for functions beyond mere physiological control. Also, as brains become larger in absolute size, an increasing percentage of the neurons appear to be dedicated to higher functions. This chapter describes an arrangement of brain circuits that allows one set of neurons to abstract qualities from responses in another set of neurons. In particular, the chapter discusses brain modularity, visual circuits, visual processing in the brain, how higher derivatives of information are made in frontal areas of the brain, how neurons convey information between different forebrain areas, and neuron counts in the cerebellum. Finally, it considers the correlation between genes and brain size.

This chapter examines cognitive evolution in hominins by identifying patterns and processes based on fossils and archaeological evidence. It first reviews the arguments for the pressures driving hominin brain expansion before quantitatively evaluating tempo changes in hominin brain size and testing environmentally based hypotheses for brain size change. Finally, it contextualises encephalisation patterns within the archaeological record for the evolution of cognition in hominins.

This chapter describes Homo habilis and its lifestyle. It clearly was smarter that the hominins that had preceded it, and also had a larger brain. The brain of Homo habilis is then described in detail, and reasons suggested why it increased in size.

This chapter explores the role of time budgets, fission-fusion sociality, kinship, and division of labour in the evolution of hominins. More specifically, it considers whether human anatomical and behavioural traits, such as bipedalism and life history traits, appeared simultaneously or, if not, in what order they appeared and how they relate to each other. Drawing on the social brain hypothesis, the chapter examines the timing of social change across the hominins, with particular reference to the close relationship between community size and brain size across the primates and modern humans. It also discusses four main phases of hominin evolution: the transition into Australopithecus; the appearance of the genus Homo sometime around 2 million years ago; the appearance of archaic humans, Homo heidelbergensis and allies, sometime around 500,000 years ago; and the appearance of anatomically modern humans around 200,000 years ago.