The Evolving Brain: social interaction and complexity

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1 LENScience Connect Senior Biology Seminar Series The Evolving Brain: social interaction and complexity Jacquie Bay, Alan Beedle, Tatjana Buklijas, Mark Hanson, Peter Gluckman The brain of modern Homo sapiens is a sophisticated organ with the capacity for complex and abstract thought, communication, and metacognition (thinking about thinking). The evolution of this capability is linked to biological and cultural factors: development of the brain as an organ and development of social complexity and cultural capabilities. The evolutionary success of humans and their direct ancestors can be linked to selection that led to the development of intelligence. This is not the result of selection relating to one characteristic. Rather it is the selection for a combination of characteristics which collectively contribute to the development of intelligence, and the accompanying evolutionary success. Bipedalism, tool making, increased brain size, the emergence of language, and the development of culture are key contributors to the success of the human species. What is Human Evolution? Human evolution is the process by which over time, humans have changed to optimise the fit between the individual members of a population and their environment. Evolution of a species (macroevolution) and evolutionary change within a species (microevolution) operate to produce an organism that is matched or adapted to its environment; that match is not dependent on a particularly long or comfortable life for an individual, but rather on the successful passage of that individual s genes to the next generation. So in contrast to a modern human perspective where success is measured by a long and healthy life, evolutionary success is measured by successful reproduction. All organisms on the planet today are here because their ancestors successfully reproduced. Lineages that did not do so are now extinct. This is the core concept of fitness, which is fundamental to evolutionary biology. Evolution is the process whereby a population changes over time to optimise fitness of its individual members within a particular environment so Homo sapiens evolved by adaptations that maximised its fitness in the environments of eastern Africa, the region in which our species first emerged. Biological fitness for a human was and still is achieved by a strategy of supporting a small number of offspring to grow successfully to adulthood, reproduce and live long enough to support their own children reproducing successfully. Evolutionary pressures on our lineage operated to ensure this. Health and longevity beyond the reproductive period of the lifecourse, or the period necessary to support offspring into adulthood, are not drivers in the process of evolutionary selection. 1

2 Humans and their Relationship to Primates There are approximately 200 existing species of primate. They are typically arboreal (tree dwelling) species generally living in tropical and subtropical environments. Some common physical features of the primates include (figure 1): opposable thumb / big toe on the hands and feet locomotion which tends to be hind limb dominated enhanced vision and a reduced sense of smell compared to many other mammals a specific pattern of dentition (teeth) a flattened face. Primate brains are large relative to body size. As a result there are associated changes in life history traits, generally reflected in relatively long life spans, late onset of reproduction, having few offspring, and singleton pregnancies. Figure 1. Young Chimpanzee (Pan troglodytes), showing the opposable thumb / big toe and flat face common to the primates. Humans or Homo sapiens are upright apes belonging within the primate group. Humans have a particular form of locomotion, communication, cognitive capacity, capacities for intentionality and foresight, and the capacity to develop and use technologies. We are a species that lives in social groups and have uniquely developed cultural capacities, defined as knowledge, behaviour and tradition within a particular community or population. The hominoid clade Hominoids include the superfamily of apes and humans. This group is made up of the currently living great apes (the orang utan, the gorilla, the bonobo, the chimpanzee, and the human), their extinct ancestors and other extinct species which had evolved from the last common ancestor of these five species. Molecular evidence shows that the orang utan lineage was the first to split from the primitive hominoid lineage some million years ago and that the gorilla lineage split perhaps 7 million years ago (figure 2). We shared a last common ancestor with the chimpanzee and the bonobo as recently as about 5 6 million years ago. The hominin group consists of humans and their direct ancestors, which split from the rest of the hominoid clade 5 6 million years ago. These dates are based on estimates from the molecular clock which measures the rate of change at a molecular level (DNA, RNA or proteins). The molecular clock assumes that the rate of mutation of these molecules is relatively constant over time and amongst different species. This means that the molecular difference between two species is proportional to the time since they split from a common ancestor. Figure 2. Evolutionary relationship between the hominoid clade (group of species with a common ancestor). Dates are in millions of years before the present. 2

3 Adaptive radiation Adaptive radiation is a term used to describe an evolutionary phenomenon where there is the rapid appearance of multiple related species from a common ancestor. This is generally thought to occur when a species enters a new ecosystem where there are multiple ecological niches available. Natural variation within the ancestral species means that over time individuals that are best adapted for a particular niche will be selected for, and thrive in that niche, eventually forming new species. A classical example of adaptive radiation is the finches of the Galapagos where at least 13 species rapidly evolved from a common ancestral species which reached the Galapagos Islands. Each variant is adapted for a particular niche so that they are usually not competing with each other for resources and they can thus diverge and eventually form distinct new species. The primates and the hominins display adaptive radiation. We cannot be sure of the factors that drove the pattern of adaptive radiation seen in hominins, but they are likely to reflect a change in forest environment, perhaps reflecting climatic change. Currently the earliest fossil for which there is reasonable evidence to suggest that it was an ancestral hominin is a specimen of Sahelanthropus tchadensis which is dated at least 6 MYA and was found in the region of Lake Chad. Ardipithecus ramidus is dated to 4.4 MYA and is the first fully bipedal hominin, which was also partially arboreal and lived in a woodland environment. The Australopithecines are several species of early hominid found in both eastern and southern Africa and which date to between 4 and 2 MYA. Their appearance and radiation has been related to a change in global temperatures, which fell during that time and led to a more open habitat in eastern Africa. The earliest members of the genus Homo are found approximately 2.5MYA. Homo rudolfensis and Homo habilis had smaller jaws and teeth, a larger brain, and are known to use tools. Exact relationships between the hominins continue to be debated, however it is clear that the hominin clade has included many species, some of which will have coexisted (figure 3). 3 Figure 3. Hominin Evolution.

4 Evolution of the Hominin Brain The size of the brain relative to the body has increased as hominins have evolved. Figure 4 shows a comparison of cranium size in the gorilla and selected hominins while figure 5 shows the change in brain volume estimated from fossil skull dimensions and compared to body size across the hominin lineage. Figure 4. Comparison of cranium size in gorilla and selected hominins 1. Gorilla 2. Australopithecus africanus 3. Homo erectus 4. Homo neanderthalensis 5. Homo heidelbergensis 6. Homo sapiens. Image courtesy of wikicommons. Creative commons Public Domain File:Craniums_of_Homo.svg Brain size, in both absolute terms and relative to body size, did not change dramatically in the earliest members of the hominin lineage. Australopithecines had absolute and relative brain sizes not greatly different from those of the modern apes. Brain size expansion started with the appearance of Homo species and showed an exponential increase from habilis, through erectus to neanderthalensis, and then a slight decrease to sapiens. During this period, average brain volume increased from about 400 cm 3 to 1250 cm 3. However brain size is not inherently linked to evolutionary success. Neanderthal brains were bigger than modern human brains, yet the species was not as successful. Brains are energetically expensive and there are many large animals with highly successful evolutionary histories yet relatively small brains (witness the dinosaur prior to the asteroid induced extinction). Thus the question of why hominins evolved large brains and Homo sapiens is endowed with its unique set of capacities is not self evident. Log Brain Volume (cm 3 ) Brain Volume Relative to Body Size Hominids H. sapiens H. erectus H. habilis Australopithecines A.boisei A.afarensis Gorilla Chimp Orang Great Apes Figure 5. Predicted brain size in the hominoid lineage. Using skull volumes to estimate brain size it is possible to compare brain volumes to body weight for humans and our ancestor species. The slope of the relationship is very different for the apes compared to the australopithecines and is different again for the Homo lineage. While there was an increase in brain size in Australopithecines compared to the existing ape lineage, there was a further development in relative brain size and presumably function across the evolution of the Homo lineage Log body weight (kg) Redrawn from Bonner J.T., Why Size Matters. Princeton University Press

5 What drove brain expansion? There are several inter related theories on the evolutionary origin of brain expansion. Effectively they either place emphasis on Homo species finding adaptive advantage in social interactions within their group, or in planning their affairs for hunting and tool making. The weight of evidence now favours the former. The adaptive advantage of social interaction There is a close relationship between social group size, brain size and levels of intentionality which can be achieved in group interactions. Essentially, fitness is advanced by maintaining stable social alliances that promote potential mating opportunities. But if this strategy is to be successful, then an evolutionary ratchet may be created where societal pressures build, requiring even greater intellectual complexity to maintain reproductive opportunity. Thus a series of feed forward loops may be created in which rising social complexity requires greater intelligence, in turn this allows development of more sophisticated technology and more complex ways of living and more sophisticated social structures, and these in turn generate the need for greater higher cognitive function still. Communication becomes a critical component of such a feed forward system, and the development of language was clearly permitted and expedited by this increasing neural complexity. Advantages and challenges of living in large groups While living in a large group has advantages it also has challenges. The advantages of large groups include: defence against predators improved food supply large pool of mating partners However the challenge of living in a large group is associated with managing complex interactions between individuals within the group. Individuals living in a group must have the capacity to consider the implications of their actions on the group or members of the group, and to interpret the intentions of others. The development of language and the communication of abstract thoughts allowed this ability to develop. This possibly gave rise to art, music, science and religion. 5

6 BOX 1: Theory of mind orders of intentionality This theory describes how individuals can interpret each others behaviour and thoughts. The basis of theory of mind is a hierarchy of orders of intentionality. 1st order: 2nd order: 3rd order: 4th order: 5th order: I am aware of my own thoughts I think I know what you are thinking I think I know what you are thinking about me I think I know what you are thinking about what I am thinking...and I think I know what will happen if you do not respond in the way I would like you to respond... This can continue. The more political our intent, the higher the levels of intentionality that we need to employ. Most adults can reach 5 6 orders of intentionality before we become confused. This serves as the structure of adult human interactions. Observational evidence suggests that some non human primates operate to the 3rd order of intentionality, practising deception in relation to sexual matters and sometimes food access. Humans today interact socially with multiple individuals within multiple communities. Consider the number of groups you interact with on a daily basis; family, school, work, friends from sports clubs, church groups, cultural groups, facebook, twitter. Technology means we now have multiple interactions with people who we do not see face to face. Figure 6. Group size vs brain size in primates. Mean social group size for different species of primates plotted against relative neocortex volume (neocortex volume divided by the volume of the rest of the brain). Ape species are shown as open symbols; the human Anthropologist Robin Dunbar from the University point is predicted from the ape regression equation of Oxford has suggested that living in larger groups (from Dunbar RIM, Br Acad Rev 2008;11:15 17, with permission). had an evolutionary advantage for our ancestors. The larger group size offered security of food supply and increased protection, Dunbar has also shown that living in large groups requires intelligence. Figure 6 shows that group size in primates is correlated positively to brain size. In particular the noecortex, a part of the brain associated with higher order functions such as sensory perception, motor control, spatial reasoning, conscious thought and in humans, language is larger in comparison to the rest of the brain in species that live in larger groups. Based on correlations with other primates such as chimpanzees who live in groups of 50, Dunbar predicted that humans have evolved to live in groups of about 150 individuals. 6

7 A similar relationship with social network size has been found with the brain s amygdala, a structure involved in functions such as interpreting other s facial expressions and trusting strangers. Among humans, amygdala size correlates with social network size (Bickart et al 2010), suggesting it too helps with the skills required for a successful complex social life (figure 7). Figure 7. The relationship between social group size, number of groups, and amygdale volume in humans. Bickart KC, Amygdala volume and social network size in humans, Nature Neuroscience 2010 doi: /nn.2724 Language One of the most contentious debates in human evolutionary biology is over the timing of the evolution of the capacity for language. The informed views range in dating this from early Homo some 400,000 years ago to its appearing as recently as 50,000 years ago. The difficulty is the limited anatomical substrates which can be used to infer language, since the soft tissues of the larynx are not preserved as fossils. The scanty anatomical evidence suggests that some form of vocalisation was possible in early Homo. The other major form of evidence has been inference from the study of material (such as tools and artefacts) and social structure and behaviour. It has been suggested, for example, that relatively consistent patterns of tool making, some of which involved quite complex but arbitrary patterns of design, imply the use of language for their transmission. A stronger argument can be derived from studies of the development of art, rituals, and social rules. The first evidence of symbolic art may date to some rudimentary markings in the Blombos cave in South Africa estimated to be about 70,000 years old. But unequivocal representative art in the form of cave paintings or carved figures dates to only about 35,000 years ago in Europe and perhaps as early as 50,000 years ago in Australia. Some of this art, particularly in Australia, clearly involves use of abstraction and suggests the capacity for thought, which is intimately related to language capacity. Language is generally considered to have evolved as a way of assisting communication within the social group. Cooperative ventures such as hunting would be aided by such communication, although many other species such as wolves can cooperate in hunting without requiring advanced language. Indeed there has been a shift in emphasis towards viewing the evolution of language in a different context: namely to aid the capacity to be conscious and to analyse the perceived world. It is widely believed that it is not possible to build a construct of the world beyond the immediate present without language in some form. Dunbar has gone further in arguing that language was key to the maintenance of larger stable social groups of the order of 150 individuals which characterised our ancestral social organisation and to some extent still do today. Whereas grooming is used to achieve social cohesion in other primate organisational groups, language would be a more effective and efficient means of doing so as social groups became larger and the capacity for grooming across the full community became limited by time. There is thus a loose interaction between social group structure, brain size and language, and while it is not possible to be definitive, the weight of evidence suggests that language evolved relatively recently, perhaps 70,000 years ago. In turn this supported and reinforced our social organisation and allowed the development of a more complex mental world which could support the development of art, music, belief and political systems. 7

8 Brain Expansion cost and benefit The benefits of brain expansion have been significant, however alongside benefits there are costs. The human brain requires a higher level of energy to sustain activity than any other tissue in the body. Humans expend around 20% of their energy intake on the brain compared to 8 10% in most other primates and 3 5% in non primate mammals. Humans have evolved to eat an energy and nutrient rich diet which supports the high level of metabolic need of the brain. Fossil evidence suggests that major changes in brain size seen in Homo erectus coincides with the evolution of hunting and gathering communities. These communities consumed a higher proportion of animal foods than ever seen before among primates, had home bases to which they transported foods and were in the habit of sharing food within a community group. These improvements in diet quality are thought to be important in the development of the large brain in relation to body size (Leonard et al 2007). The other significant consequences of the development of the large brain are the problems associated with giving birth to a baby with a large head through the narrow and rigid birth canal that arose as a result of the pelvic changes associated with bipedalism. If the human infant was born at the same stage of maturity as other primates, then pregnancy would last about 21 months: this would require a pelvic canal so wide that it would be impractical for efficient bipedal locomotion. The compromise has been that humans give birth to an infant at a stage when the head can fit through the birth canal, but this means that the human infant is entirely dependent on its mother for many months after birth. This need to give birth to a totally dependent infant has determined human social structure if the mother is to support the infant she in turn needs to be confident of support from the father. Thus while early hominids showed great sexual dimorphism, with the males much larger than the females, implying a harem type mating system with fighting between males for mating rights, Homo sapiens has a much lesser degree of sexual dimorphism, suggesting that in general females were able to have continued support from one male. Box 2: Why are human babies so fat? Human babies are born with higher levels of fat than any other species (figure 8). Traditionally this was explained in relation to our hairlessness. It was assumed that natural selection compensated for our loss of fur with a layer of insulative blubber. A newer perspective notes that this excess adipose tissue is well suited to serve as a backup energy supply for another distinctive human trait our large brains. Brains have among the highest metabolic rates of any tissue or organ in the body, and they are quickly damaged in the event of even temporary disruption in energy supply. Humans have exceptionally large brains, especially early in life. Roughly 70 80% of the body s metabolism is devoted to this costly organ in the newborn. The developmental pattern of high fat levels seen in humans is likely to have arisen to cope with the very high demands of the proportionally large infant brain. Figure 8. Proportion of resting metabolic rate (RMR) allocated to the brain during human growth and development. Brain metabolism represents >60% of RMR in infancy, and about 20% in adulthood (from Leonard WR, et al. Comp Biochem Physiol A Mol Integr Physiol 2003;136:5 15). 8

9 Culture and Society Homo sapiens is a social animal. We are adapted to living in groups which requires cooperation and adherence to rules and customs; in turn this is dependent on reciprocal altruism (unselfish behaviour that benefits others) and overt ways of dealing with freeloaders individuals who take from but do not contribute to the group. Much of human behaviour can be understood in terms of the requirements for successfully living in such a social structure. Culture is an amalgam of knowledge, behaviour and tradition within a particular community or population. It can be manifest in technology and tools, in art and music, in belief, myth, stories and tradition, in behaviour and in social structure and organisation. There is an intimate relationship between our biological and cultural evolution. With the development of the capacity to communicate, observe and learn comes the potential for a different mode of inter individual and inter generational transmission of information: cultural inheritance. This is not restricted to humans, since other species show the capacity to adopt behaviours. One classic example is that of potato washing in macaques on a Japanese island. Scientists were in the habit of placing potatoes on a sandy beach to attract the animals out of the forest. Once one female macaque had learnt to remove sand from her potatoes by washing them in the sea, other monkeys and eventually all the monkeys in the troop adopted the same behaviour. In contradistinction to genetic inheritance, cultural inheritance need not involve vertical transmission between generations. Horizontal transmission is the norm for example when young people adopt a particular form of dress we can see that it is rapidly transmitted through the peer group. But culture itself undergoes evolution. Every aspect of human culture from belief, to art, language, music and technology shows a process of change which is termed cultural evolution. There is variation in a culturally determined characteristic and there will be selection by the society as to which variants are preferred and thus which become successfully spread. However the fidelity of replication need not be sustained, unlike most genetic replication for which there are repair mechanisms which generally maintain fidelity. Thus change in culture can be rapid. Trade offs and costs The evolution of humans has involved complex interactions between pre existing variation, environmental conditions, life history, and selection. No one factor has dominated the resultant shape of modern Homo sapiens, however we can identify some key factors and tradeoffs. Selection for traits such as bipedalism, increased brain size, tool making, the emergence of language and the development of culture have all played a significant role in the evolution of humans. However, each of these groups of adaptations has involved costs and tradeoffs. Overall, the collective advantage from the adaptations outweigh the costs to the individual and therefore promote the adaptive success of the species. References and further reading: Gluckman, P.D., Beedle, A.S., Hanson, M.A. (2009) Principles of Evolutionary Medicine Oxford University Press Bonner J.T., Why Size Matters. Princeton University Press 2006 Princeton, NJ. Bickart KC, Amygdala volume and social network size in humans, Nature Neuroscience 2010 doi: /nn.2724 Leonard WR, et al Metabolic correlates of hominid brain evolution. Comp Biochem Physiol A Mol Integr Physiol 136: Dunbar, R.I.M (2008) Why humans aren't just great apes British Academy Review 11, Leonard, W. R., Snodgrass, J. J. androbertson, M. L., (2007) Effects of Brain Evolution on Human Nutrition and Metabolism Annu. Rev. Nutr., 2007, 27, For further information contact: LENScience LENScience@auckland.ac.nz Copyright Liggins Institute