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Brain Expansion: Advantages and Challenges

This page focuses on factors that have led to the development of intelligence in humans and to the evolutionary success of the human species.

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.



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 cm3 to 1250 cm3 .    


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.



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. 


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: I am aware of my own thoughts

2nd order: I think I know what you are thinking

3rd order: I think I know what you are thinking about me

4th order: I think I know what you are thinking about what I am thinking

5th order: ... 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, Instagram etc. Technology means we now have multiple interactions with people who we do not see face to face.


Anthropologist Robin Dunbar from the University of Oxford has suggested that living in larger groups 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.


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).




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.


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.

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.


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 trade offs. 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 trade offs. 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

Bickart, K. C., Wright, C. I., Dautoff, R. J., & Dickerson, B. C. (2011). Amygdala volume and social network size in humans. Nature Neuroscience, 14(2), 163-164.


Bonner, J. T. (2006). Why size matters. Princeton, NJ: Princeton University Press.


Dunbar, R. I. M. (2008). Why humans aren't just great apes. Issues in Ethnology and Anthropology, 3(3), 15‐33.


Gluckman, P. D., Beedle, A. S., Hanson, M. A.  (2009). Principles of Evolutionary Medicine. Oxford, England: Oxford University Press.


Leonard, W. R., Robertson, M. L., Snodgrass, J. J., & Kuzawa, C. W.  (2003). Metabolic correlates of hominid brain evolution. Comp Biochem Physiol A Mol Integr Physiol, 136(1), 5-15.


Leonard, W. R., Snodgrass, J. J., & Robertson, M. L. (2007). Effects of brain evolution on human nutrition and metabolism. Annual Review of Nutrition, 27, 311–328.