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But there was never any doubt as to why the human brain could not continue to grow.

"Fountain of Ceremonies", Machu Picchu, Peru
"Fountain of Ceremonies", Machu Picchu, Peru


The answer lies in the biology of human reproduction. Robert D. Martin explains the problem in the essay The Evolution of the Human Body:

The dimension of the birth canal that passes through the pelvis represents a limit to the circumference of a newborn child’s head, and in humans the relevant dimensions are even smaller to enable the pelvis to be remodelled for bipedal walking.

At birth, the size of the human brain has already reached the limit set by the pelvis. The human birth process has therefore become exceptionally complex compared to that of other primates. It includes the continuous twisting and turning that a newborn has to do in order to be born head-first when he or she turns towards the mother’s back.
(Die Evolution des menschlichen Körpers, in: Fischer/ Wiegandt: Evo­lu­tion, pp. 74-109, p. 105)

This is where the development into a human being should have come to an end. The birth channel of the early human woman, who was still far from Homo sapiens and its culture, blocked brain growth and thereby preventing the development of the great ideas of a Descartes, Darwin, Kant and Mayr.

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The brain was already perfect, why should it keep growing?

Entrance to the "Mausoleum", Machu Picchu, Peru
Entrance to the "Mausoleum", Machu Picchu, Peru


Is the answer to the question of the end of brain growth really that simple? In the passage already quoted, Jürgen Habermas also overlooks the problem of the end of the growth of the human brain:

Once the growth of the human brain stopped, cultural learning processes began to take the place of genetic adaptation. What other animal species lack is the transfer of symbolically stored knowledge from generation to generation, such that it can be revised and expanded light of new experiences.

If genes cannot induce human adaptation, does culture have to take over? But where does culture come from? Who discovered cultural learning processes?

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Hypothesis 11

Extended brain growth

Round Tower, Machu Picchu, Peru
Round Tower, Machu Picchu, Peru


In humans, brain growth is limited by the width of the woman’s birth canal, through which the child must pass at birth. The selective advantage of an individual having a large brain reaches a natural limit here.

To overcome this natural limitation, evolution has found two solutions in human beings: The first solution is to prolong brain growth into the post-natal period. This solution is acquired by a further increase in effort because of the required and time-consuming care of the helpless infant.

Explanation:

Why, during the course of evolution, does the human brain simply stop growing? This question is just as fascinating as the question related to the causes of growth in previous periods. Surprisingly, it does not get discussed very much.

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Another advantage of large social groups is that they are attacked less often by predators.

Palace District, Machu Picchu, Peru
Palace District, Machu Picchu, Peru


Dunbar comments,

that the main reason for the development of large groups of primates is the risk of ending up as prey. This risk of becoming a victim has even contributed directly to the selection of large brains, as it has been shown in many instances that predators attack members of species with small brains with a disproportionate frequency, relative to the frequency in their respective habitats. (p.250)

Dunbar compared the neocortex volume of monkeys and great apes with the sizes of the respective groups and determined the correlation discussed above. Applied to humans, Dunbar comes with a group size for humans of 150 individuals (the Dunbar Number).

He finds this number in many forms of human organisation: in the average size of hunting and gathering clans, in the size of European villages before the industrial revolution, in the size of personal networks, etc.

Obviously, the relationship between brain and social complexity in humans is similar to that of the great apes.

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In primates, the group size correlates with brain size. In other mammals and birds this is not the case.

Machu Picchu, Peru
Machu Picchu, Peru


There, however, it is apparent that the monogamous species (in pairs) have a larger brain than the polygamous species. Dunbar’s interpretation of this finding is,

that the original incentive for the evolution of larger brains can be found in the development of bonding in couples, which usually goes hand in hand with the fact that both parents care for the offspring (…)

We can imagine that in the event that when pair bonding became an established practice, it led to larger brains and the cognitive ability to deal with complex relationships (relations). Primates have succeeded in exploiting these cognitive abilities by generalising them so that they would be available to all members of social groups. (pp. 249s)

So with the larger brain, which was developed through pair bonding, more complex social systems could be mastered with friends i.e. with non-reproductive partners.

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The larger the brain, more precisely: the neocortex (historically the youngest part of the cerebral cortex) in a primate species,

Machu Picchu, Peru
Machu Picchu, Peru

the larger the group in which this primate species can live. That implies that the size of the brain depends on the number of social contacts that individuals of this species manage. Dunbar continues:

Furthermore, additional analyses have shown that a number of behavioural patterns that are particularly associated with the social complexity of primates are also correlated with the relative size of the neocortex.

These include the size of the grooming clique (grooming: mutual body and fur care), the use of alternative mating strategies in males, the use of coalitions and alliances, manoeuvring for tactical deception, and the quality of social play. (pp. 247s)

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Hypothesis 10

The social brain

Parade, Cusco, Peru, 27. 8. 1989
Parade, Cusco, Peru, 27. 8. 1989


With the brain size of monkeys, the potential size of the social group increases, as individuals with the larger brains can establish and maintain social relationships with a larger number of group members (Robin Dunbar).

The advantage of the larger social group and the advantage of greater contact ability lead to a selection of individuals and groups with the larger brains. Selection promotes brain growth, as confirmed by hominid fossils.

Explanation:

The brain is an expensive organ because it consumes a lot of energy that the living being has to provide. Why did it paid off in evolution to invest in large brains?

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Another phenomenon that Achim Peters describes is body downsizing,

Parade, Cusco, Peru, 27. 8. 1989
Parade, Cusco, Peru, 27. 8. 1989


a phenomenon that can already be detected in lower vertebrates such as fish. When food shortages or a protracted illness occur, tissue recedes. Peters:

In the history of the human species, which ranges from Homo Australopithecus to Homo erectus to us, there were long periods of crisis such as ice ages and periods of drought (…)

If we look at the era some 50,000 years ago as an example of such a long-term supply crisis in the tribal history of humans, it perhaps provides us with the reason why the human body underwent such a sustained transformation, became more delicate, therefore providing the brain with greater access to energy in modern Homo sapiens. This is supported by the fact that about 50,000 years ago, with the beginning of the last ice age, progressive body downsizing began. (p. 49)

Hypothesis 9 dealt with the special position of the brain among the organs of animals and humans. The high energy consumption makes it a risky organ for living beings. The effort is only worthwhile if there are major advantages that offset the negatives.

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In evolution, however, the enlargement of the brain has not only taken place in apes and hominids

Parade, Cusco, Peru, 27. 8. 1989
Parade, Cusco, Peru, 27. 8. 1989


but also seems to be a general principle of evolution. Carel van Schaik and Karin Isler explain the problems that animals have with the growth and maintenance of their brains and then write:

Therefore, there are good reasons to assume that each animal species has the largest brain that it can energetically afford. Despite these costs, brain size has gradually increased over evolutionary periods. This is what palaeontologists call the Law of Marsh. It was formulated as early as 1879. (p. 155)

To regulate the energy of the human brain, Achim Peters formulated the Selfish Brain Theory, which is important in our context.

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Carel van Schaik and Karin Isler explain:

Parade, Cusco, Peru, 27. 8. 1989
Parade, Cusco, Peru, 27. 8. 1989

Perhaps the most important limitation of brain size is the fact that brain tissue is metabolically very intensive and it, therefore, consumes a lot of energy. The heart, the liver and the kidneys need a similar amount of energy per gram of tissue. However, their size is much more determined by body weight than brain size i.e. it varies much less between species. Other organs, bones, muscles, skin etc. consume per gramme a fraction of the energy the brain consumes (…)

That means that it is more difficult to achieve a certain increase in brain size through natural selection than it is, for example, to achieve an equivalent increase in muscle mass or general body size.

Another difficulty is that an organism cannot temporarily shut down its brain to save energy because the brain needs exactly the same amount of energy at rest.
(Carel van Schaik, Karin Isler 2010: Gehirne, Le­bens­läufe und die Evolu­tion des Menschen, in: Fischer/ Wiegandt: Evolution, pp. 142-169, pp. 153s)

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