Summary on brain size and human intelligence

Human evolution over the past 3.6 million years involves the increase in brain size.  There is, however, no threshold brain size to being human – over and above the brain size of apes.

The reason for the increase in cranial capacity is to accomodate a brain with a larger volume of white matter.  White matter consists of the fat-based myelin sheaths of neuron axons.  Myelin sheaths insulate neuron fibres so that signals can be passed along the fibres at greater speed.  It is the myelination of neurons that requires more volume.

Our small-brained ancestors could have a high level of talent or dexterity in one or a limited number of areas.  Increased brain size allows for more pathways in the brain, so individuals have a greater repertoire of talents.  Diversity in talents is positively selected as society becomes more complex.

The actual basis to human intelligence are the brain cells in the grey matter of the cerebral cortex.  As a person learns and makes new connections, the cortex grows.  The cortex is only 3 mm thick, and as it grows it convolutes within the confines of the skull.

Thus, intelligence is dependent upon the surface area of the cerebral cortex.  This varies between individuals; it is not genetically determined but developed by learning done by the individual; the volume and the surface area of the cerebral cortex is independent of overall brain size.

The mechanisms by which skulls evolve larger size include:

  • Delayed development such that fontanelles which are holes between cranial plates stay open for a year after birth allowing for brain growth at fetal rates in the human infant for the first year of its life.
  • Thinner skull bones and fontanelles allow for a baby’s head to be moulded during childbirth.  The deformed skull returns to a normal shape a few days after birth.  In this way an infant can be born with a larger initial size of head and not die in the process of birth.
  • Later species of mankind have wider bodies and a more sedentary life style.  This can be linked to locomotion involving canoes rather than walking.  The female pelvis has evolved towards ‘child-bearing hips’ specialized for giving birth to babies with larger heads.

Sutures – the joins between cranial plates can remain open for a long time in humans, far into adult life.  Sutures close and fuse together when an individual ceases to learn things. 

A growing brain puts pressure on sutures such that they pull apart with cartilage forming in the gap.  This process causes sutures to take on a serrated, denticulate pattern.  The complication of cranial sutures is an indication of the intelligence of the individual and for how long in life they continued to learn new things.

There are examples of fossil skulls going all the way back which show imprints of convolutions and a degree of complication of sutures.  The convolutions of the cortex leave an imprint on the endocasts of skulls, while the sutures are observed on the outside of crania.

As a final confirmation of the humanness of early species of mankind, endocasts show some revealing features.  Firstly, an asymmetry of the cerebral cortex indicates that Homo primocreatus and Homo habilis were right-handed and busy making something.  Secondly, there is an increase in size of the language areas of the brain going right back to earliest species with a clear development of Broca’s Area in Homo habilis.  Broca’s Area located in the third inferior convolution controls the motor aspects of speech.

Gorillas have a cranial capacity of 500 cc.  There have been known people afflicted by microcephalic dwarfism with a cranial capacity of 500 to 700 cc who used articulate language.  Thus, there is no threshold for the use of human language in terms of brain size.

The evolution of brain size did not make us human.  Something else makes a person human.  What the evolution of brain size did was fit beings who were already human to the demands of an increasingly complex society.

Cro Magnon type skull from after the Ice Age – the Eguisheim skull – showing complicated, denticulate sutures

What is the actual basis to human intelligence?

  • There is no direct link between cranial capacity and intelligence in modern humans.  A large brain is not the cause of intelligence.
  • There is a minimum cranial capacity for normal intelligence which is about 1000 cc.  So cranial capacity is a factor that allows for intelligence, but does not cause it.
  • More volume inside the skull is taken up with white matter than with grey matter as it houses the myelinated axons of neurons.

It is well-known that humans are distinguished by having a highly developed cerebral cortex.  We’ve always been told to ‘use our grey matter’ when faced by intellectual problems.

What I have realized is that intelligence is linked to brain surface area rather than to overall brain size.  In humans the cerebral cortex is highly convoluted and this extends its surface area. 

A human baby may be born with ten thousand million brain cells located in the grey matter of the cerebral cortex.  The cortex is, however, only 3 millimetres thick.  The cortex grows by dendritization as each brain cell sends out dendrites to receive information.  These dendrites connect to axons from other neurons whose myelinated fibres occupy the white matter of the brain.  As the cortex grows, it starts to fold into ridges and furrows beneath the confines of the bones of the skull, and this increases its surface area.

The total volume of a modern human cerebral cortex can vary between 230 cc and 561 cc.  The surface area varies between 1469 and 1670 cm2.  This variation between individuals occurs independently of skull size.

It was noted in the 19th century (from autopsies) that very intelligent individuals had more convolutions to their cerebral cortex than was commonly observed.  When he died Cuvier’s brain (French palaeontologist of the 19th century) was observed to have an extraordinary complication of convolutions and deep sulci.  Each convolution was noted to be doubled by a rounded ridge.

This is what I believe happens: as an individual learns things, their cerebral cortex grows and when they are young their skull grows as well.  But when the skull growth slows down and the individual continues to learn, the cortex bound by the skull bones starts to convolute within the cranium.  In this way the cortex can grow in surface area within the space provided by the skull.

The point is that growth of the cortex is not set by inheritance, while cranial capacity and skull shape is largely set genetically.  Thus, just as muscles get bigger and stronger with exercise, the convolutions of the cerebral cortex would form from use of the brain for thinking.  The true basis to human intelligence depends on mental effort on the part of the individual and on educational opportunities not simply on inherited traits such as brain size.

This means that an endocast of a skull that shows the imprints of cerebral cortex convolutions is showing signs of intelligence exercised by that individual.

It is interesting to note that some fossil skulls have imprints of cerebral convolutions on the endocasts taken from them.  These are some examples classified as Homo ignis / Homo erectus:

  • Sangiran 2 from Java with a cranial capacity of 813 cc
  • Trinil 2 from Java with a cranial capacity of 940 cc

These skulls showing the imprints of cerebral convolutions on the internal bones of the skulls are both dated to 500 000 to 830 000 years ago. 

The following fossil is classified under my scheme as Homo praecursor heidelbergensis, but by the archaeologist who found these fossils as Homo erectus:

  • Arago XXI from France with an estimated cranial capacity of 1100 – 1200 cc

This skull has well-developed convolutions on the inner surface of the frontal bone and is dated to 450 000 years.

It is my belief that prehistoric species of mankind were learning skills that caused their cerebral cortex to grow as their neurons made new connections – imprints of convolutions of the cortex in these fossil skulls is an indication of a human environment for learning.

Convolutions of the cerebral cortex grey matter. Redrawn from Gray’s Anatomy