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

When did the chatter start? Human language

If primordial Man had a cranial capacity of only 450 cc, a third of the modern average, this appears to exclude a human level of intelligence for this creature.

It has been calculated, however, that the volume of the cerebral cortex for Australopithecus could have been 242 ml, for Homo habilis 366 ml and for early Homo erectus 540 ml (Blumenberg 1983, Table 3, page 591).  The modern range for the volume of the cerebral cortex varies from 230 to 561 ml or cc (Blinkov 1968, page 172).  This means that all species in the human line fall within the range of modern man.  The reason for this is that the volume of the cerebral cortex depends on how much an individual uses their brain, though all people, whether high usage or low usage are human.

The crucial faculty of humans is the ability to use language. 

What is the evidence for use of language in early species of mankind?

The parts of the brain involved in language are Broca’s Area and Wernicke’s Area.  These areas are dominant on the left side of the brain.

Features of the endocasts of Taung and Hadar 162-28, fossil skulls of Homo primocreatus / Australopithecus have markings showing that the language areas had increased in size.

Symbolic language is associated with handedness.  Right-handed people have greater development of the lobes on the left side of the brain.  Asymmetry of the cortex in Homo primocreatus / Australopithecus such as in the fossil MLD 1 indicates that they were using their right hands for intricate tasks.  These tasks are later associated with tool-making.

It is really use of language that would secure a place for Australopithecus in the Homo genus among anthropologists.  This genus has human adaptation rather than animal adaptation.

Homo habilis fossils come from Olduvai Gorge, East Rudolf and Omo in East Africa dating from 2 million to 1.3 million years ago.  Cranial capacity has increased to 700 cc although this species is very similar to previous australopithecines / Homo primocreatus. The Homo habilis fossils KNM-ER 1470, 1805 and 1813 show clear asymmetries of the cortex relating to right-handedness.  The specimen ER-1470 endocast has a more complex pattern of the third inferior convolution which includes Broca’s Area.  Broca’s Area controls the motor aspects of speech.

This leaves me in no doubt that these early species of mankind were right-handed, using their hands to make things, and used speech to communicate to each other.

Small brain size does not preclude the ability to use language, and this has been observed in living people.  There have been examples of people born with genetic problems causing them to be microcephalic and have dwarfism.  There have been known people with cranial capacities of 500 to 700 cc who nevertheless used articulate language.

The chatter started a long time ago with people only a metre high living a primitive, but a human life.

CT scan of a right-handed adult showing asymmetry of the cerebral cortex

Sutures of the skull

I’ve made lots of claims for prehistoric species of mankind being intelligent and even meriting the label ‘sapiens’.  There is circumstantial evidence for this; is there any direct evidence for it?

Complex sutures seen on both modern and fossil skulls may be direct evidence of a human level of thinking.  Ape skulls have sutures but these are much simpler.

The bony plates of the cranium are joined together by cartilage.  When the brain stops growing, this cartilage changes into bone and the cranial plates fuse together.  When this happens the sutures start to disappear and the skull appears to made of one piece.

In apes the sutures fuse early in life, but in humans the sutures may remain open well into adult maturity.

As an individual learns things, their cerebral cortex grows and both the grey matter and white matter increase in volume.  This causes pressure that stops the sutures from closing.  Not only do the sutures with their cartilage not close, but they also pull apart.  This process causes the sutures to have a serrated effect.

Paul Broca provided a typology of suture complication in 1875 so that skulls could be described.  Sutures may be simple, harmonious and denticulate.  The width of sutures show various grades of complication.

Sutures are developmental features that increase the size of the skull through behaviour rather than through genetic inheritance.

As an individual thinks, the cerebral cortex expands as new connections are made between neurons; the cortex convolutes beneath the cranial plates and pushes on the plates; the joins between the plates at the sutures pull apart with cartilage forming in the gaps, and the process continues sometimes late into life forming extremely complex sutures.

The degree of sutural complication relates to how much that individual used their brain for problem-solving.  A number of prehistoric skulls possess these features.

  • Sterkfontein 5 and MLD 37/38 from South Africa are classified as Homo primocreatus / australopithecus.  Sutures show a fair degree of complication, although they started to fuse at the end of youth.
  • Cranium 3, 4 and 5 from Sima de los Huesos classified as Homo praecursor heidelbergensis / Homo heidelbergensis.  The sutures are quite complicated in places.
  • Eguisheim skull is a Cro Magnon / Homo aequalis fossilis from the Post Ice age period.  It has highly complicated posterior sutures infilled with wormian bones.  The skull can be extended by 1 -2 cm in the places of these extensions.

Denticulate sutures in fossil skulls give an indication of human intelligence in prehistoric species as these developmental features only develop with use of the brain in a human degree of thinking.

I rest my case for prehistoric human intelligence upon these fossilized brain cases.

SH 4 from Sima de los Huesos, Atapuerca showing suture complication for Homo praecursor heidelbergensis

Childbirth the limiting factor

Birth in humans is difficult compared to birth in most mammals.  The limiting factor is the adjustment between size of baby and its head, and size of the bony pelvic inlet and outlet – the birth canal of the mother.  In humans cephalopelvic disproportion is a frequent problem.  Death often occurs in childbirth.

From the point of view of human evolution and increases in brain size, giving birth to a baby that starts off larger with a larger head is the direction of the trend.  Thus, a major factor in human development is those ‘child-bearing hips.’

Human evolution shows an increase in pelvic size due to increase in stature.  The first human species was only a metre high.  I’m 1.7 metres tall and many women are taller than me.  As overall height and size increase, pelvic size increases, so bigger babies can be born.

My thesis concerning human evolution is that human life style has been the main factor in changes to the body.  Early human species such as Homo ignis / Homo ergaster were adapted with narrow bodies to run fast to escape predators and to hunt animals by running them down.  Later species were far more sedentary.  Species such as Homo praecursor heidelbergensis had wider bodies.  Homo centralis / Neanderthal had medium stature and a well-built, wide body.

I see several ways of adopting a more sedentary way of life even in prehistory:

  • The first way is to use a canoe for locomotion instead of walking and running.
  • The second is to hunt using traps rather than pursue the quarry animal.
  • The third is the sexual division of labour – with women remaining in and around the camp, harvesting vegetable crops, cooking and looking after young children.  Thus, the distance that women would walk would be much less than for male hunting parties.  Later on small animals were kept for food in the camp.

When women ceased to hunt, they became specialized in the gestation of  large fetuses.  A wide body is an advantage in childbirth.

An increasingly sedentary life is an on-going trend in human culture.

Therefore, human culture and ways of life have changed with human inventions and use of technology.  The evolution of increased cranial capacity was dependent upon what human culture had to offer.  This goes back to prehistory, at least 400 000 years ago, and it is a trend that continues today.

Reconstruction of the pelvis of KNM-WT 15000 belonging to Homo ignis / Homo erectus

How did the human skull evolve increasing size?

Human evolution shows an increase in cranial capacity – the question is, how did this come about?

The answer is:

  • The development of the skull became delayed so that the fetal condition continued long after birth with fontanelles remaining open for longer.
  • The skull bones became thinner in modern humans compared to most prehistoric species and this allows for moulding during birth.
  • The female pelvis became bigger in wide-bodied populations so babies with larger heads could be born.

Compared to animals, human babies are born helpless and dependent on the parents for protection to a much higher degree.  Human babies are born with holes in their skulls that have only cartilage and membranes covering the brain, with no bone.  These holes are called fontanelles. 

Fontanelles allow for two things:  Firstly, they allow the baby’s head to be deformed during birth as it moulds during the descent down the birth canal.  Secondly, the skull grows outwards from these holes and from sutures between cranial plates.  The longer the fontanelles stay open, the more the skull can grow outwards from them.

This delayed pattern of growth – with the fetal condition being retained after birth – is the human pattern of growth.  It is called secondary altriciality.  By the time ape babies are born their fontanelles have almost closed up.  In the human baby the fontanelles remain open for the first year and the infant’s brain grows at fetal rates during this year.

The human pattern of growth is found in Homo ignis / Homo erectus.  We can infer that babies of this species were born with open fontanelles from two million years ago.  This allowed cranial capacity to increase to 800 cc or more.

My own calculations, which will appear in a book I am currently editing, show that Homo primocreatus – Primordial Man had an intermediate degree of secondary altriciality compared to apes.  There would be a small delay in development.

Homo ignis / Homo erectus had thick skull bones and well-developed browridges.  He and she lived in a world where being clubbed over the head by an enemy was a real possibility.  Survival was dependent on a reinforced skull.

Gracilization of the skull came with Homo aequalis / Homo sapiens  100 000 years ago – I think due to internal tribal peace and a new set of weapons used against enemies.

Thinner skull bones meant that the head of a baby could mould during childbirth, thus a child with a larger head survived the birth process.  It also meant a more delicate infant requiring more protection from falls.

An alternative way of having babies with larger heads was for the female bony pelvis to become more ample.  This seems to have been the way forward for Homo centralis / Neanderthal.  These were wide-bodied people with ample pelvis for childbirth.  The skulls of Neanderthals had thick bones so they must have achieved large cranial capacity by this alternative method.

In modern people all of the above traits are combined such that women give birth to infants whose skulls mould as they are born, and continue to develop at a high rate after birth.  The combination of traits has increased cranial capacity over the course of human evolution to what we consider normal today.

Top view of skull of human new born showing the fontanelles
Cranial plates and fontanelles in the human new born

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

More white matter, more talents

If a creature can have human capacities right back to those with brain sizes comparable to those of apes – on condition that this creature has a soul – then I appear to be saying that brain size is irrelevant to being human.  If this were the case, why would the brain have evolved an increase in size from the first known examples of upright creatures 3.6 million years ago to the appearance of Homo aequalis / Homo sapiens 100 000 years ago?

The first answer is that increased brain size does serve a purpose.

The second answer is that humans evolved increased brain size because they were already human.

The brain consists of white matter and grey matter around a primitive brain that runs basic functions and instincts.  The grey matter is the cerebral cortex which is only 3 mm thick surrounding the white matter.  Each neuron consists of a cell body located in the grey matter with extensions to it called dendrites.  A long fibre extends from each neuron cell into the white matter called an axon.  Dendrites receive signals to the neuron while axons send signals to other neurons in the brain and some axons lead to the spinal cord for connection with the rest of the body. 

Thus, the brain cells are connected via this wiring system with in-coming signals and out-going signals.  Learning consists of making new connections between brain cells.

Axons are myelinated with a protective sheath to improve their function.  The myelin sheath which is made of fat is white in colour so this is why white matter is white.  Signals can be passed more rapidly along a neuron axon when it is myelinated.  The more that signals are passed along a certain pathway in the brain, the more it becomes myelinated.

It is myelination of neuron axons that takes up the most volume in the brain.  The more that pathways in the brain are myelinated through use of the corresponding functions, the more necessity there is for space to be available.

What this means is that as brain size increases, the number of pathways and diverse functions performed by the brain can increase.

The way I see human evolution is that you start with a creature given a soul.  This creature is agile – apes are also agile.  This creature has mind and language ability.  Nevertheless, this creature lives a life of a very primitive sort, mainly catering for immediate needs though with a human perspective on those needs.

The early forms of mankind could have been very skilled at a limited number of activities.  As inventions were made, and human culture advanced, the human environment became more complex.  This brought about selection pressure for individuals with greater capacity and a greater diversity of talents.  More pathways could be myelinated with larger skull size.

Therefore, small brain size does not exclude the possession of human traits, but it does make the number of talents more limited.  The evolution of bigger brain size has allowed for more connections and more pathways in the brain.  For the individual this meant a greater repertoire of talents that were productive in a human environment.

Neuron showing short dendrites and a long, myelinated axon with junctions to another neuron

The evolution of brain size

Human evolution in the direction of Homo sapiens has essentially been couched in terms of an increase in brain size.  Measurements of prehistoric fossil skulls done by anthropologists show the cranial capacity for different prehistoric species of mankind.  The volume of the brain case was measured with no. 8 shot or with mustard seed in the 19th century.  Cranial capacity is larger than the size of the brain that would have been inside it.  For example, the average cranial capacity today is 1400 cc and the average brain weight is 1330 grams.

There was a steady increase in cranial capacity in prehistoric species.  This is a summary of the facts of physical anthropology:

  • Chimpanzee average brain size is 380 cc (cubic centimetres) [included for comparison]
  • Homo primocreatus / Australopithecus 450 to 500 cc
  • Homo habilis 700 cc
  • Homo ignis / Homo erectus / Homo ergaster 800 to 1100 cc
  • Homo ignis antecessor / Homo erectus 1100 cc
  • Homo praecursor heidelbergensis / pekinensis / afarensis / Homo heidelbergensis 1200 cc with a range of 1100 to 1400 cc
  • Homo centralis / Neanderthal range of 1250 to 1750 cc
  • Homo aequalis / Homo sapiens of 100 000 years ago 1500 cc
  • Homo aequalis fossilis Cro Magnon man  1570 cc
  • Homo aequalis / Modern Man  1400 cc

The evolutionary thesis is that as the brain evolves and increases in size, the animal evolves into a human.  The bigger the brain, the more human the creature.

There is, however, a problem with this.  The brain of Neanderthal was larger than the global average today but he was denied the status of being human.  It was said he could only grunt and not talk, no wonder that this brute went extinct when our species came along.

It was later found, with advances in genetics, that we in Europe carry a percentage of Neanderthal genes.  This either means that we are part-animal or that Neanderthal was human after all.

Christians who adopt an evolutionary perspective on creation and an evolutionary origin for humans have more of a problem than most with human evolution.  From a Christian perspective a human person is not an animal and a human person has a soul.  Some Christians have resolved the issue by dropping belief in a human soul, but for those who still believe that to be created in the image of God is to have a soul – at what point did souls enter the human being?

The narrative goes that an ape-man managed to become far enough evolved and get a big enough brain for God to consider this creature to be worthy of being given a soul.  From then on ensouled humans had moral responsibility for their actions.  Ironically they wiped out all the hominids without souls causing them to become extinct.

For me the above question is totally meaningless.  And the answer given has quite horrific implications – firstly that there could be a creature looking just like a human that had no soul and therefore was simply an animal – although you would have no way of distinguishing between the human and the animal just by looking at them.  Secondly, the highest gift of a soul becomes the justification for mass hominidcide, and this is presented as a good thing.

My alternative thesis is that:

  1. There is no threshold brain size for possession of a soul or for being a morally responsible being.
  2. If you have a soul, you are human.  The soul is whole and in every part, there are no part-souls.  Likewise there were never any hominids part-way to being human.
  3. Even with a cranial capacity only marginally above that of apes, a human life is still possible.

Therefore, all prehistoric species that have been classified as ‘hominids’ or ‘hominins’ have, in fact, been human beings adapted to various sets of circumstances that are not found today.  They have all been peoples with culture, and persons with souls.

I had already reached this conclusion when Homo floresiensis was found on the Island of Flores, Indonesia in 2004.  The skeleton found had a cranial capacity of 380 cc and a stature of one metre.  The find was classified as a dwarfed Homo erectus, but the surprise was that it was dated to only 18 000 years old. 

Homo floresiensis is confirmation that a being with a cranial capacity the same as chimpanzees, can, in fact, be human and have lived not long ago.  The human bones were found in a cave with evidence of fire, stone tools and animals that had been hunted.

Humans are human because they have a soul.  Even the most primitive humans had souls so any being looking like a human had a soul.  There was never a situation of Homo sapiens possessing a soul confronting hominids lacking a soul.  Human evolution has brought about changes to the physical body, but not to the fundamental identity of the human being.