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