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.