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.