Evolution of the Brain
The human brain carries the story of its evolution within its very anatomy.
Fish Brain
It began in water when fish developed a tube to carry nerves from the distant parts of their body to a central control point. First there was just a bulge on top of the spine, then the nerves started to arrange themselves into specialized modules. Some became sensitive to molecules and formed what is now our senses of taste and smell. Others became light sensitive and formed eyes.
The Reptilian Brain
These were connected to a clump that controlled movement; the cerebellum. This collection formed the reptilian brain, mechanical and unconscious. Its basic parts are still intact and form the lowest of the three-tier system that has developed since.
The Mammalian Brain
On top of this, more modules developed: the thalamus, allowing sight, smell and hearing to be used together; the amygdala and hippocampus, creating a crude memory system; and the hypothalamus, making it possible for the organism to react to more stimuli. This is the mammalian brain, known as the limbic system. Emotions are generated here but not experienced here as it is not conscious.
During mammalian evolution the sense modules triggered the development of a thin matrix of cells, whose shape allowed for many neural connections to be formed between them with only a small increase in size. This skin became the cortex and it is from this that consciousness emerged.
The Human Brain
The mammals that were to evolve into humans developed an ever larger cortex, pushing the cerebellum back to the position it now occupies. Australopithecus Africanus had a fairly human-shaped brain 300 million years ago, but it was only a third the size of a modern brain. One and a half million years ago the hominid brain underwent an explosive enlargement. So sudden was it that the bones of the skull were pushed outwards, creating the high, flat fore-head and domed head that distinguish us from primates. The areas that expanded most are those concerned with memory recall, thinking, planning, organizing and communicating.
The development of language was almost certainly the springboard for the leap from hominid to human. Combined with memory recall, language enabled our ancestors to ‘evolve’ as they no longer had to discover everything for themselves ‘from scratch’. They, for the first time, could learn from their predecessors and that led to the exponential development that we still observe today. For this rapid development new and specialised brain tissue was needed.
Skull Development
The frontal lobes of the brain duly expanded by some 40 per cent to create large areas of new grey matter: the neocortex. This spurt was most dramatic at the very front, in what are known as the prefrontal lobes. These jut out from the front of the brain, and their development pushed the forehead and frontal dome of the head forward, reforming it to the shape of a modern skull.
What are ‘we’?
Where does this development leave us? We still have our ‘fishy’ reflexes, allowing us to survive by automatically snatching our hand from a flame to avoid it being burned, our reptilian instinct to trust those who look similar to ourselves or our mammalian intuition to care for our young in a family environment.
