In ancient mythology, the goddess Minerva sprang fully armed from the head of Jupiter. The brain was not so fortunate. Far from being created in a single instant, it evolved into its present complex system over a period of millions of years. It came into existence at quite a primitive level of evolution. Single celled organisms show certain behaviour patterns (e.g. movement towards light or nutrients). With the advent of multi-cellular life, a sharp division takes place between animal and plant life. While possessing internal signalling devices that enable plants to “communicate”, plant evolution turned away from the evolution of nerves and brain. The movement in the animal kingdom required rapid communication between cells in different parts of the body.

The simplest organisms are self sufficient, possessing all their requirements within a single cell. Communication between one part of the cell and another is relatively simple. On the other hand, multi-cellular organisms are qualitatively different and permit the development of specialisation between cells. Certain cells can deal primarily with digestion, others providing a protective layer, and others circulation, etc. Chemical signalling (hormones) exists in the most primitive multicellular organisms. Even at such a primitive level specialised cells can be found. It is a step towards a nervous system. The more complex organisms, such as flatworms have developed a nervous system, where the neurons are clustered together into a ganglion. It has been established that the ganglion is the evolutionary link between nerves and the brain. These clumps of nerve cells occur in insects, crustaceans and molluscs.

The development of a head and the location of eye spots and mouth are an advantage in receiving information about the direction in which the animal is moving. In conformity with this development a group of ganglia are clustered in the head of a flatworm. It represents the evolution of the brain – despite its primitive form. The flatworm also exhibits learning – a key property of the developed brain. It represents a revolutionary leap forward in evolutionary terms.

American neuroscientists have found that the basic cellular mechanisms for the formation of memory in humans are also present in snails. Professor Eric Kandel of Columbia University studied the learning and memory of a marine snail called Aplysia californica, and found that it exhibited some basic features found in humans. The difference is that, while the human brain has some 100 billion nerve cells, Aplysia only has a few thousand, and they are large. The fact that we share these mechanisms with a marine snail is a sufficient answer to the stubborn attempts of idealists to present humankind as some kind of unique creation, separate and apart from other animals. For almost every function of the brain depends in some way upon memory. No divine intervention is required to explain this phenomenon. Natural processes tend to be very conservative. Having hit upon an adaptation which proves useful for performing certain functions, it is constantly replicated throughout evolution, enlarged and improved upon to the degree that this bestows an evolutionary advantage.

Evolution has introduced many innovations in the brains of animals, especially the higher primates and humans with their very large brains. Whereas Aplysia can “remember” something for several weeks, its memory only involves a level of mental activity known as habit in humans. Such a memory is involved in, say, remembering how to swim. Research into brain-damaged people suggests that the faculty of remembering facts and habit are stored separately in the brain. A person can lose his memory for facts, but still ride a bicycle. The memories that fill a human mind are, of course, infinitely more complex than the processes that go on in the nervous system of a snail.

The continued enlargement of the brain required a drastic change in animal evolution. The nervous system of arthropods or molluscs cannot develop further as a result of a fundamental design problem. The nerve cells are arranged in a ring around the gut, and if expanded would increasingly restrict the gut – a limit sharply revealed in the spider, where the gut is so narrowed by its nerve ring that it can only digest its food as a thin liquid. Insects cannot grow beyond a certain size because their structures would break under their own weight. The brain size has reached its physical limits. Giant insects in horror movies are confined to the realms of science fiction.

The further development of the brain requires the separation of the nerves from the gut. The emergence of vertebrate fish provides the model for the subsequent development of the spinal cord and brain. The skull cavity can house an enlarged brain and the nerves run from the brain through the backbone down the spinal cord. From the eye pits developed an image-forming eye, which could present optical patterns to the nervous system. The emergence of amphibians and reptiles on land saw the great development of the fore-brain region which takes place at the expense of the optic lobes.

Harry Jerison of the University of California developed the idea of the correlation of brain size to body size, and tracked its evolutionary development. He discovered reptiles were small-brained 300 million years ago and remain so today. His graph of reptilian brain size against body size produced a straight line, which includes the dinosaurs. However, the evolution of the early mammals some 200 million years ago marked a leap in relative brain size. These small nocturnal animals were four or five times brainier than the average reptile. This was largely due to the development of the cerebral cortex, which is unique to mammals. This remained the same relative size for some 100 million years. Then, some 65 million years ago, it developed rapidly. According to Roger Lewin, within 30 million years brain development “had increased four to fivefold, with the biggest increases coinciding with the evolution of ungulates (hoofed mammals), carnivores and primates.” (New Scientist, 5 December 1992.)

As monkeys, apes and humans evolved, brain size became much bigger. Taking body size into account, monkey’s brains are two to three times the average for modern mammals, whereas the human brain is about six times the size. The development of the brain was not of a continuous gradual development but one of fits, starts, and leaps. “Though this broad-brush picture misses important details, the main message is clear enough”, says Roger Lewin, “the brain’s history involves long periods of constancy punctuated by bursts of change.”

In under three million years – an evolutionary leap – the brain tripled in relative size, producing a cortex that accounts for 70-80 per cent of brain volume. The first bipedal hominid species evolved somewhere between 10 and seven million years ago. However, their brains were relatively small, on a par with the ape. Then, about 2.6 million years ago, a rapid expansion took place with the emergence of Homo. “A leap in the evolution of the ancestors of modern humans took place,” says geologist Mark Maslin of Kiel University. “What evidence there is,” explains Lewin, “suggests that brain expansion began some 2.5 million years ago, a period coinciding with the first appearance of stone tools.” With labour, as Engels explained, came the expansion of the brain and the development of speech. Primitive animal communication gave way to language – a qualitative advance. This must have also depended upon the development of vocal cords. The human brain is capable of making abstractions and generalisations beyond that of the chimpanzee, to which we are closely related.

With the increase in brain size came the increase in complexity and the reorganisation of neural circuitry. The main beneficiary is the front section of the cortex, the prefrontal zone, which is about six times the size of that in apes. Because of its size, this zone can project more fibres to the midbrain, displacing connections there from other brain regions. “This may be significant for the evolution of language”, says Terrence Deacon of Harvard University, who notes that the prefrontal zone is home to certain human speech centres. Rose observes that for humans, this reality of consciousness is revealed in self-awareness and thought:

With the emergence of consciousness, a qualitative evolutionary leap forward has occurred, making for the critical distinction between humans and other species, so that humans have become vastly more varied and subject to complex interactions than is possible in other organisms. The emergence of consciousness has qualitatively changed the mode of human existence; with it, a new order of complexity, a higher order of hierarchical organisation, becomes apparent. But because we have defined consciousness not as a static form but as a process involving interaction between individual and environment, we can see how, as human relationships have become transformed during the evolution of human society, so human consciousness too has been transformed. Our cranial capacity or cell number may not be so different from the early Homo sapiens, but our environments – our forms of society – are very different and hence so too is our consciousness – which also means that so too are our brain states.[56]

  1. Rose, S. The Conscious Brain, p. 179.