THE BRAIN FROM TOP TO BOTTOM

Our Evolutionary Inheritance

The Connection between Ontogeny and Phylogeny

Brain Imaging

The prefrontal cortex seems to contain an especially large number of long axons, which can make connections between various regions of the cortex that are far from one another. The larger the prefrontal cortex, the more long axons it contains, and the more likely consciousness is to emerge. Brain imaging also shows that the prefrontal cortex is highly active when tasks of memorization and deductive reasoning are being performed.

Many researchers, such as anthropologist Robin Dunbar, say that the main selective pressure that has favoured the growth of the neocortex in primates has been the growing complexity of their social groups. According to Dunbar, the dramatic increase in the size of the prefrontal cortex (compared with the various sensory cortexes, for example) is explained by the properties of this frontal part of the brain, which have a great deal to do with social skills.

Though primates are not capable of elaborate systems of ethics, they do display many moral behaviours, according to Dunbar. These behaviours of mutual assistance and co-operation require them to trade short-term costs for long-term gain, even though this may make them vulnerable to exploitation by more selfish members of their society.

That is why morality may be advantageous from an evolutionary standpoint. It can strengthen group cohesion and provide a harmonious social climate that benefits the greatest number of individuals. And what part of the brain performs the cognitive functions needed to establish such a climate by restraining selfish tendencies? The prefrontal cortex.

A study was conducted of men who had antisocial personality disorder, characterized by irresponsible actions, cheating, impulsiveness and lack of affect or remorse, and who all had committed violent crimes. Images of their brains revealed that the neuronal volume of their prefrontal cortexes was 11 to 14% lower than in normal men.

The prefrontal cortex is recognized as playing an important role in individuals’ moral sensibility and self-restraint. The results of this study provide strong evidence for this role, and raise some questions about the notion of free will that is the basis for all law.

THE EVOLUTIONARY LAYERS OF THE HUMAN BRAIN

The various species of vertebrates are very similar in the way that their brains are organized. For example, all vertebrates have a forebrain, midbrain and hindbrain, within which are found all the major neural systems that have evolved to perform functions common to all species.

However, the various species also have areas of the brain that have specialized in distinctive ways in response to the specific constraints of their environments. The human brain is about three times larger in volume than we would expect in a primate of comparable size, and the proportions of its parts to one another are different than in other primates. For instance, in humans, the olfactory lobe is only 30% of the size it would be if it were in the same proportion to the entire brain as in other primates. It follows logically that if the human brain overall is nevertheless much larger than would be expected in a primate our size, then some of its other structures must be proportionately far larger. And indeed, when we trace the brain’s evolution from fish to amphibians to reptiles to mammals and finally to humans, we see that the parts of the brain that have grown the most in human beings are in the neocortex, and more specifically the prefrontal cortex.

The prefrontal cortex is the most rostral region of the cortex. In other species, it is dedicated to voluntary motor control, but in primates, it has developed considerably. For many years, scientists believed that humans’ unequalled abilities for planning and abstract reasoning were attributable to their having a more developed prefrontal cortex than other primates.

But studies conducted in the first few years of the new millennium have called this idea into question. Earlier studies had compared the human brain to those of other primates, but had not included most of the great apes. In these more recent studies, magnetic resonance imaging has been used to measure the relative size of the prefrontal cortex in all species of great apes, including humans. When this new method was applied to this broader range of species, the relative size of the prefrontal cortex was found to be almost the same in humans as in the great apes who are our closest cousins (chimpanzees, bonobos, gorillas, and orangutans).

According to the authors of these recent studies, humans’ superior abilities to anticipate and to plan can more correctly be attributed to other specialized regions of the cortex and to denser interconnections between the prefrontal cortex and the rest of the brain. The main reason that the prefrontal cortex is slightly larger relative to the rest of the brain in humans than in most other primates is that humans have a larger volume of white matter in their prefrontal cortex. This white matter is composed of myelin-covered axons that communicate with other parts of the brain, thus providing greater connectivity between the prefrontal cortex and the rest of the brain than in other species.

The high, straight forehead that characterizes modern humans, superceding the prominent brow ridges of our ancestors, is due to the expansion of the cortex, and especially the prefrontal cortex, in our species.

1. Australopithecus robustus 2. Homo habilis 3. Homo erectus
4. Homo sapiens neanderthalensis 5. Homo sapiens sapiens

This connectivity is essential for the proper functioning of our working memory, in which the prefrontal cortex plays an active role. Working memory is involved in many of the cognitive abilities that are so highly developed in humans, such as the abilities to retain information while performing a task, to verify the relevance of this information to the task in progress, and to keep the objective of the task in mind at the same time. Patients who suffer severe injuries to their prefrontal lobes can experience difficulties in relating the past, present, and future and hence in planning their actions. This phenomenon, known as “frontal syndrome”, confirms the primary role that this part of the cortex plays in anticipation and in choices of all sorts.

The recent expansion of the prefrontal cortex, together with the increased plasticity and associative capacities of the neocortex, thus seems to be the source of many of the most typically human cognitive abilities.

Though an increase in the size of the brain does not automatically confer any evolutionary advantage on the species concerned, it has been observed that in the process of hominization, the hominid species with smaller brains were gradually replaced by species with larger ones.

Some inventive hypotheses have been advanced to explain this phenomenon. For example, some theorists argue that the greater associative capacities of individuals with larger brains enabled them to make more unpredictable behavioural responses. In turn, selective pressure would then have been exerted on other members of their species to develop larger brains so that they could better predict these responses — an essential survival skill among social species. These larger brains would then have generated even more unpredictable behaviours. The positive feedback loop thus established would have been responsible for this tendency toward increased brain size in primates.

The concept of a positive feedback loop provides another way of accounting for evolution’s apparent tendency toward greater complexity. In this case, the selective pressure is seen as being generated by the evolutionary changes themselves. This pressure is also entirely dependent on the particular context of the human evolutionary line, with its highly developed social behaviours.

Close parallels can be drawn between the way that the brain has evolved in our species and way that it develops in an individual. In individuals, just as in species, mutations can arise, and they can have a significant impact on the adult’s morphology. Even though it is the adult individual who has been subjected to the selective pressure of the environment, it is that individual’s genetic development program that will eventually be selected and passed on to his or her descendants.

Many researchers think that the expansion of the neocortex in general and the prefrontal cortex in particular might be explained by mutations in a limited number of genes at an early stage of development. These mutations would have resulted in the duplication of certain areas of the cortex, exactly as is observed for certain genes in the genome.

As in the case of genes, one of the possible advantages of this duplication would be that one particular cortical region could evolve rapidly while its duplicate continued to perform the basic function originally assigned to it.