Despite the diversity in brains that is inevitable during development, evolution still produces offspring that resemble their parents more than they resemble another species. The majority of genetic information must be conserved and yet remain flexible enough to allow for individual variation arising from events that occur during the developmental process. One way to consider the influence of genes and environment is to think about the journey through life as an epigenetic landscape.
In 1940, the brilliant British polymath Conrad Waddington used a metaphor of a ball rolling down a corralled landscape made of up troughs and valleys of different depth. The diagrams overleaf represent two paths of development in two individuals (A and B) who have the same starting genotype, as in the case of identical twins. These two individuals therefore inherit the same probability of developing a certain phenotype – the expression of those genes into characteristics that emerge over one’s lifetime. However, they may have different actual phenotypic end points, determined by chance events and environmental effects, especially at critical points. At each junction, development can take a different path but whether it stays on course depends on the depth of the gully. Some gullies or canals are very deep, so the ball has no option but to follow that trajectory and it would take a mighty upheaval to set it on another course. These are the genetic pathways that produce very little variation in the species. Other canals are shallower, so that the path of the ball could be more easily set on another route by a slight perturbation. These are aspects of development that may have a genetic component but outcomes can be easily shifted by environmental events.
Figure 7: Waddington’s epigenetic landscape
(After Kevin Mitchell, PL.S Bristol © 2007)
Waddington’s metaphor of canalization helps us to think about development as a probabilistic rather than deterministic process. Most of us end up with two arms and two legs but it is not inevitable. Something dramatic during foetal development could produce a child with missing limbs, as happened during the 1960s when the drug thalidomide was given to pregnant women to prevent morning sickness. Other individual differences are much more susceptible to the random events in life that can set us off on a different course. This can happen at every level, from a chance encounter with a virus as a child to growing up in an abusive household.
Unravelling the complexity of human development is a daunting task and it is unlikely that scientists will ever be able to do so for even one individual, because the interactions of biology and environment are likelihoods and not certainties. There are just too many ways that the cards could stack up. More importantly, as the vernacular saying goes, ‘Shit happens’, which is a very succinct and scientifically accurate way of saying that random events during development can change the course of who we become in unpredictable ways.
One day a scorpion and a frog met on a riverbank. The scorpion asked the frog to carry him across the water on his back because he could not swim. ‘Hold on,’ said the frog, ‘How do I know that you won’t sting me?’ ‘Are you insane?’ the scorpion replied. ‘If I sting you, then we will both die.’ The frog, reassured by this, agreed to carry the scorpion on his back across the river. Halfway across, the frog felt the sharp, fatal puncture from the scorpion’s tail. ‘Why did you do that? Now we are both doomed,’ cried the frog, overcome by venom, as they both began to sink below the water. To which the scorpion replied, ‘I can’t help it. It’s in my nature.’
The story of the scorpion and the frog has been retold for thousands of years. It is a tale about urges and impulses that hijack our behaviours despite what is often in our best interests. We like to believe that we are in control and yet our biology sometimes gets in the way of what is best for us. As animals with the capacity to reason, we think that we are capable of making informed decisions as we navigate the complexity of life. However, many of the decisions we make are actually controlled by processes that we are not always aware of or, if conscious, seem beyond our volition.
Impulses are the drives behind the four Fs that we evolved to keep us alive long enough to reproduce. However, impulses and drives are not always appropriate – especially when they conflict with the interests of others. After all, there is a time and a place for everything. The impulses and drives that served us so well in our early evolution became problems in the modern era as we became domesticated. Being domesticated includes having to conform to social rules of what is acceptable in polite company. We may be highly evolved animals with more flexible behaviours than scorpions, but we also retain automatic urges that, unless regulated, can lead to self-defeat.
Many drives can become self-defeating. Some of us eat too much despite the warnings that we are ruining our health, whereas others starve themselves to death. Fighting often gets us into trouble and fleeing is not always the best thing to do when one should stand one’s ground. Making unwelcome advances or engaging in sexual acts in public is not acceptable in decent society. Addicts knowingly abuse legal and illegal substances that will put them in an early grave. Chronic gamblers can squander away their families’ futures and still believe that they can turn their luck around. There are those who feel the need to scream obscenities in public when it is the last thing anyone wants to hear. When we cannot stop these urges, we are no longer in control of our actions.
We all have the potential to be slaves to our urges and impulses and every so often we fail to keep them under control. How many of us have lost our tempers behind the wheel of a car, said things that should have remained as inner thoughts or acted in ways that we never thought we could? In the cold light of day, we often know what we should think, say or do, but sometimes our urges and impulses get the better of us in the heat of the moment.
Scorpions may be rigid and inflexible, but humans possess a much greater capability to control our urges because we have evolved brain circuitry that plays a critical role in regulating our thoughts and actions. These mechanisms of self-control, shaped and strengthened by domestication, are essential for regulating behaviours in social settings. Without this self-control, we are in danger of being ostracized from the group.
The executive suite in our brain
The capacity for self-control is supported by neural mechanisms that traverse through the frontal lobes. Throughout human evolution, the frontal lobes expanded as our brains grew bigger and account for one-third of the human cortical hemispheres.1 Although the human frontal lobes are bigger than those of the great apes, they are not proportionally larger than would be expected for an ape brain of human size.2 However, as we noted in the preface, it is not so much the overall size but rather the way the microcircuitry of our frontal lobes is organized that is crucial to processing power. If you put a brain through a meat slicer, the cross-section reveals that there is more surface area where the cortical neurons are, tucked in the deep grooves and folds of the human brain in comparison to other apes. If you unfold the brain, humans would have more grey-matter surface area and therefore more potential for connectivity in the frontal lobes.3 However, it is the change in this connectivity of the grey matter with experience that makes us so different from our closest primate cousins.4