The evolving brain

Human beings' ability to represent the world is far superior to that of our nearest evolutionary cousins, so much so as to make it qualitatively different. We can even generate mental representations of things that do not exist. The computer I am typing this on did not exist before people could think about such things. We can give voice to our internal mental representations and thereby share them with others and re-re-present them externally as written text or imagery. How did this qualitative difference form other animals come about?

Natural selection solves the problems the physical, social, pathological, and geographical environment throws at organisms by adapting them to that environment given time. For human beings, such environmental pressures caused us to be very clever compared to other animals, and this is demonstrated by the size and organization of the human brain (we may have been very lucky since there appears to have been a bottleneck in the human population around 75,000 years ago when it shrunk to fewer than 10,000 individuals). Generally speaking, the brain size of mammals tends to increase with increasing body size. The brain to body ratio is known as en-cephalization. It is well known that the brain size of our evolutionary cousins, the Neanderthals, was larger than that of humans but the encephalization was about the same. Bigger bodies need more brain capacity to control their bigger bodies, although it's not a neat straight line correlation. Nevertheless, the bigger the brain, the greater the complexity of cognitive tasks the animal can do. Over the course of human evolution, brain size has increased some 350% with the most recently evolved structure in the brain being the neo-cortex. This structure covering the surface of the brain is so densely packed that it has many folds so that it can fit itself inside the skull. The ability to think is related not only to the size but also the organization of the brain, although this is not always neatly captured by encephalization. A more accurate way of assessing the relation between brain size and body size is to use a measure called encephalization quotient (EQ). For mammals the mean EQ is about 1 with primates scoring above that and in-sectivores and herbivores below it. The EQ of humans is 7.6 ± 0.2. The nearest mammal is the dolphin (5.3) followed by the chimpanzees (2.4), so dolphins are quite smart too.

However, increasing human brain size might not be the whole story. Seymour, Bosiocic, and Snelling (2016) found that, not only did brain size increase, but blood flow rate (perfusion) to the brain increased over the course of evolution by 600%. They argue that this high cerebral metabolic rate in humans is associated with increasing energetic connections between neurons (as high as ten times that of other cells in the body) which, in turn, has helped promote the evolution of human cognitive ability. For humans, therefore, thinking is demanding in terms of its energy requirements since the brain along with the liver and heart generates the most heat in the body at rest. Apart from the energy requirements, there are other costs related to the development of human intelligence and big brains.

The costs of thinking

One obvious cost for human beings of being really clever is their utter helplessness when they are born. They spend a very long time learning but at the cost of being very vulnerable. This, in turn, requires the expenditure of significant resources from the parents. An extended childhood co-evolved along with strengthening feelings of attachment and bonding both between parents and offspring but also between mates. However, the benefits to the species of a big brain must surely outweigh the costs, as can be seen by the fact that we have spread across the entire planet and into a variety of often extreme environments. Our numbers have increased steadily since the Pleistocene and rocketed up after the depredations of the Black Death. However, evolution has never been too concerned about some of the costs so long as the genes survive to reproduce. As well as the costs in terms of the pain involved in human childbirth, there was the attrition rate - deaths during childbirth. In the USA in 2015, the sixth most common cause of death among women aged 24 35 was due to complications in childbirth (https://www.cdc.gov/women/lcod/2015/all-females/index.htm). In the 17th century, one woman died for every 40 births, so evolution must have had a really very good reason for this attrition rate (Schwartz, 2011).

This state of affairs was not helped by cultural changes. For millennia, the people with the expertise concerning the problems of childbirth were midwives. Over the years, deaths in childbirth in the West reduced until the end of the 19th century when male doctors began to take over from midwives (there were no female doctors). As a result, deaths in childbirth increased due to infections known as puerperal fever. It has been argued that this was because male doctors were physically moving from diseased patients or from dissecting corpses to women giving birth. Evidence for this came from the fact that, although poor people were more likely to die early than the rich, more rich women were dying at childbirth than poor women because they could afford the doctors whereas the poor continued to rely on midwives. Furthermore, women giving birth in hospitals, where there were both male doctors and diseases, were more likely to die than those giving birth at home (Helmuth, 2013).