by Myra Barrett
They start the chapter with a short explanation about the many theories within the scientific field in relation to the evolution of the brain. The many theories surrounding brain evolution come with their own challenges and limitations. As they put it, “evolutionary theory also needs to be checked against biological and anthropological plausibility” (2012). The remainder of this chapter attempts to check the plausibility of several theories, as well as explains the many mechanisms that make our brains so unique.
The authors then analyze the brain's size in proportion to the human body. One of the more puzzling facets about the human brain is how much energy it uses. It actually seems that it would be maladaptive to supply that much energy for just one organ. However, the amount of energy it uses provides us humans with the capacities to form complex societies and sophisticated cognition. However, Downey and Lende (2012) write, “human cognitive distinctiveness presents a problem as well, however, since smaller animals have proportionally larger brains.” The solution to this problem is to measure brain size using “encephalization quotients” because this method of measuring brain size incorporates allometric scaling.
Comparatively, the phylogeny of the brain is correlated with its growth. For example, Downey and Lende (2012) say that starting around 2 million years ago, our ancestors' brains tripled in size within a short amount of time. However, chimps' brains are quite similar to ours, meaning that we can see them as homologous to our own in the evolutionary context.
Considering this, the human brain is not a new structure, but a homologous organ that corresponds to many other primates'. However, Downey and Lende (2012) point out that “as brains grow larger among related species, every part of the brain does not increase at the same rate, so species with large brains demonstrate predictable changes in the size of regions relative to each other.” Despite all the similarities in ape and human brains, ours are still the largest, comparatively.
Along with the structural qualities of our brains, it also grows and develops in some interesting ways. There are two main growth patterns described in this chapter: mosaic and concerted. Both are believed to have had a significant role in the evolution of the brain. Also, the brain grows, or develops, connections between neurons. These connections are an important piece of how our brains have developed. For example, brains can go through “connectional invasion,” in which they develop connections between regions that were not common in ancestors. These connections can also be maintained through “neuronal group selection,” which allows for connections that are not in use to essentially fade away.
Just as connections can be naturally selected for, so too can social mechanisms. However, social traits also have an affect on the environment that individuals are in. With the environment having influence over brain connections, and the social traits of a group having an affect on the environment, humans have been able to manipulate our own situations.
Similarly, individuals could be in competition within a group. This requires the development of certain social traits in order to survive in an environment. Downey and Lende suggest that in this kind of situation, those certain traits will become advantageous to develop, which in turn will adapt the brain of those exhibiting some trait. However, they also present a different perspective of the affect of sociality on the brain. There are some theories that suggest that learning and participating in a culture is advantageous for groups. If there are many people to pass on beneficial traits, not just by reproductive means, but by simply teaching and imitating, that would allow for the success of that group. Also, the ability to assess the emotions of other people would be beneficial in social context. The brain supplies humans with a good amount of empathy, making it possible to share in experiences and understanding.
Downey and Lende conclude this chapter with the thought that information on the evolution of the brain is vital in understanding ourselves in a neuroanthropological perspective. Evolutionary theory of the brain requires many different fields of study, including biology, anthropology, and physiology. There is new information being discovered every day on the human brains history and present. Our brains are constantly revealing new things about social structure as well as cognition.
Lende, Daniel H.; Downey, Greg (2012-08-24). The Encultured Brain: An Introduction to Neuroanthropology (Kindle Locations 2312-2809). The MIT Press. Kindle Edition.