EVOLUTION AND THE BRAIN
It has long been appreciated that there is something about the human brain that makes it unique amongst other primates and mammals in general. Dr. Greg Downey and Dr. Daniel Lende explore how and why the human brain has evolved the way that it has in Chapter 4 of The Encultured Brain: An Introduction to Neuroanthropology. The authors are well-qualified to provide an overview on this topic as both have a wealth of publications in this area, as well as being leaders in the development of the field of Neuroanthropology.
What makes a human brain unique? Is it simply the sheer size of it? Well, no. Anyone who has visited the American Museum of Natural History in New York City can clearly see that the enormous blue whale hanging from the ceiling has a brain much larger in size than that of a human’s. Perhaps the issue is not sheer size then, but the size relative to one’s own body. Unfortunately, we once again do not have a satisfactory explanation for human’s unique cognitive capabilities. While looking at relative size does work to explain the blue whale example (a blue whale’s brain only accounts for 0.01% of its body’s mass while a human brain accounts for 2%) we see other species that are an exception to this rule. For instance, a pocket mouse has a brain that comprises 10% of their body mass, much more than that of a human and yet we don’t see the unique functionality of a human brain expressed in a mouse.
However, when we turn instead to the encephalization quotient (i.e. the ratio of predicted brain mass to observed brain mass) we see that humans do stand out in this respect. In fact, humans exhibit an encephalization quotient that is between five to seven times higher than what is predicted for a mammal of our size. While greater encephalization is found across primates, humans are still an outlier and it appears this has been true for quite some time. Around two million years ago the genus Homo appears and with it we see a tripling in brain size in our ancestors as compared to other apes. However, it is not just the increase in size that is notable here–brain organization is a key component in better understanding our cognitive evolution.
So, do humans simply have brains that have a ton of new structures that other primates don’t possess? This is once again an incorrect assumption. Rather than humans and primates differing in existing regions of the brain, our current evidence suggests that the differences are actually proportional which has fascinating implications for our evolutionary understanding of cognitive function. Instead of evolving new structures, it appears that humans have modified or repurposed existing structures so that certain brain regions have expanded at a different rate than others. This evolutionary trade-off has resulted in decreased development in areas like the human olfactory bulb, while structures like the cerebellum which is involved in frontal lobe functioning has shown great expansion.
In addition to size and structure changing across evolutionary time, connections among regions of the brain have also seen significant changes.
In particular, we have seen an increase in the total number of neurons and with this, we see that larger brains tend to develop areas that are increasingly independent or modular which requires an increase in white connective matter. Understanding the brain’s connectivity is likely a key component of understanding human consciousness. Further, many researchers are now emphasizing the failure of previous metaphors such as the brain being “hard-wired” which does not capture the way in which brains are shaped through interactions and development (i.e. “wet-wired”).
NOT A BRAIN ALONE
To better understand how it is that experiences help shape the brain, Downey and Lende draw on the concept of niche construction which emphasizes the role that organisms play in shaping their own environment and subsequent selective pressures.
The authors argue that niche construction provides a place for cultural researchers within evolutionary studies–an interdisciplinary relationship that is too rarely created. This relationship is absolutely necessary since an understanding of human “intelligence” cannot be obtained by looking simply at the size and structure of the brain. Rather, we must also consider how our social relationships allow us to transfer and amass all of the components that we regard as forms of “intelligence” (e.g., technology, skills, information). Moreover, the authors emphasize how emotions, motivation, and perception are all factors that play into our social and cultural complexity and, thus, cognitive evolution.
This last section of the chapter was by far my favorite as I feel the authors made a convincing argument for the role of culture and social relationships in our understanding of human evolution. Additionally, I think that they do a great job of not allowing those who are skeptical or critical of previous evolutionary research to “throw the baby out with the bathwater.” I think their point is best summed up in the following quote:
Powerful, but overly simple, models of evolution that assume evolutionary traits will necessarily result in human universals need to give way, not to erase evolutionary explanations, but to provide richer accounts that incorporate data emerging from genetics, paleoanthropology, comparative neuroscience, and anthropology, including research on human diversity (p. 124).
EVOLUTION OF THE CEREBELLAR CORTEX: THE SELECTIVE EXPANSION OF PREFRONTAL-PROJECTING CEREBELLAR LOBULES
The lead author for this paper is Dr. Joshua Balsters whose research interests are in the area of social and emotional decision making. While not covered in the article, Dr. Balsters states that his specific interest is in Autism Spectrum Conditions (ASC) which he studies using a combination of fMRI, EEG, and computational modeling.
At the broadest level, the researchers are interested in whether the process of brain evolution is mosaic (i.e. evolutionary pressures act on individual neural structures) or concerted (i.e. evolution acts on interconnected parts of the brain that comprise whole functional systems). To test this, the researchers examine the cortico-cerebellar system in three different primate species: humans, chimpanzees, and capuchin monkeys.
The study consisted of obtaining high-resolution MRI scans from 10 primates from each of the previously mentioned species (5 females and 5 males). All of the included primates had either reached sexual maturity or were close. The researchers were able to isolate the cerebellum and examine the lobules related to the primary motor cortex and the prefrontal cortex.
The data demonstrate that the lobules related to motor and prefrontal cortex occupy a greater proportion of the human cerebellum (83.87%) as compared to chimpanzees (67.1%) and capuchin monkeys (56.82%). Moreover, the results show that where there were increases in the prefrontal cortex, there were proportional decreases in the motor cortex. Since the volume of areas of the prefrontal cortex increased relative to cerebellar lobules connected to the motor cortex, these data suggest that these associated functional systems evolved together.
This study provides support for the idea that brain systems evolve in a concerted fashion. The results from this study are important as they suggest a potential route to find clues regarding the evolutionary pressures that may have contributed to various expansions in the brain. Additionally, this research demonstrates how comparative MRI can be utilized to examine differences across primates.
I was able to somewhat follow the methodology of this study; however, I found myself both intrigued and somewhat intimidated by what I couldn’t grasp. This makes me wonder about some of the practical issues with interdisciplinary collaboration. I loved Downey and Lende’s description of how cultural researchers could and should be involved in evolutionary research, but there will likely be some limitations to this collaboration. In many ways, Balsters et al. (2009) is speaking a different language with words and acronyms that will have no meaning to someone who is not well-versed in the cognitive literature. Even simply grasping the hypothesis or overall finding for the study would likely be quite difficult for someone outside the field to grasp. Here is our challenge: if we were to reduce the complexity of the article, perhaps more researchers could understand the results; contrastly, researchers most likely to utilize this study will need a detailed report of the methodology and results in order to replicate or expand on this study. How do we find this balance?
- How would our understanding of human brain evolution be different if we didn’t consider it in terms of niche construction?
- What are some arguments against the idea that humans have “unusual cognitive abilities?”
- In light of new ideas regarding “dual-inheritance,” what are some reasons why anthropologists might be uniquely qualified to examine human cognitive evolution?
- How might we define “culture” in evolutionary terms?
- With the full acknowledgement that there is very likely more than one explanation for human brain encephalization, what is your favorite theory for why humans evolved such large and complex brains?
- How can we encourage interdisciplinary research when each field has their own “language”?