The Evolving Human 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.


Blue Whale at The American Museum of Natural History

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.

Pocket Mouse at White Sands National Monument

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.

Human Olfactory Bulb


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”).


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).


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.


Capuchin Monkey

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.

Cerebellum in Humans


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”?

18 thoughts on “The Evolving Human Brain”

  1. Great summary! A question that I came across while I was reading this was how come humans have the most complex social structures? I understand that we do and our brains evolved in accordance with this theory but how come this didn’t happen to other non-human primates? It almost seems like we have a retroactive explanation for why we are more intelligent. We certainly know we are and social complexity is a large reason for that, but why did this complexity evolve only in humans? Which came first, the social complexity or the intelligent brain? Regardless of the answer, why only in humans?

  2. The inclusion of both more traditional evolutionary theories and newer, less explored ones gave more depth to the reading that I enjoyed. I also agree it is important to emphasize the idea that rather than throwing out older evolutionary ideas we should build on and expand them. This seems especially important when it comes to the brain as the human brain is dependent not just on genetics but on the development it goes through and the environment’s influences on that development. The human brain’s plasticity is remarkable and can help to explain why humans were able to spread themselves across the globe so effectively. Human brain evolution in a modern context is discussed at the end of the Downey and Lende chapter which was very interesting. Selective pressures influencing human brain evolution in the contemporary era is exciting, albeit a little frightening especially in the case of the “super-bug” scare, but can make one wonder how the human brain will continue to change.

    1. This topic is certainly interesting especially considered with all the topics we have continued to cover through the course. Seeing how complex the brain is and discussing different topics such as embodiment really puts in perspective how our brains have truly become able to excel at understanding complex ideas. It is just interesting to think what makes our brains unique and how they came to be which gets more intriguing with each class.

  3. I really like the extended, evolutionary version of the “nature vs nurture” debate presented in the last part of Downey and Lende’s chapter. I thought they covered some major theories about it really well, outside of just the Niche Construction theory. There are many co-factors outside of just the environment and genetics that play into the evolution of the brain, but the chapter also does a great job qualifying the plethora of interaction theories and points out they both support and contradict each other. I liked their comments about semantics within the field, and that of course it’s good to be mindful about how we’re labeling things, but at the end of the day their all extremely similar and overlapping.

    I was also lost for a lot of the Balsters article. There was way too much jargon without an explanation, and without a solid foundation of that particular topic I found it extremely daunting to get through. It also seemed like they had to continually qualify their own math, as if they had done a different analysis they would have gotten different results. That doesn’t seem very scientific to me, and while I understand they were proposing a fairly new idea, it seems to me your results should be more robust than that.

    1. I really liked hearing some about epigenetics in this context. I’m not very familiar with epigenetics so it was great to hear other people’s thoughts on it! I also really enjoyed the rat licking demonstration. Since I have a pet rat I know that they’re extremely social creatures, but I guess I didn’t have an idea of what that meant in early development. It’s nice knowing that nurture plays such a big role in development! (Also, my rat is super friendly and playful so he must have had an extremely attentive mother).

  4. This is a very well written and intriguing post. I’m curious about the academic language changing between fields as well. I feel as if most disciplines of advanced study come with their own lexicon, and I’ve rarely seen them translate well. “Reciprocal determinism”, to a psychologist, is the same as “socioconigitve theory” to a neuroscientist. But then again, I’ve heard both neuroscientists and psychologists use the latter.

    While I grant that it is important to give new discoveries and theories new names, it makes me wonder about the intention behind the names. I understand a molecular biologist using an acronym for a protein they’re studying, and using niche terms to explain the mechanisms of it. However, I can’t help but feel that some discipline-specific words (across all specialized disciplines, from law to biology,) are intentionally overcomplicated or buzzworthy. Someone mentioned the concept of “neuroinflation”in an earlier post, or the use of the word “neuro” in a term to make the term /study seem more important or noteworthy.

    It’s unfortunate that a large amount of terms we use have such specialized rhetoric, but theories and new discoveries must be named -something-. I just think it limits the amount of information that we can communicate across disciplines, or to people trying to learn about the field. The two methods of integration I can currently foresee are either everyone agreeing on a term and sticking to it (unlikely), or a huge, encompassing thesaurus/dictionary mashup of various terms (more unlikely still). So then, I come back to the original question posed here- how -do- we get everyone on the same page, if we even can?

  5. I thought that this was a very well written summary of the two readings that we had to do for this week and I’m looking forward to discussion tomorrow. I am hoping to discuss the theories presented by Lende and Downey in the last section of the chapter during class tomorrow. I thought the idea of emotions playing a role in cognitive development was interesting and I just wanted to hear other opinions on how the lines were drawn between the social intelligence and emotional changes theory.

    I also thought the section on wet wiring was extremely interesting just because I never thought to think of our social interactions as having that large of an impact on the brain structure.

    1. I’m also interested by how social intelligence and emotional changes theory are related. I know that humans have facial expressions that are globally recognized and that is an additive factor to why our brains are so intelligent, however, don’t some animals have facial expressions also? Besides the bearing of teeth to show anger, don’t they also have expressions that are recognizable amongst each other? I feel like they have to somehow be able to convey feelings to each other, otherwise, why or how, for example, would a male know if a female wants to copulate? Some literature states that some non-human primates participate in rape. How would we know that if they don’t show emotion? This question leads me to see the emotional aspect of the intelligent brain theory as less valid.

  6. I want to respond to the question about why humans evolved such large and complex brains. Obviously, there are a lot of conflicting theories about this, but most people who study human evolution agree that there were around 20 or so species of early humans. Many disagree about relatedness and extinction of the species, but it seems generally accepted that most died out and left no descendants behind. Therefore, our brain structures are clearly a favored adaptation. What’s interesting to me is whether or not the changes in our brains were a result of adaptions for specific purposes. This highly specified functions that we think of as adaptations could ultimately just be a part of a general process. There is a lot of debate about this in cognitive science, especially when it comes to higher function.

    1. This isn’t totally related, but I enjoyed the tangent on birth and detecting female ovulation. I’ve read studies that show both men and women can detect female ovulation, but consider its importance to be relative only to female competition for mates. I think more research needs to be done in this area.

      I also really liked the ted talk on Brain Soup. Counting neurons is a really interesting way to compare brain sizes and capability across species. She also did an interesting study about the relationship between the cerebral cortex area and thickness and the number of cortical folds. Her model relates this in a method that also explains the scaling of the folding index of crumpled paper balls.

  7. Has anyone discussed the theory of extended cognition in relation to brain evolution? In terms of “Not a Brain Alone,” we could consider that cognition could have been increasingly extended into the environment, technology in simplistic ways, and, especially, social networks to affect how early human used their brains and how cognitive structures could have evolved. This idea, if it works, could extend niche construction and the social brain hypothesis. Rather than saying that “niche modifying creates a feedback loop from behavior to environment to selection” (Downey & Lende 2012:118), why not think about it as cognition to social group to environment to selection, where, effectively, cognition is socially distributed rather than considered isolated in individual brains?

    1. Further thought, we could then talk about cognition as extended into culture as well… ritual, for example, or even cultural models as types of extended cognition.

      1. This is still something I’m interested in talking/thinking about more. It seems like a cool way to marry cultural consensus/consonance theory with other aspects of cognitive science. However, I’m not sure how one would include it in a research design other than a theoretical perspective. How does one study extended cognitive methodologically?

  8. I question their suggestion that motivation, perception, and emotions help to explain why our cognitive evolution is so much more developed than other animals because in my opinion, some of these things are not unique to us. Many animals seem to be able to recognize emotions within their own species, but some new studies suggest that dogs can even recognize human emotion. This would make sense when you consider how closely entangled their lives are with ours. It could also help explain how emotional support dogs are able to recognize human sadness or fear.

  9. I guess I just don’t understand how motivation, emotion, and perception make our brains unique, when some studies show that these elements can be observed in other animals. A study from the University of Buffalo suggested that some animals may even be capable of metacognition, a process previously only believed to be seen in humans. If these higher-level social and cognitive processes are being seen in other animals does that suggest that their brains are evolving to be more like ours, or are our brains just not as unique as we thought? If so, then what is it that actually makes our cognitive evolution so advanced?

  10. The thing that struck me about this lesson the most was when Mandy showed us the “lick the mice” simulation, as weird as that sounds. What we learned was that the more a mouse pup was licked (how a mouse mother cares for its pup), the better of a mother it becomes. I like Epigenetics a lot, and always have. I like the idea that more than hard genetics at birth determine and inform things like maternal care, and even social intelligence.

  11. I also thought that the ted talk on brain soup was super interesting. The brain is a super cool organ that I feel I don’t give enough credit to sometimes. I think the idea that our upbringing and our culture does have as large an impact on our neural connections as it does. Its reassuring that our personality and way of thinking is not predetermined.

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