by Casey FulkersonIn “Evolution of the Primate Brain,” Falk seeks to answer the question of “how humans came to be the largest-brained primate but also the most intelligent species on Earth” (Falk, 2014 pg. 1496). To best answer this question, we must study the evolution of the primate brain and the adaptations made early in primate brain evolution and how those early adaptations have influenced brain physiology of primates and humans. This is done by direct and comparative methods. The direct method is to study endocasts, or casts of the interior of braincases. These can be physical, fossilized endocasts and virtual endocasts created by 3DCT data, both allowing for the study of living and extinct species of primates (pg. 1498). The comparative methods are used to study living species and include histochemical, immunocytochemical, positron emission tomography (PET) scans, and functional magnetic resonance imaging (FMRI) scans. Using these methods, researchers can learn about neurons, including their types, sizes, densities, distributions, and connections, and about functional processing in the brain and how it works in terms of movement, sensation, sleep, preparation for action, thinking, and emotions (pg. 1499).
Apparently, there is some division among paleoanthropologists regarding the importance of absolute brain size and brain reorganization. Specifically, the argument is about which is superior. Falk provides in-depth analysis of both areas. In the sections devoted to the evolution of primate brain size, Falk discusses how to best interpret primate brain size, which is quite the undertaking due to the wide variation among primates. This wide range of brain size makes comparative study between primates difficult, so relative brain size, or RBS as it is referred to as in Falk’s paper, is used. RBS is a ratio between brain and body size. Falk discusses the methods that comparative studies use to eliminate the effects of allometric scaling on the brain. The quotients mentioned are the index of progression (IP) developed by Bauchot and Stephan and Jerison’s encephalization quotient (EQ). It should be noted that EQ heavily depends on the group being studied for baseline data and can easily overestimate the EQ of smaller-bodied species and underestimate the EQ of larger-bodied species (pg. 1501, 1503). In later sections further delving into brain size of primates, Falk discusses studies done by Leigh that investigated two life-history strategies and their effects on infant brain grown and also two hypotheses that attempt to explain why primates (humans are included in this too) are able to grow large energetically expensive brains (the maternal energy hypothesis proposes that the mother’s basal metabolic rate during gestation determines neonatal brain mass and the expensive-tissue hypothesis says that encephalization is able to occur because of an evolutionary “trade-off” where amount of brain tissue has increased while the mass of other energetically expensive organs, like the heart, gut, liver, and kidneys has decreased) (pg. 1503-04). It should be noted that both hypotheses do not hold for bats and so their broader application in mammals is questionable. Falk also gives an overview of encephalization in hominins, providing the detailed Table 1 on pages 1506 and 1507. Table 1 lists cranial capacities for Hominins as evidence to show the encephalization of hominin brains, but Falk says on page 1505 that “error may be introduced, however, because fossil endocasts are rarely whole and, thus, usually require partial reconstruction.” Later he writes “Although many workers have estimated EQ’s for fossil hominins, these estimates must be taken with a grain of salt because of the difficulty of determining surrogates for body mass” (Falk 2014, pg. 1507). Few intact femurs of hominins provide few accurate measures for determining body mass and size and therefore RBS. Falk also discusses neurological reorganization because many believe that “size along is not enough to account for the observed diversity in primate behavior and that circuitry, neurochemistry, and subsystems must have been reorganized within brains to accommodate evolving behavioral repertoire” (pg. 1509). On page 1514 he mentions a study done by Semendegeri which dispels the idea that humans have abnormally large frontal lobes, which are larger due to allometric scaling, not because humans have better cognitive abilities (pg. 1514). It is also important to note that Neurological reorganization was not isolated to one area but was instead spread across multiple structures in the brain, dispelling the idea of a “mosaic evolution” evolution of the brain (pg. 1512). This is supported by the fact that high-order cognitive tasks engage areas of the brain across the cortical mantle and are not focused on one specific area (pg. 1512). Falk also mentions the role of mirror neurons in manual and orofacial communication in apes and humans and emphasizes the role of cortical asymmetries that are related to unique human behaviors like the tendency of right-handedness, symbolic language, and humanlike abilities for music, art, and technology (pg. 1516). Given that this is a Neuroanthropology class, the question of how this relates to Neuroanthropology and to the human brain should be at the forefront of our minds. This article seeks to answer the question of how humans came to be not only the largest-brained primate, but also the most intelligent - without ever having defined what, for the purposes of this paper, qualifies or quantifies the term “intelligence” or reaching any real conclusions. This is a valuable paper filled with information about endocasts and direct and comparative methods and the incredible range in primate brain size and the theorized ways that primates were able to support such an energetically expensive organ, but the answer to the ultimate question, how humans came to be the largest-brained primate and most intelligent animal, remains unanswered. We have theories and suggestions that seem to have merit because of the genetic closeness of humans and primates, but we do not have any concrete data that confirms these hypotheses. Paper: Falk, D. (2014), Evolution of the Primate Brain. Handbook of Paleoanthropology, 1495:1518.
9 Comments
Elisabeth Nations
1/29/2019 03:09:52 pm
Great overview of the article! I also thought this was a very thorough and well-done article. I felt like I have a much better understanding of how brain encephalization happens and the differences between general increasing brain size and the change and reorganization of certain structures within the brain. My interest was piqued by Falk's very brief mention that there is a difference between the anatomies of men and women's brains, and that these differences may have come about because of different reproductive strategies. What are these different strategies and how do they relate to the shaping of the human brain? Do they have to do with investment in parenting? I just thought that topic was particularly interesting and I hope we explore it further in class.
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Elisabeth Nations
2/26/2019 08:47:21 pm
Casey, you express at the end of this review that we don't have any concrete data to explain exactly how and why human brains came to be so unique, and I would definitely agree with you, and even go a step further to say that I doubt we'll ever have anything approaching concrete evidence. I'm a bit of a skeptic myself when it comes to making a lot of inferences based off of a few scarce fossils, but I have to say, especially after reviewing our class discussion, that I'm very impressed that we can make so many reasonable claims about the uniqueness of the human brain and what likely occurred over the course of encephalization. Allometric scaling and the calculation of the encephalization quotient prove that our brains are very much unique, even among other primates. It's fascinating that we've been able to compare ourselves to primates and figure out what parts of our brains have been reorganized, like our visual system and frontal lobe. So although it's highly unlikely we'll ever find evidence that all adds up to create a logical picture of encephalization throughout hominin history, I think the comparisons we've made to other primates by studying them today has given us enough valuable information for us to have a pretty good idea of how brain evolution occurred.
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Zach Obaji
1/29/2019 03:56:55 pm
Casey, I think you did a great job summarizing and analyzing Falk's article. From an evolutionary perspective, cranial capacities increased across species during hominin evolution. However, as you mentioned, this reason alone not fully answer why human primates have evolved to become among the most intelligent beings. I found the analyzation of neurological reorganization by Semendegeri quite fascinating. As mentioned in your post, his research using MRI scans to perform comparative statistical analyses of both relative and absolute brain volume among 30 humans and apes brings both answers and further questions. The enlargement of the frontal lobe is understandable when reflecting back to hominin evolution; however, his findings found smaller cerebellum in humans. Since this deviated from allometric predictions, you are indeed correct when you mentioned that we still need more concrete data and research. Although research in the evolution of brain development has advanced, future research is essential to answer many of these unanswered questions. Great blog post!
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Zach Obaji
2/27/2019 06:23:55 pm
I took ANT 270 with Dr. DeCaro last semester, and I remember learning about increasing cranial capacity throughout primate evolution. This was my favorite assigned reading so far this semester since I was able to read about the significance of primate brain evolution and how neuroanthropology gives us a greater understanding of how the human brain has evolved aside from just cranial capacity.
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Jennifer Fortunato
1/29/2019 07:21:30 pm
Thanks Casey for a great review of this paper! I agree with you that Falk does not actually explain what they mean by "intelligence" which would have given more weight to the content of the paper. However, I do enjoy the comparative approach that they take to this paper by analyzing the many different way of quantifying brain size, the organization of the brain and evolution and neurochemical pathways of both. This comparative approach made me curious to see what other research has been done on visual systems as that has come up multiple times in the readings. I found another research article by Orban et al.(2004) that compared higher visual areas in monkeys and humans using fMRI and found that certain functional regions of the brain, like the early visual system and motion area, are conserved in humans. I believe more research, like Orban et al. could be very useful for the field of neuroanthropology because it can insert biological relevance into an anthropological viewpoint and bring us closer to the question that you pose " how humans came to be the largest-brained primate and most intelligent animal?"
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Moe Prince
1/30/2019 09:25:08 am
This was an awesome overview of the article. Falk's article was incredibly detailed when explaining the different ways brain size can vary. I found the amount of detail to be confusing at times, since they would bring up interesting points (like decreasing brain size in new world/old world monkeys) but didn't elaborate. However, I do agree that one of the details they lacked on were what specifically classified as intelligence. It was kind of implied to be complex social structures, language, and infrastructure. However, it wasn't entirely clear.
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Samuel Scopel
1/30/2019 09:36:43 am
Thanks for the review Casey. I do appreciate Falk mentioning that these different comparative approaches (ie brain size vs reorganization) are not mutually exclusive when examining these relationships, nor is one approach "better" than the other in every case. I believe this speaks to the more holistic, interdisciplinary approach that is emphasized within neuroanthropology. A given method might yield more useful information than another within a specific context.
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Jennifer Fortunato
2/26/2019 01:04:55 pm
After our classroom discussion, I understand a little bit more about how absolute and relative brain size are helpful in understanding primate brain evolution. Absolute brain size has been shown through the fossil record to have been grown in different primates. However, from our classroom discussion I have realized that relative brain size is more important when looking at primate brain evolution. Though it is not as well recorded in the fossil record because of assumptions about how large the skull is based upon relationships between skeletal structures, relative brain size has increased in primates as well. Understanding encephalization quotient and reorganization of different brain structure is even harder because we would need soft tissue samples to analyze the differences between ancestral species and current species. However, we can make some assumptions, though not great ones, about reorganization from looking at extant species. The classroom discussion has imbued upon me the importance in taking different aspects of brain size to understand how primate brain evolution has occurred and the why it is important to study non-human primate brain evolution to better understand human brain evolution. Using comparative approaches, a strong aspect of neuroanthropology, can help us gain an even better understanding of primate brain evolution.
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10/14/2021 02:42:36 am
i like this article, it has a lot of info i need to know, thanks for sharing
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AuthorThis blog is group authored by Dr. DeCaro and the students in his ANT 474/574: Neuroanthropology. Archives
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