by Casey Fulkerson
In “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.