by Michelle Bird
Mankind has been plagued by questions of our origin from the beginning of time. Many different suggestions have been provided, but Falk argues that the intelligence inherent in modern primates is associated with changes that occurred over deep time, which consist of not only an increase in absolute brain size juxtaposed with a decrease in relative brain size (or RBS), but also asserts that the fluctuations in size alone are not enough to explain the amount of behavioral diversity observed among primates. She proposes that a sort of reorganization of the brain occurred in regards to the composition of cranial circuitry, neurochemistry, and subsystems, which reassembled to accommodate evolutionary behavioral changes.
Falk captures her audience by describing a uniquely primate propensity towards curiosity, and explains the proposed origins of how humans came to be both the largest-brained and most intelligent species. She describes the first monumental changes as “shifts” from leading nocturnal, terrestrial lives to diurnal, arboreal ones, thus explaining our “enhanced visual ‘modules’” as opposed to those of the olfactory, and the increase in visual prominence that “led to improvements in sensory-motor coordination in conjunction with a variety of locomotor patterns that evolved in different arboreal species (Falk 1496). Following descriptions of established direct (studying fossilized endocasts) and indirect (comparative) methodologies in the field of paleoneurology, Falk introduces new methods that “have major implications for normal as well as diseased human brain function, employ more and more complex methods and reveal the workings of smaller brain components” (Falk 1499).
Determining the true significance of primate brain size has been challenging, as Falk points out by describing the relationship between relative and absolute brain sizes. The absolute measure varies so greatly among living primates that comparison is virtually useless, but RBS presents its own issue of not accounting for allometric scaling that leads to a tendency to “[overestimate encephalization] for smaller-bodied species, but underestimated larger ones” (Falk 1502). In addition, this change in size does not account for the increase in intelligence observed among humans, for which Falk presents an explanation of the evolution of neurological reorganization. She states that early primate evolutionary trends included not only increased brain-to-body size ratios but also increases in the relative sizes of the neocortices and the amount of visual cortex, a decrease in the size of olfactory bulbs, and the development of a central rather than coronal sulcus in anthropoids (Falk 1509). This information paired with a comparative study of cortical specializations that showed evolution occurring independently in both New and Old world monkeys as well as the retention of similarities from a common ancestor suggest that “the addition of new cortical areas may have provided an opportunity for the evolution of new behavioral capacities” (Falk 1510).
She continues to describe how neurological reorganization has been evidenced to have occurred in the cerebellum through comparisons of the lateral cerebellar system among primates, which is described as “relatively large in chimpanzees and gibbons, while a central nucleus is larger in humans than in apes” but is smaller in humans that predicted (Falk 1510). The implication here is that an increase in size is not enough to explain all of the advances in motor coordination and cognitive thinking, and also that it is not necessary for a part of the brain to be “’new’ or grossly enlarged for reorganization to occur” which implies that these evolutionary advances may not have to be selected for by nature at all. There is also evidence that a complex social life could have influenced the selection for a larger brain, meaning the organization of human brains “is quantitatively different from any other living primate” and is not related to brain size alone. Another study has suggested that human cognitive ability, contrary to prior belief, has not “evolved in conjunction with differentially enlarged frontal lobes”, explained by quantifying the allometric nature of the human frontal lobe” (Falk 1514).
Falk’s main purpose is to bring to light some of the lesser known intricacies that drive primate brain evolution and how they have influenced both neurological structure and function, and dispute the claims that alterations in brain size alone are enough to account for the variety of behaviors that occur across time among primates. She asserts that “arguments about the relative evolutionary merits of brain size versus neurological reorganization are unnecessary” and that “the sizes of different brain structures are a consequence of overall brain size” and not necessarily the result of a specific kind of trade-off (Falk 1517). For all of the unknowns concerning the origins of human brain evolution, Falk has clearly shown that the “high intelligence of today’s primates flowered from trends in primate brain evolution that reach back into deep time” and has effectively introduced the possibility that “neurological reorganization can take place with, or without, an increase in brain size” and suggests that because of this, there is “the potential for evolving internal functional interactions, loops, or modules” within the brain (Falk 1518).
Falk, D., 2014. Evolution of the primate brain. Henke W., Tattersall I. Handbook of Paleoanthropology, continually updated edition, Springer-Verlag Berlin Heidelberg. DOI: 10.1007/SpringerReference_135072