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17 A Hierarchical Model of the Evolution of Human Brain Specializations--H. Clark Barrett
Pages 313-334

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From page 313...
... However, if brain mechanisms evolve through processes of descent with modification, they are likely to be het erogeneous, rather than coming in just two kinds. They are likely to be hierarchically organized, with some design features widely shared across brain systems and others specific to particular processes.
From page 314...
... Theoretically, although it is clear that the brain is the product of evolutionary processes, including natural selection, we cannot move past this simple truism if we are unable to answer the question of what adaptations it contains, or to distinguish the results of natural selection from the results of other processes. Empirically, a variety of methods have been developed for studying brain specializations, including studies of developmental disorders and brain lesions, brain mapping techniques, experimental psychology tasks, comparative studies of brain anatomy and development, and more recently, studies of gene expression and gene regulation in the brain.
From page 315...
... shared more widely across organismal structures, and relatively recent ones (e.g., properties of specialized brain regions) more narrowly distributed, in a hierarchically organized fashion (Carroll et al., 2005)
From page 316...
... Type and Token Outcomes of Developmental Processes Because natural selection acts on phenotypes, developmental processes are selected based on the phenotypic outcomes they produce. However, the plastic nature of mammalian brain development means that actual phenotypic outcomes may vary substantially between individuals along some dimensions, while exhibiting similarities along others.
From page 317...
... . Because human brains contain multiple developmental processes, they are likely to contain different reaction norms for different functional regions and processes.
From page 318...
... and the developmental procedures (i.e., reaction norms) that shape development as a function of inputs.
From page 319...
... . As brain specializations evolve through descent with modification, they inherit ancestral design features -- including underlying genomic building blocks and regulatory machinery -- that were present before recently derived changes.
From page 320...
... Given the location of the FFA within a larger region known to be active in object recognition more generally, it is likely that face recognition abilities are a specific token within a type category of object recognition procedures, akin to claws as a token of crustacean limbs more generally. Thus, processing signatures characteristic of objects in general are of limited use in testing between the domain-specific and domain-general hypotheses because, like limbs, specialized brain structures are likely to exhibit a combination of specialized and general properties.
From page 321...
... If derived brain specializations evolve from ancestral ones via processes of descent with modification, and if these historical processes leave a signature in the design and organization of brain mechanisms, this has implications for the study of human brain architecture and the evolution of so-called uniquely human traits such as language and complex culture. Varieties of Homology Homologous traits are traits that descended from a single ancestral trait.
From page 322...
... human brain mechanisms and processes are likely to exhibit relationships of homology (Kaas, 1989; Striedter, 2005)
From page 323...
... . Specialized, category-specific object recognition capacities may have evolved via duplication and divergence from a previously undifferentiated object recognition system.
From page 324...
... . More generally, other signatures of neural processing might be widely duplicated across brain mechanisms and regions, for example, Bayesian updating procedures, statistical learning, effects of magnitude such as those described in Weber's law, and others (Kirkham et al., 2002; Nieder and Miller, 2003; Chater et al., 2006)
From page 325...
... . Changes in one part of the brain can alter how information is routed to or processed by other parts of the brain, potentially altering how natural selection acts on those areas, as in the scenario described earlier in which increased attention to faces might alter selection on object recognition areas.
From page 326...
... . This is consistent with a hierarchical specialization view: word recognition is a token, albeit an evolutionarily novel one, of an evolutionarily specialized type of brain mechanism, that is, a category-specific object recognition module.
From page 327...
... One possibility is that the prime mover in brain expansion was selection for increased neural processing power per se. However, if increased brain size forces increased modularity for architectural reasons, this may set the stage for natural selection to favor further specialization of the resulting brain regions.
From page 328...
... . Comparisons with modern chimps and bonobos suggest that the CHCA was likely to be a social species with a relatively long lifespan and a sophisticated cognitive toolkit including social learning of tool use, "Machiavellian" social intelligence, and some elements of theory of mind, such as tracking others' knowledge of food in food competition and sensitivity to intentional communication in contexts such as aggression and reconciliation (Call and Tomasello, 2008; Whiten, 2011)
From page 329...
... , suggesting that modifications in how brain regions communicate with each other may have played an important role in hominin brain evolution. If the hierarchical specialization view is correct, we should expect to see selection on genes with different patterns of expression or activity in different parts of the brain.
From page 330...
... Modified Orthologies Many derived features of human cognition may be the result of modifications to older brain systems, in the form of modifying the design of those systems per se or modifying how they interface with other and perhaps newer systems. Such modifications are likely to be involved, for example, in the evolution of human language abilities.
From page 331...
... There may be other mechanisms of higher-level cognition that have evolved through duplication and divergence -- a possibility suggested by expansion of prefrontal cortex in humans -- but there remains controversy over how to characterize specializations in this area. Given the many apparently unique aspects of human cognition, including ratcheting cultural evolution, language, the ability to cooperate in large groups, morality, and a unique elaborated theory of mind, there are likely to be many additional examples of derived specializations in humans that we have not yet discovered.
From page 332...
... Many psychologists believe that evolutionary considerations imply a tradeoff between a few generalized processes and many specialized ones, and that the former is more likely because generalized processes yield more flexibility. However, if it turns out that the way evolution creates more flexible brains is by proliferating specialized brain regions that carve up computational problems via specialized division of labor, this widely held assumption may turn out to be wrong.
From page 333...
... For example, a particular phenotypic outcome in the brain may be contingent on developmental input, and also the result of a reaction norm selected to produce that phenotype given that input. To properly test evolutionary hypotheses about brain specialization, then, it is important to compare apples against apples and oranges against oranges: to compare hypotheses posed at equivalent levels of the ultimate–proximate continuum of evolutionary causation.


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