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Figurę 1 Neocorto vatume as a ^rsoc & medulia vołume in differerit groups of primates (aftar Pasingriarri®) Souce Stephan et aS²*

the brain such as the medulia. the neocortex shows dramatlc and łnereas-ing expansion across the rangę of primates (Fig. 1). The neocortex is ap-proximately the same size as the medulia in insectłvores; however, it is about 10 times larger than the me-dulla in prosi rnians and 20-50 limes larger in the anthropoids, with the human neocortex being as much as 105 times the size of the medulia.

This suggests that rather than look-łng at total brain size, as previous studies have done. we shouid in fact be considering the brain system, namely the neocortex. that has been maJrdy responsible for the expansion of the primate brain. From the point of view of all the hypotheses of primate brain evolutk>n, this makes sense: The neo-

ccwnpłcnty of socaal groups. the fact that Information-proce&sing dernands can be expected to increase as the number of relabonships invołved in-creases. Morę importantly. perhaps, this measure has the rfisfrinrt merit of bemg easily quantified and wideły a.ailabk Ałthough it is possible to conceśve of a number of bet ter mea-sures o# sociai compkudty, the appro-priate data are rarely ava2łabie for morę than one ar two species.

The second problem concerns the most appropriate measure of brain evołution. Hitberto, most studies have ccrtsidered the brain as a single func-tionai unit This view has been rein-forced by F tniay and Dariington,¹¹ who argued that the ewołution in brain part size closely correlates with the evoki-tion of total brain size and can be espiamed simply in terma of allome-tric consequences of increases in total brain size. However. Finlay and Dar-ttngton failed to consider the possibil-tty that changes in brain size might actuaDy be driven by changes in its parts racher than in the whołe brain. This is especialiy true of the neocor-to. for its vol ume accounts for 50% to 80% of totai brain . ol ume in primaies. Thus. changes in the vokune of the neooortes mevitabły have a large di-rect effect on apparent change in brain vołume that rnay be quite unrełated to changes in other brain components This point is given weigm by the fact that Finlay and Dariington themsehes showed that neocorte* size is an expo-nential function of brain size. whereas other brain components are not Finlay and Dariington¹¹ notwith-standing, there is evidence that brain evołution has not been a hlstory of simple expansion in total volume. Rather, brain evolution has been mo-saic in character, with both the ratę and the extent of evolution having varied between components of the system. MacLean¹⁷ pointed out many years ago that primate brain evolution can be viewed in terms of three major Systems (his concept of the triune brainj. These systems correspond to the basie reptilian brain (hind- and midbrain systems), the mammalian brain (palaeocortex, subcortical systems). and the primate brain (broadły. the neocortex). A morę important point, perhaps. is that variations can be found within these broad catego-ries in the rates at which different components expanded. which, in at kast some cases. have been shown to correlate with ecological factors.¹⁰ Par tialling out the effects of body size on the size of brain components suggests that the story may be morę complex than Finlay and Dariington¹¹ sup-posed, with some remodeling of brain growth patiems occurring in the tran-sitions between insectivores. prosim-ians, and arithropokis.-*

The iroporta/j’. point in the present contect is that* as Passingham¹⁴ noted, relative to the morę pnrnitive parts of

... brain evolution has not been a history of simple expansion in total volume. Rather, brain evolution has been mosaic in character, with both the ratę and the extent of evolution having varied between components of the system.

cortex is generally regarded as being the seat of those cognitive processes that we associate with reasoning and consciousness, and therefore may be expected to be under the moja intense seleclion from the need to increase o: improve the efTectiveness of these processes.

One additional problem needs to be resolved. In his semina! study of brain evolutlon. Jerison²⁰ argued that brain size can be expected to vary with body size for no other reason than fundamenta! ałlometric relationships associ-ated with the need to manage the physiokjgical machinery of the body. What is of interest, he suggesied, is not absołute brain size. but the spare brain capacity over and above that needed to manage body mechanisms.

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